Master's degrees completed
The following is a list of master's dissertations that were recently completed in this Department. The list is linked to the Powerpoint presentations that were used during the public defences of these dissertations.
All these dissertations can be found on the the University of Pretoria electronic theses and dissertations service, UPeTD.
F.P.A. Prinsloo, 2017 "Investigation of turbulent heat transfer and pressure drop characteristics in the annuli of tube-in-tube heat exchangers (horizontal lay-out)"
Tube-in-tube heat exchangers are commonly used in many applications and are generally operated in a counterflow configuration. Unfortunately, existing correlations developed for heat transfer and pressure drop predictions for the outer annular flow passage have been found to sometimes produce large discrepancies between them.
In this experimental study research was performed to obtain experimental data with the lowest possible uncertainties associated with it in order to validate existing correlations and to identify the core aspects that influence the heat transfer and pressure drop characteristics in annular flow passages that have neither uniform wall temperatures nor uniform wall heat fluxes. Focus was placed on the turbulent flow regime and temperature and pressure drop measurements were taken at different fluid velocities, annular diameter ratios, and inlet temperature of water.
Four horizontal test sections with annular diameter ratios of 0.327, 0.386, 0.409 and 0.483 and hydraulic diameter of 17.00, 22.98, 20.20 and 26.18 respectively were constructed from hard drawn copper tubes. The test sections were equipped with industry standard inlet and outlet configurations and had pressure drop lengths of between 5.02 m and 5.03 m and heat transfer lengths of between 5.06 m and 5.10 m. This resulted in length to hydraulic diameter ratios of between 194 and 300. A wide range of annular flow rates were considered and Reynolds numbers ranges from 15 000 to 45 000 were covered for both heated and cooled annulus operating conditions. Specific attention was given to the influence of the inlet fluid temperature. For heated annulus cases an inlet temperature range of 10°C to 30°C was covered, while for cooled annulus cases an inlet temperature range of 30°C to 50°C was covered.
Since one of the main focuses of the study was to provide accurate temperature measurement, especially local wall temperature measurements of the inner tube, an in-situ calibration technique of the wall thermocouples were used. This enabled continuous verification of the measurement accuracy and allowed re-evaluation of readings.
Based on the processed experimental results, it was found that the direction of heat transfer did not affect the average heat transfer coefficient across the inner tube wall. Longitudinal local heat transfer coefficients were found to not be constant along the test section length, but continually decreased towards the annulus outlet, indicating undeveloped thermal flow. Heated annuli had a larger average heat transfer coefficients compared to cooled annuli at similar Reynolds numbers. This can be attributed to a dependency on fluid properties, which were less at higher bulk temperatures. Analysis showed although both had about the same local Nusselt numbers at the exit region, the heated annuli had much larger Nusselt numbers at the entrance region of the test section. The friction factor was mostly affected by the fluid velocity, but at low velocities higher friction factors were detected when inlet temperatures were lower.
For the data sets considered in this study, the average Nusselt number and the Colburn j-factor decreased somewhat with increase in annular diameter ratio. It seemed that the friction factor was also not influenced by the annular diameter ratio.
Supervisor:Prof. J. Dirker
Co-supervisor: Prof. J P. Meyer
JC Pieterse, 2017 "High Pressure Feedwater Heaters Replacement Optimisation "
Widespread uncertainty exists regarding the ideal replacement time of installed feedwater heaters in coal fired power plants. Eskom consequently identified the need for this research project to find the optimal age at which to replace high pressure (HP) feedwater heaters. Previous work has failed to quantify the unique financial risk of tube failures, which varies for individual heaters. Using life cycle cost (LCC) methodology, a framework is developed for the optimisation of the HP feedwater heater replacement age in Eskom coal fired power plants and integrated into existing software used in the organization. This entails identifying the most significant cost factors involved in the lifecycle of HP heaters and determining how they evolve over time by conducting a case study. Minimum life cycle cost for an actual HP heater is calculated in the case study based on failure data and cost information supplied by the power plant. This optimisation of replacement time can realise significant savings in annualised LCC compared to current practice.
Supervisor: Prof Jasper L. Coetzee
S Schmidt, 2017 "A cost-effective diagnostic methodology using probabilistic approaches for gearboxes operating under non-stationary conditions"
Condition monitoring is very important for critical assets such as gearboxes used in the power and mining industries. Fluctuating operating conditions are inevitable for wind turbines and mining machines such as bucket wheel excavators and draglines due to the continuous fluctuating wind speeds and variations in ground properties, respectively. Many of the classical condition monitoring techniques have proven to be ineffective under fluctuating operating conditions and therefore more sophisticated techniques have to be developed. However, many of the signal processing tools that are appropriate for fluctuating operating conditions can be difficult to interpret, with the presence of incipient damage easily being overlooked.
In this study, a cost-effective diagnostic methodology is developed, using machine learning techniques, to diagnose the condition of the machine in the presence of fluctuating operating conditions when only an acceleration signal, generated from a gearbox during normal operation, is available. The measured vibration signal is order tracked to preserve the angle-cyclostationary properties of the data. A robust tacholess order tracking methodology is proposed in this study using probabilistic approaches. The measured vibration signal is order tracked with the tacholess order tracking method (as opposed to computed order tracking), since this reduces the implementation and the running cost of the diagnostic methodology.
Machine condition features, which are sensitive to changes in machine condition, are extracted from the order tracked vibration signal and processed. The machine condition features can be sensitive to operating condition changes as well. This makes it difficult to ascertain whether the changes in the machine condition features are due to changes in machine condition (i.e. a developing fault) or changes in operating conditions. This necessitates incorporating operating condition information into the diagnostic methodology to ensure that the inferred condition of the machine is not adversely affected by the fluctuating operating conditions. The operating conditions are not measured and therefore representative features are extracted and modelled with a hidden Markov model. The operating condition machine learning model aims to infer the operating condition state that was present during data acquisition from the operating condition features at each angle increment. The operating condition state information is used to optimise robust machine condition machine learning models, in the form of hidden Markov models.
The information from the operating condition and machine condition models are combined using a probabilistic approach to generate a discrepancy signal. This discrepancy signal represents the deviation of the current features from the expected behaviour of the features of a gearbox in a healthy condition. A second synchronous averaging process, an automatic alarm threshold for fault detection, a gear-pinion discrepancy distribution and a healthy-damaged decomposition of the discrepancy signal are proposed to provide an intuitive and robust representation of the condition of the gearbox under fluctuating operating conditions. This allows fault detection, localisation as well as trending to be performed on a gearbox during fluctuating operation conditions.
The proposed tacholess order tracking method is validated on seven datasets and the fault diagnostic methodology is validated on experimental as well as numerical data. Very promising results are obtained by the proposed tacholess order tracking method and by the diagnostic methodology.
Supervisor: Prof. PS Heyns
Co-supervisor: Dr. JP de Villiers
M S Cowley, 2017 "Optimising pressure profiles in superplastic forming"
Some metals, such as Ti-6Al-4V, have a high elongation to failure when strained at certain rates and temperatures. Superplastic forming is the utilisation of this property, and it can be used to form thin, geometrically complex components. Localised thinning occurs if the specimen is strained too quickly, and components with locally thin wall thickness fail prematurely. The superplastic forming process is investigated with the finite element method. The finite element method requires a material model that describes the superplastic behaviour of the metal. Several material models are investigated in order to select a material model that can predict localised thinning at higher strain rates.
An optimisation algorithm is developed to minimise the forming time of some component by prescribing the pressure profile, subject to a lower limit on the minimum thickness. This algorithm involves fitting a metamodel to simulated data (using the finite element method), and using the metamodels to search for the optimum pressure profile. The final forming time of the superplastic forming of a rectangular box was successfully minimised while limiting the final minimum thickness. The metamodels predicted that allowing a 4% decrease in the minimum allowable thickness (1.0 mm to 0.96 mm) that the forming time is decreased by 28.84%. The finite element verification indicates that the final minimum thickness reduced by 3.8% and that the forming time reduced by 28.81%.
Supervisor:Prof S. Kok
R. Kombo, 2017 "Qualitative analysis of flow patterns: Two-phase flow condensation at low mass fluxes and different inclination angles "
A great deal of work has been conducted on in-tube condensation in horizontal and vertical smooth tubes. The available literature points to mechanisms governing two‑phase condensation heat transfer coefficients and pressure drops, which are directly linked to the local flow pattern for both horizontal and inclined configurations. However, the work has been limited to flow pattern observations, heat transfer, pressure drops and void fractions for both horizontal and inclined tubes at high mass fluxes. No work has been conducted on the analysis of the observed flow patterns and the effect of temperature difference between the average wall temperature and average saturation temperature for different inclination angles at mass fluxes of 100 kg/m2.s and below. The purpose of this study is to carry out a qualitative analysis of flow patterns, and show the effect of temperature difference on the heat transfer coefficient for inclination angles from +90° (upward flow) to ‑90° (downward flow) at mass fluxes below 100 kg/m2.s. An experimental set-up provided the measurements for the two-phase condensation of R‑143a in a smooth tube with an inside diameter of 8.38 mm and a length of 1.5 m. The mass fluxes were 25 kg/m2.s to 100 kg/m2.s, the saturation temperature was 40 °C and the mean qualities were 0.1 to 0.9. A high‑speed camera was used to visually analyse and determine the flow patterns for both the inlet and the outlet of the test section. Through the results, eight flow patterns were observed: stratified‑wavy, stratified, wavy, wavy‑churn, intermittent, churn, annular and wavy‑annular. The maximum heat transfer was observed for downward flow between inclination angles of ‑15° and ‑30°. The Thome-Hajal flow pattern map correctly predicted horizontal flow patterns, but failed to predict most of the inclined flow patterns. Various flow pattern transitions were identified and proposed for all the investigated inclination angles in this study. Finally, the heat transfer coefficient was found to be dependent on quality, mass flux, temperature difference and inclination angle.
Supervisor: Pro. J.P. Meyer
B. W. Kohlmeyer, 2017 "Development of an improved design correlation for local heat transfer coefficients at the inlet regions of annular flow passages"
Several applications, including those in the energy sector that require high thermal efficiency, such as those in the solar energy industry, require a careful thermal analysis of heat exchange components. In this regard, thermal resistance is a major cause of exergy destruction and must be minimised as much as possible, but also adequately designed.
In the past, a number of correlations have been developed to predict heat transfer coefficients in compact heat exchangers. The designers of such heat exchangers often exploit the development of thermal boundary layers to achieve higher overall efficiency due to increases in local heat transfer coefficients. However, most of the correlations that have been developed for heat exchangers neglect the specific effect of the thermal boundary layer development in the inlet region, and instead only offer effective average heat transfer coefficients, which most users assume to be constant throughout the heat exchanger. This is often an over-simplification and leads to over-designed heat exchangers.
In this study, focus is placed on annular flow passages with uniform heating on the inner wall. This geometry has many applications. This study aims to collect experimental heat transfer data for water at various flow rates and inlet geometries, to process the data and determine local and overall heat transfer coefficients, and to develop an improved local heat transfer coefficient correlation.
Experimental tests were performed on a horizontal concentric tube-in-tube heat exchanger with a length of 1.05 m and a diameter ratio of 0.648. The surface of the inner tube was treated with thermochromic liquid crystals (TLCs), which allowed for high-resolution temperature mapping of the heated surface when combined with an automated camera position system in order to determine local heat transfer coefficients. Conventional in-line and out-of-line annular inlet configurations were evaluated for Reynolds numbers from 2000 to 7 500, as well as the transition from laminar to turbulent flow for a single in-line inlet configuration.
It was found that the local heat transfer coefficients were significantly higher at the inlets, and decreased as the boundary layers developed. With the high resolution of the results, the local heat transfer coefficients were investigated in detail. Local maximum and minimum heat transfer coefficients were identified where the thermal boundary layers merged for high turbulent flow cases. The annular inlet geometries only influenced the heat transfer for Reynolds numbers larger than 4000, for which larger inlets are favoured. Out-of-line inlet geometries are not favoured for heat transfer. A new heat transfer correlation was developed from the experimental data, based on an existing heat transfer correlation for turbulent flow in an annular flow passage, considering the boundary layer development. The new correlation estimated the area-weighted heat transfer coefficients within 10% of the experimental data and closely followed trends for local heat transfer coefficients.
Supervisor: Prof. J. Dirker
Co-supervisor: Prof. J P. Meyer
M Kandindi, 2017 "Heat transfer and pressure drop investigation for prescribed heat fluxes on both the inner and outer wall of an annular duct"
Heat exchangers are used in industrial processes to recover heat between two processes fluids and are widely used. Although the equations for heat transfer and pressure drop characteristics in a double-pipe heat exchangers are available, there is still need to completely understand how these characteristics interact which geometrical factors like annular diameter ratio or some thermal boundaries conditions which have not yet drawn more attention from the research community.
The purpose of this study was to experimentally measure the heat transfer and pressure drop characteristics of a concentric annular duct of ratio 0.593 for different heat fluxes simultaneously on the inner and outer tube in the turbulent flow regime and to describe or discuss the impact or interaction of heat flux ratios on the flow and heat transfer behaviour.
An experimental set-up was designed to achieve this goal. It consisted of an overall facility and a removable test section. The test section allowed for the measurement of the temperature along the length of the test section, the pressure drop, the heat flux inputs and the flow rate. These quantities were used to determine the heat transfer coefficients and friction factors of the system.
The concentric duct was an annulus formed of a single (15.88-mm-outer diameter and 14.46-mm-inner diameter) copper tube inserted inside a 0.91mm- thick- copper tube of 26.76 mm of inner diameter. The overall length of the annular duct was 4.84 m. To transfer heat, a heating element made of constantan wire was wrapped around each heat transfer area.
Heat transfer and pressure drop data were obtained on heating the inner and the outer wall separately with four different heat flux densities and eight heat flux ratio were used for the case of simultaneously heating both walls. Reynolds numbers for unilateral heating range from 5 800 to 12 000 while bilateral heating were focus around two Reynolds numbers, 6 500 and 9 500.
Satisfactory results were found between the measurements of this experiment and currently available literature for the case of unilateral heating. An estimate of the accuracy of the experimental setup showed the maximum relative error was about 5 % in the determination of the Nusselt number and 1.8 % for the friction factor.
Diabatic friction factors have been presented using adiabatic friction factors with a correction term which considered the effect of temperature difference between the fluid and walls. Heat flux density ratio showed to have an impact on the heat transfer characteristics. The Nusselt number on the inner wall could be enhanced by 19% with increasing the heat flux ratio up to 2.3 times.
Supervisor: Prof. J. Dirker
Co-supervisor: Prof. J P. Meyer
K J Mujanayi, 2017 "Thermal Management and Optimization of Heat Transfer from Discrete Heat Sources"
These days, the cooling of new generation electronic servers is a challenge due to the immense heat generated by them. In order to avoid overheating caused by the important rise in temperature appropriate cooling procedures must be used in order to meet the thermal requirement. The current study aims at addressing the issue of overheating in this field, and focuses on the thermal management of electronic devices modelled as a discrete heat sources (mounted in a rectangular cavity) with uniform heat flux applied from the bottom. A review of the literature published regarding the convective heat transfer from heated sources as well as a thorough background on the theory of the cooling of discrete sources by forced convection in rectangular channel is provided in this study. It was showed that the heat transfer performance in channel is strongly influenced by the geometric configurations of heat sources. Therefore, the arrangement and geometric optimisation are the main considerations in the evaluation of thermal performance. Unlike experimental methods that were carried out widely in the past, which provided less cost-effective and more time-consuming means of achieving the same objective, in this study we first explore the possibilities and the advantages of using the CD-adapco’s CFD package Star-CCM+ to launch a three dimensional investigation of forced convection heat transfer performance in a channel mounted with equidistant heat-generating blocks. Numerical results were validated with available experimental data, and showed that the thermal performance of the heat transfer increases with the strength of the flow. The second objective was to maximise the heat transfer density rate to the cooling fluid and to minimise both the average and the maximum temperature in the channel by using the numerical optimisation tool HEEDS/Optimate+. The optimal results showed that better thermal performance was not obtained when the heated sources followed the traditional equidistance arrangement, but was achieved with a specific optimal arrangement under the total length constraint for the first case. Subsequently, for the second case study, on the volume constraints of heat sources, the results proved that optimal configurations that maximise the heat transfer density rate were obtained with a maximum of either the height-to-length ratio or the height-to-width ratio. It was concluded that the heat transfer rate to the cooling fluid increases significantly with the Reynolds number and the optimal results obtained numerically are found to be fairly reliable.
Keywords: Thermal management, discrete heat sources, CFD package, forced convection, numerical optimization, maximise heat transfer, optimal configuration, volume constraints
Supervisor: Prof. T Bello-Ochende
Co-supervisor: Prof. J P. Meyer
J. C. P. Brits, 2017 "An Experimental and Stochastic Approach to Estimate the Fatigue Crack Life of a Turbomachinery Blade using Finite Element Modelling"
Large rotating machines are expensive and not easily replaceable or repairable. If the useful life of turbomachinery blades has been reached, failure can occur and lead to unplanned downtime, repair, and maintenance costs. If a crack is found on a component during an inspection, the extent of the damage is not always certain. It is also not always possible to replace the cracked component or allow downtime while waiting for a replacement to be manufactured or shipped. Costly inspections will also be needed on the damaged blades, until they are replaced. Since a cracked component can still be in service, the fatigue crack life should be “known” before re-commissioning to improve safety and make proper budgeting and planning for maintenance possible.
An approach to include modelling uncertainties and material variations in the input parameters when predicting the fatigue crack life of a turbomachinery blade during resonance conditions has been developed in the present study. As result, the reliability of the estimated lifetime is quantifiable. The determination of the fatigue crack life of a component is affected by various factors and these unknowns are generally taken into account by using conservative assumptions in deterministic models and rarely include a measure of uncertainty.
A FE model, built in MSC.Marc/Mentat 2016, was used to create a library of cracks with associated stress intensity factors of representative cracks within axial fan blades under cyclic loading. An experimental setup was designed to initiate and propagate a crack on multiple different blades to characterize the blade material using the Raju-Newman formulation on a simplified geometry. To stimulate crack growth, a base excitation, at resonance, was applied to the test specimens and the crack growth was measured with digital image correlation. A Monte Carlo simulation was employed to assess the sensitivity of the lifetime estimation to material variations and modelling uncertainties.
Supervisor:Prof P. Stephan Heyns
Co-supervisor: Dr Helen M. Inglis
ATC Hall, 2017 "The Effect of Inlet Header Geometry on the Heat Transfer Performance of Smooth Horizontal Tubes in the Transitional Regime"
Heat exchangers are seen to bear significance in many different industries, especially in the generation of energy in its various forms. Accurate design information is therefore required in order to improve the efficiency of these systems. Heat exchangers often end up operating in the transitional flow regime, or close to the transitional flow regime. Previous studies in this flow regime concentrated on single tube test sections with a variety of inlet geometries. In some heat exchangers, such as in chillers with a large number of tubes, not every tube has its own inlet but an inlet header feeds the tubes. However, no work has been done to study the effect of such an inlet header geometry on the heat transfer in adjacent tubes. It was therefore the purpose of this study to experimentally investigate the effect of an inlet header on heat transfer in the transitional flow regime. An experimental test set-up was constructed and commissioned for this purpose, that operated on water as working fluid and was validated against existing literature using results obtained from a single tube test section. A three-tube inlet header was then used to obtain heat transfer measurements on three tubes in parallel across Reynolds numbers ranging from 950 to 6 200, Prandtl numbers of 3.6 to 5.7, at a heat flux of 3 kW/m². The tube inner diameter was 3.97 mm and the tube length was 6 m. Inlet and outlet temperatures were recorded, in addition to surface temperature measurements along the length of each tube, as well as flow rate in each individual tube. Comparisons were made of the heat transfer coefficients over the last 2 m of the tube where the flow was fully developed. An uncertainty analysis was done, revealing uncertainties to vary between 11 and 16% in the Nusselt numbers and between 4 and 6% in Colburn j-factors, while uncertainty in the Reynolds numbers remaining less than 3 and 5% throughout the testing range. It was found that the use of a three tube inlet header resulted in increased heat transfer performance in the centre tube of the test section. In addition, transition was seen to occur earlier in the centre tube, followed by a secondary transition that aligned with the transition observed in the outer tubes. It was noted that the heat generated by the outer tubes may have influenced the heat transfer performance of the centre tube.
Supervisor: Professor JP Meyer
T L Ottermann, 2017 "Experimental and Numerical investigation into the natural convection of TiO2-Water nanofluid inside a cavity"
This Master of Engineering investigation focuses on the natural convection of nanofluids in rectangular cavities. The governing equations applied to analyse the heat transfer and fluid flow occurring within the cavity are given and discussed. Special attention is given to the models that were developed to predict the thermal conductivity and dynamic viscosity of such nanofluids.
A review concerning past investigations into the field of natural convection of nanofluids in cavities is made. The investigation is divided into experimental works and computational fluid dynamics (CFD) numerical investigations.
Through the literature review, it was discovered that many numerical models exist for the prediction of the thermophysical properties of nanofluids, specifically thermal conductivity and viscosity. Depending on the nanofluid and the application, different models can be used.
The literature study also revealed that most previous works were done in the CFD field. Very few experimental studies have been performed. Numerical CFD investigations, however, need experimental results for validation purposes, leading to the conclusion that more experimental work is needed.
The heat transfer capability and thermophysical properties of the nanofluid are investigated based on models found in the literature. The investigation includes measuring the heat transfer inside a cavity filled with a nanofluid and subjected to a temperature gradient. The experiment is performed for several volume fractions of particles. An optimum volume fraction of 0.005 is obtained. At this volume fraction, the heat transfer enhancement reaches a maximum for the present investigation.
The investigation is repeated as a numerical investigation using the commercially available CFD software ANSYS-FLUENT. The same case as used in the experimental investigation is modelled as a two-dimensional case and the results are compared. The same optimum volume fraction and maximum heat transfer is obtained with an insignificantly small difference between the two methods of investigation. This error can be attributed to the minor heat losses experienced from the experimental setup as in the CFD adiabatic walls considered.
It is concluded that, through the inclusion of TiO2 particles in the base fluid (deionised water), the thermophysical properties and the heat transfer capability of the fluid are altered. For a volume fraction of 0.005 and heat transfer at a temperature difference of 50 °C, the heat transferred through the fluid in the cavity is increased by more than 8%.
From the results, it is recommended that the investigation is repeated with TiO2 particles of a different size to determine the dependency of the heat transfer increase on the particle size. Various materials should also be tested to determine the effect that material type has on the heat transfer increase.
Supervisor: Prof. Mohsen Sharifpur
Co-supervisor: Prof. Josua P Meyer
E Grove, 2017 "Feasibility study on the implementation of a boiling condenser in a South African fossil fuel power plant"
The South African electricity mix is highly dependent on subcritical coal-fired power stations. The average thermal efficiency of these power plants is low. Traditional methods to increase the thermal efficiency of the cycle have been widely studied and implemented. However, utilising the waste heat at the condenser, which accounts for the biggest heat loss in the cycle, presents a large potential to increase the thermal efficiency of the cycle. Several methods can be implemented for the recovery and utilisation of low-grade waste heat.
This theoretical study focuses on replacing the traditional condenser in a fossil fuel power station with a boiling condenser (BC), which operates in a similar manner to the core of a boiling water reactor at a nuclear power plant (Sharifpur, 2007). The system was theoretically tested at the Komati Power Station, South Africa’s oldest power station. The power station presented an average low-grade waste heat source. The BC cycle was theoretically tested with several working fluids and numerous different configurations. Several of the theoretical configurations indicated increased thermal efficiency of the cycle. The BC cycle configurations were also tested in two theoretical scenarios.
Thirty configurations and 103 working fluids were tested in these configurations. The configuration that indicated the highest increase in thermal efficiency was the BC cycle with regeneration (three regenerative heat exchangers) from the BC turbine. A 2.4% increase in thermal efficiency was obtained for the mentioned theoretical implementation of this configuration. The working fluid tested in this configuration was ethanol. This configuration also indicated a 7.6 MW generating capacity.
The increased thermal efficiency of the power station presents benefits not only in increasing the available capacity on South Africa’s strained grid, but also environmental benefits. The mentioned reduction of 7.6 MW in heat released into the atmosphere also indicated a direct environmental benefit. The increase in thermal efficiency could also reduce CO2 emissions released annually in tons per MW by 5.74%.
The high-level economic analysis conducted, based on the theoretically implemented BC cycle with the highest increase in thermal efficiency, resulted in a possible saving of R46 million per annum. This translated to a saving of R19.2 million per annum for each percentage increase in thermal efficiency brought about by the BC cycle.
The theoretical implementation of the BC, with regeneration (three regenerative heat exchangers) from the BC turbine and ethanol as a working fluid, not only indicated an increase in thermal efficiency, but also significant economic and environmental benefits.
Supervisor: Prof. Mohsen Sharifpur
Co-supervisor: Prof. Josua P Meyer
KG Katamba, 2017 "Investigation into waste heat to work in thermal systems in order to gain more efficiency and less environmental defect"
In most previous studies that have been conducted on converting waste heat energy from exhaust gases into useful energy, the engine waste heat recovery system has been placed along the exhaust flow pipe where the temperature differs from the temperature just behind the exhaust valves. This means that an important fraction of the energy from the exhaust gases is still lost to the environment. The present work investigates the potential thermodynamic analysis of an integrated exhaust waste heat recovery (EWHR) system based on a Rankine cycle on an engine’s exhaust manifold. The amount of lost energy contained in the exhaust gases at the exhaust manifold level, at average temperatures of 500 °C and 350 °C (for petrol and diesel), and the thermodynamic composition of these gases were determined. For heat to occur, a temperature difference (between the exhaust gas and the working fluid) at the pinch point of 20°C was considered. A thermodynamic analysis was performed on different configurations of EWHR thermal efficiencies and the selected suitable working fluids. The environmental and economic aspects of the integrated EWHR system just behind the exhaust valves of an internal combustion engine (ICE) were analysed. Among all working fluids that were used when the thermodynamic analysis was performed, water was selected as the best working fluid due to its higher thermal efficiency, availability, low cost and environmentally friendly characteristics. Using the typical engine data, results showed that almost 29.54% of exhaust waste heat can be converted. This results in better engine efficiency and fuel consumption on a global scale by gaining an average of 1 114.98 Mb and 1 126.63 Mb of petrol and diesel respectively from 2020 to 2040. It can combat global warming by recovering 56.78 1 011 MJ and 64.65 1 011 MJ of heat rejected from petrol and diesel engines, respectively. A case study of a Volkswagen Citi Golf 1.3i is considered, as it is a popular vehicle in South Africa. This idea can be applied to new-design hybrid vehicles that can use the waste heat to charge the batteries when the engine operates on fossil fuel.
Supervisor: Prof. Mohsen Sharifpur
Co-supervisor: Prof. Josua P Meyer
J Joubert, 2017 "Influence of a magnetic field on magnetic nanofluids for the purpose of enhancing natural convection heat transfer"
Natural convection as a heat transfer mechanism plays a major role in the functioning of many heat transfer devices, such as heat exchangers, energy storage, thermal management and solar collectors. All of these have a large impact on the generation of solar power. Considering how common these devices are – not only in power generation cycles, but in a majority of other thermal uses – it is clear that increased performance for natural convection heat transfer will have consequences of a high impact. As such, the purpose of this study is to experimentally study the natural convection heat transfer behaviour of a relatively new class of fluids where nano-sized particles are mixed into a base fluid, also known as nanofluids. Nanofluids have attracted widespread interest as a new heat transfer fluid due to the fact that the addition of nanoparticles considerably increases the thermophysical properties of the nanofluids when compared to those of the base fluid. Furthermore, if these nanoparticles show magnetic behaviour, huge increases to the thermal conductivity and viscosity of the nanofluid can be obtained if the fluid is exposed to a proper magnetic field. With this in mind, the study aimed to experimentally show the behaviour of these so-called magnetic nanofluids in natural convection heat transfer applications.
In this study, the natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is investigated with and without an applied external magnetic field. The effects of volume concentration and magnetic field configuration are investigated. Spherical Fe_2 O_3 nanoparticles with a diameter of 20 nm are used with a volume concentration ranging between 0.05% and 0.3%, tested for the case with no magnetic field, while only a volume concentration of 0.1% was used in the magnetic cases. The experiments were conducted for a range of Rayleigh numbers in 1.7 ×〖 10〗^8<Ra<4.2 ×〖 10〗^8. The viscosity of the nanofluid was determined experimentally, while an empirical model from the literature was used to predict the thermal conductivity of the nanofluids. An empirical correlation for the viscosity was determined, and the stability of various nanofluids was investigated.
Using heat transfer data obtained from the cavity, the average heat transfer coefficient, as well as the average Nusselt number for the nanofluids, is determined. It was found that a volume concentration of 0.05% showed an increase of 3.75% in heat transfer performance. For the magnetic field study, it was found that the best-performing magnetic field enhanced the heat transfer performance by 1.58% compared to the 0.1% volume concentration of the nanofluid with no magnetic field.
Supervisor: Prof. Mohsen Sharifpur
Co-supervisor: Prof. Josua P Meyer
S Roberts, 2017 "Characterising the Behaviour of an Electromagnetic Levitation Cell using Numerical Modelling"
Experimental investigations of high temperature industrial processes, for example the melting and smelting processes taking place inside furnaces, are complicated by the high temperatures and the chemically reactive environment in which they take place. Fortunately, mathematical models can be used in conjunction with the limited experimental results that are available to gain insight into these high temperature processes. However, mathematical models of high temperature processes require high temperature material properties, which are difficult to measure experimentally since container materials are often unable to withstand high enough temperatures, and sample contamination often occurs. These difficulties can be overcome by employing containerless processing techniques such as electromagnetic levitation to allow for characterisation of high temperature material properties.
Efficient design of electromagnetic levitation cells is challenging since the effects of changes in coil design, sample size and sample material on levitation force and sample temperature are not yet well understood. In this work a numerical model of the electromagnetic levitation cell is implemented and used to investigate the sensitivity of levitation cell operation to variations in coil design, sample material and sample size.
Various levitation cell modelling methods in literature are reviewed and a suitable model is chosen, adapted for the current application, and implemented in Python. The finite volume electromagnetic component of the model is derived from Maxwell's equations, while heat transfer is modelled using a lumped parameter energy balance based on the first law of thermodynamics. The implemented model is verified for a simple case with a known analytical solution, and validated against published experimental results. It is found that a calibrated model can predict the lifting force inside the levitation cell, as well as the sample temperature at low coil currents.
The validated model is used to characterise the operation of a levitation cell for a number of different sample materials and sample sizes, and for varying coil geometry and coil current. The model can be used in this way to investigate a variety of cases and hence to support experimental levitation cell design. Based on model results, operating procedure recommendations are also made.
Supervisor: Prof. S. Kok
Co-supervisors: Dr J. H. Zietsman, Dr H. M. Inglis
M.J.R. Schoeman, 2017 "Development and comparison of strategies for the reconstruction of full and partial skull geometries"
The development and comparison of strategies for the reconstruction of full and partial surface mesh based skull geometries is presented. The intended application is to aid the South African Police Service Victim Identification Centre (SAPS VIC) with forensics, specifically prediction of a mandible when only the cranium is available.
Various methods for the registration of surface meshes are outlined. A new non-rigid iterative closest point (NR-ICP) algorithm based on an adaptively refined least square Radial Basis Function (RBF) approximation of the forward and backward nearest neighbour correspondence is developed. The newly developed non-rigid registration strategy is demonstrated and characterised for various parameters using an artificial mandible dataset constructed through Monte-Carlo (MC) sampling of a quadratic displacement field. Various suitable parameters are shown to result in imperceptible visual registration differences, with the correspondence error mainly distributed in-surface.
Multivariate regression techniques suited to the application of geometry prediction are considered, specifically for cases where the data is expected to be multi-collinear and the number of variables are far greater than the number of observations. Two regression approaches based on spatial information are considered. The first is the classical use of Procrustes Analysis where the Cartesian coordinates are used directly for regression. The second is a new Euclidean distance based approach utilizing pair-wise distances to consistent reference points. The proposed regression methods' time-space complexity is investigated to limit system sizes that result in time tractable cross-validation and model comparison. Pre- and post-processing required for tractability considerations are also developed for both approaches.
Proof of concept of the registration based prediction strategies are demonstrated. This is accomplished through the use of an artificial dataset with embedded covariance and the use of registration targets without point-wise correspondence. The registration based prediction strategy is shown to be capable of accurate predictions for data with strong underlying structure/covariance.
The proposed registration based prediction strategy is demonstrated on a real cranium and mandible dataset, where the mandible geometry is predicted from the cranium geometry. Marginal improvement over the geometric mean is obtained. Observation scaling suggests that model accuracy is improved for increased observations, which merits expanding the dataset.
The proposed registration strategy has the limitation that it is not capable of registration of significant partial/incomplete geometries. A new regression-registration hybrid strategy is developed for use with partial geometries, when a full dataset of the given geometry is available. The regression-registration hybrid strategy is demonstrated on a real mandible dataset and mandible fossil.
Supervisor: Prof. S. Kok
Co-supervisor: Dr. D.N. WIlke
MD Marais, 2017 "Computational fluid dynamics investigation of wind loads on heliostat structures"
Heliostats, mirrors tracking the sun and reflecting onto a target, make up the solar collector field of a central receiver solar plant. Since a large amount of these structures are needed to achieve high temperatures at the receiver, they make up a significant portion of the initial capital investment. The optimum design of heliostats is, therefore, an important research field as it has been identified as a key cost saving area in the bid to make concentrating solar power a viable alternative to current fossil fuel technologies.
Due to topology considerations, central receiver plants are generally constructed in flat, open country environments where the heliostat fields are subjected to atmospheric winds. The structural design of the individual heliostats has to take into account the wind-induced forces and moments, being able to resist structural failure during storm loads with the mirrors in stow position. Heliostats should also not suffer static deflections or vibrations which causes reflected solar radiation to miss the target during operation, a time when the mirror is at varying angles to the attacking wind. Over-design should be avoided in order to limit expenditure.
The prediction of mean and peak wind loads on heliostat structures has, therefore, been a key research area for more than three decades. Experimental wind tunnel studies, in which the atmospheric wind profiles for velocity and turbulence are replicated, have been the favoured method to obtain wind loading data. Computational fluid dynamics (CFD) have been less popular, but have the potential to serve as a cost effective analysis and optimization tool, provided that such models are properly validated using experimental data.
Supervisor: Prof KJ Craig
Co-supervisor: Prof JP Meyer
J.C. Smit, 2017 "Flow regime identification and compensation for solid flow measurement using concave capacitive sensors"
This dissertation describes research and experimental work done to improve the accuracy and applicability of capacitive sensors which are used to evaluate the mass flow rate of solid material within pneumatic conveying environments in power plant engineering applications. The research is focused on creating a measurement methodology on which solid mass flow rate can be evaluated without being dependent on variable flow regimes. The proposed research investigates the process of identifying four different types of flow regimes using a decision tree that utilises support vector machines and cut off values. During the study an investigation was also made into what electrode setup would provide proficient information regarding the flow distribution and orientation. Compensation according to the identified regime is then proposed by means of using a nonlinear fraction curves determined through calibration experiments.
Supervisor: Prof P.S. Heyns
Co-Supervisor: Mr H Fourie
AJ Vogel, 2017 "Comparing direct and indirect methods for low-budget tuning of heuristic optimisation algorithms"
Heuristic algorithms have parameters that control their performance on specific optimisation problems. By choosing optimal parameter values researchers can drastically reduce the time it takes to solve optimisation problems. Unfortunately, these optimal parameters depend on several factors such as the algorithm used, the problem being solved, and the fitness budget available. So, while some optimal parameter values are published for many algorithms, they are only applicable to a very specific case. For this reason researchers have spent some time developing methods to find these optimal parameter values, a process known as tuning. The performance of heuristic algorithms are stochastic and tuning methods must compensate for this in some way. Most algorithms do this by evaluating a single parameter set several times (usually 30 to 50) and taking the average performance of this parameter set as the nominal for comparison. We believe this approach is sub-optimal as the tuning budget is wasted on parameter sets that are sub-optimal.
In this thesis we propose an alternative approach that uses regression through spatially distributed points to compensate for the stochastic response of heuristic algorithms. This approach allows more efficient use of the tuning budget. In our approach we use a radial basis function response surface to create a regression surface through the points. This surface is then optimised to find a new candidate parameter set.
In a series of numerical experiments we show that our approach outperforms commonly used tuning methods, especially when tuning budgets are low.
Supervisor: Dr DN Wilke
D Kafka, 2017 "Investigation into regression strategies to address model errors in inverse analysis of creep models"
When solving inverse problems, the model error is a critical aspect to be considered, as it affects the validity of the solution. The forward method is the de facto standard technique used to solve inverse problems. It is, however, limited in the accuracy of its fit by the validity of the model used. If global convergence is achieved, it constitutes the best possible fit for the given model. This strategy can be computationally expensive in the context of engineering technologies such as the Finite Element Method. This is often alleviated using response surfaces to approximate the response of the desired analysis. An alternative approach is direct inverse mapping strategies or inverse regression. Inverse regression strategies offer a computationally efficient means of solving inverse problems in particular when they need to be solved multiple times. The absolute accuracy of these methods is dependent on the data used to construct the inverse regression. However, they offer the ability to extract features from the data used to construct the inverse regression. A linear combination of the extracted features is then used to construct the inverse regression. This study shows that in the case of using heterogeneous training data, with data from two different models, the mappings constructed are capable of accurately predicting the response, which is a linear combination of measured responses related to the models independently present in the training dataset. Thus, it is shown that inverse regression is capable of reducing the model error by combining multiple independent models to characterize a single problem.
Supervisor: Dr DN Wilke
Y Chae, 2017 "Optimal sensor placement approaches for the design of inverse experiments by simulation"
This dissertation serves to present the research conducted on sensor placement optimisation (SPO) using sensitivity analyses of virtual experiments in order to design virtual inverse problems. Two classes of SPO methods are considered namely mode-based and mode-free methods. The mode-based methods make use of SIMPLS and SVD to extract useful data by examining the correlation between the target variables (characterising variables) and the sensor measurement variables, while the mode-free methods eliminate the need of spending the extra time required to extract modes, which ultimately leads to successful sensor placement for solving inverse problems.
The aim of the mode-free approach is to maximize the variance explained subject to uniqueness of the information of each sensor. Both approaches aim to maximize the potential of an experimental setup to solve an inverse problem by using the right number of sensors and placing them at the optimal spatial positions. SPO is not only capable of designing an experiment but it is also capable of classifying the well-posed or ill-posed nature of an existing experiment that can be modelled, which saves both the time and cost. The approach followed in this study was to design a simple virtual inverse problem for which the (well or ill)-posedness of the problem can be controlled.
Numerous virtual experiments were conducted that varied from well-posed to severely ill-posed to allow for rigorous testing of the various approaches. The effect of model error and stochastic noise on ability to reliably place sensors is also investigated.
Supervisor: Dr DN Wilke
R. Strauss, 2017 "Brake Based Integrated Rollover Prevention and Yaw Control for an Off-Road Vehicle."
Sport utility vehicles typically feature high ground clearances that allow them to be used in off-road conditions. Their use is not limited to off-road conditions and they are often used as day-to-day family vehicles. On the road, where high friction surfaces are prevalent, their high centres of gravity can make them susceptible to un-tripped rollovers during severe dynamic manoeuvers such as an emergency obstacle avoidance. The detection of a high risk of rollover and the avoidance thereof has great potential to improve vehicle safety, as the consequences of rollover incidents are generally quite severe.
Rollover mitigation systems are triggered when a rollover threshold index is exceeded, indicating a high risk of rollover. The metric implemented in this study is known as the zero-moment point method, which allows for vehicle parameters and terrain to be taken into account. Previous research has indicated that mitigation systems that trigger braking intervention are some of the most successful methods in reducing rollover risk, as it not only stabilises the vehicle, but also reduces the speed.
Brake based rollover prevention systems typically implement electronic stability program methods that use yaw rate reduction as the primary tool for reducing rollover risk, which often comes at the expense of the vehicle’s path following ability. This means that the stability control system may lead to the vehicle leaving the road and causing an even more severe accident. The control algorithm implemented in this study gives preference to reducing the forward speed of the vehicle which in turn reduces lateral acceleration, a major contributor to rollover propensity. Braking is however apportioned to all four wheels and distributed so as to achieve vehicle yaw rate targets. Emphasis is placed on maintaining good path following capability to prevent the vehicle from leaving the road.
The detection and mitigation system was tested on a Land Rover Defender 110 for a variety of manoeuvers in simulation as well as experimental testing. The results indicate that the rollover mitigation system managed to successfully reduce the rollover threshold index of the vehicle during the manoeuver whilst simultaneously maintaining the path following ability of the vehicle and improved the yaw rate tracking.
Supervisor: Prof. P. S. Els
GJ Howard, 2017 "Finite Element Modelling of Creep for an Industrial Application"
Thermal power stations operate at elevated temperatures and pressures in order to attain maximum available steam energy. At these high temperatures creep becomes a dominant mechanism that needs to be considered. However, for many components, the locations where peak stresses occur are unreachable to apply the commonly used Non-Destructive Testing (NDT) techniques. This encourages the use of Finite Element Analysis (FEA) to better predict the creep state in these complex components.
Commonly, creep damage models are used in conjunction with accelerated creep tests to develop material models that can be implemented into a FEA to determine failure. These approaches are often infeasible for industrial decision-making, leaving a gap for more accessible commercially available models to be developed. This paper focuses on using openly available creep data from the Japanese National Institute for Material Science (NIMS). A creep strain model capable of modelling only the primary and secondary creep regimes was then chosen from the ANSYS database to fit this data. In order to fully characterise the experimental data a multi-creep-model approach was adopted that uses a family of creep models, instead of a single creep material model, to characterise the probable range of responses. This methodology was applied to an industrial application, namely an Intermediate Pressure (IP) valve operating under creep-prone conditions. The multi-creep-model approach was incorporated into FEA to analyse the variation in stress distributions. It was interesting to see that a variation of 153% in the creep strain models only resulted in a 21% variation in the relaxed stress. Worst case scenario life time calculations were then conducted using both a time-based Larson-Miller approach and a strain-based ASME code approach. Both sets of results showed that, for the specific component of interest, creep rupture lifetimes were in excess of 3000 years. It was therefore noted that, for the IP valve of interest, the operating temperature and pressure combination were such that no worrisome creep damage occurred. In conclusion, for the specific component analysed, the operating conditions are such that creep based failure will not occur.
Supervisor: Dr HM Inglis
Co-Supervisor: Mr F Pietra
SB Leith, 2016 "An Investigation into the External Flow Boiling Phenomena on the Surface of Water Cooled Zircaloy-4 and Silicon Carbide Nuclear Fuel Cladding"
The materials in use inside modern nuclear reactors have all been subjected to a large amount of research and development. This is necessary due to the challenging environment found inside the core of a nuclear reactor as well as the stringent safety requirements imposed on their construction. The materials need to be able to handle a multitude of different circumstances and there are many attributes which a core material needs to have in order to be sustainable. Nowadays, the focus of most reactor material research is on safety. For this reason, coupled with the advanced materials available today, a new cladding material has been proposed. This material is Silicon Carbide (SiC). Therefore the purpose of this study was to experimentally investigate, and compare, the heat transfer characteristics of SiC versus current nuclear grade Zircaloy-4 hereinafter called Zircaloy®. The experiments were thus focused on external flow boiling. The setup used for the experimentation was designed by a colleague and the test section was designed purposefully for surface temperature measurements to be taken at various mass flow rates, inlet temperatures and pressures. The setup was constructed so that the cladding rods are positioned vertically with the heat transfer medium flowing vertically upwards around it. In order to replicate the heat generated by nuclear fuel, a 3 kW electric resistance cartridge heater was inserted inside both the SiC as well as the Zircaloy®. This resulted in heat fluxes ranging from 20 kW/m2 to 145 kW/m2 for Zircaloy® and from 12 kW/m2 to 90 kW/m2 for silicon carbide. A total of twenty four tests were conducted each with six heat flux set points and three flow rates. The test specimens were two Zr-4 and SiC tubes, with diameters of 9.8 mm and 15.5 mm, supported inside a square test section. Water at 100°C, 11.37°C subcooling, was pumped through the test section as the heat transfer medium with the test section held at 1.5 bar. An uncertainty analysis indicated that the heat flux uncertainty varied between 20% and 52% while the heat transfer coefficient uncertainty varied between 25% and 52%. Overall the results indicate the Zircaloy® specimen is superior to the silicon carbide specimen. The results differ from previous research indicating that a higher roughness and thermal conductivity do not necessarily lead to a greater heat transfer coefficient. When the results are placed in specific boiling equations from previous research it is found that the silicon carbide is closely predicted by the Shah and Kandlikar correlations while the Zircaloy® is more closely predicted using the Gungor and Winterton correlation. These correlations were developed for internal flow though and were simply extended to external flow boiling for predictive purposes. In conclusion it can be stated that the Zircaloy® cladding material outperforms the silicon carbide test specimen evaluated in this study. It remains however a promising replacement. To fully evaluate silicon carbide’s readiness for nuclear applications many unknowns still need to be answered though. For future research the power supply needs to be upgraded to provide a constant input power as well as a new data logging system to provide a constant readout of data. The inlet water temperature is crucial and needs to remain constant for all sets of tests. The test section should also be made smaller for single rod experiments and the heater power needs to be greatly increased to higher surface temperatures and possibly film boiling.
Supervisor: Prof J. Slabber
Co-Supervisor: Prof J.P. Meyer
H. Ghodsinezhad, 2016 "Experimental Investigation on Natural Convection of Al2O3-water Nanofluids in Cavity Flow"
The thermophysical properties of nanofluids have attracted the attention of researchers to a far greater extent than the heat transfer characteristics of nanofluids have. Contradictory results on the thermal-fluid behaviour of nanofluids have been numerically and experimentally reported on in the open literature. Natural convection has not been investigated experimentally as much as the other properties of nanofluids. In this study, the characteristics and stability of Al2O3-water nanofluids
(d = 20–30 nm) were analysed using a Malvern zetasizer, zeta potential and UV-visible spectroscopy. The natural convection of Al2O3- water nanofluids (formulated with a single-step method) was experimentally studied in detail for the volume fractions 0, 0.05, 0.1, 0.2, 0.4 and 0.6% in a rectangular cavity with an aspect ratio of 1, heated differentially on two opposite vertical walls for the Rayleigh number (Ra) range 3.49 x 108 to 1.05 x 109. The viscosity of Al2O3-water nanofluids measured between 15 and 50 °C. The effect of temperature and volume fraction on viscosity was also investigated. A detailed study of the nanoparticle concentration effect on the natural convection heat transfer coefficient was performed. It was found that increasing the concentration of nanoparticles improves the heat transfer coefficient by up to 15% at a 0.1% volume fraction. Further increasing the concentration of nanoparticles causes the natural convection heat transfer coefficient to deteriorate. This research also supports the idea that “for nanofluids with thermal conductivity – more than the base fluids – an optimum concentration may exist that maximises heat transfer in an exact condition as natural convection, laminar force convection or turbulence force convection”.
Supervisor: Dr, M Sharifpur
Co-Supervisor: Prof J.Meyer
J Otto, 2016 "Nuclear Fusion of Li-6 H-2 Crystals"
The inception of this study lies in mankind’s need for energy in order to drive commerce, industry and development. Although several sources have been exploited in the production of energy, it is the personal opinion of the author that the most novel of these is atomic energy. It is known that humanity has become quite proficient in using nuclear fission, as is evidenced in the world’s nuclear fleet; however the use of fusion for controlled energy production has not yet developed sufficiently for commercial use. It is, however, a popular anecdote that the production of energy from nuclear fusion is only forty years away – and always will be. During the literature survey, several forms of fusion producing processes are considered ranging from stellar bodies and magnetic confinement based reactors to percussive wave driven fusion as is used in fission-fusion warheads. This study focusses on the prospect of fusion at room temperature. The aim of this study is to further investigate a specific fusion process in order to determine if this may become a viable source for the production of energy. In the literature survey it is determined that the atoms in certain moleculessuch as Li-6 H-2 and H2O can spontaneously overcome the Coulomb barrier and fuse to form new elements; releasing energy in the process. The mechanism for these two atoms are perceived to be similar, and the Li-6 H-2 reaction was selected based on the ease of observability, of the two α-particles that would form , should a fusion reaction take place. Furthermore, it is theorised in the literature survey that the reaction rate of the Li-6 with H-2 can be increased by exciting the molecules with X-ray radiation. In this study, an experiment is devised and conducted in order to determine if the reaction rate can indeed be accelerated as proposed. The results are compared to studies where the molecules were not excited in this manner as a baseline, and an additional determination lies in the practicality of energy production for commercial use in this manner. In the experiment, an X-ray source is used to bombard several samples of L-6 H-2 with wide spectrum Bremsstrahlung for several hours. Mathematical approximations, as well as simulations, are used in order to determine the energy deposition of the X-rays into the samples. This is done in order to determine if the process has viability as an energy production source, and in an attempt to determine if there is a specific wavelength that the process is partial to. In order to record possible fusion reactions; the samples are layered with several plastic detectors. These detectors are chemically etched and studied using microscopy, the appendix deals with experiments aimed at calibrating the process used to study these detectors. It is successfully shown that the reaction rate is increased by introducing X-rays to the Li-6 H-2 powder, however, the magnitude is far lower than was hoped for. Additionally, due to the low number of fusion reactions that took place, enough data is not available to determine an electromagnetic wavelength that is of particular interest. Finally, it is thus shown that the configuration used in this study is not a viable assembly for the production of power.
Supervisor: Prof J. Slabber
Co-Supervisor: Prof J.P. Meyer
Saboura Yousefiaboksari, 2016 "Experimental investigation and theoretical analysis on the effects of nanolayer on nanofluids’ thermo-physical properties"
Nanofluids, which are suspensions of nanoparticles in conventional heat transfer fluids, attracted research studies on different heat transfer applications, while they enhance thermal transport properties in comparison with conventional base fluids.
Recently, the use of these new fluids has been growing increasingly. However, the ambiguities of their thermo-physical properties cause them to function inefficiently in industrial design. The recognised important parameters that affect the properties of nanofluids include the volume fraction of the nanoparticles, temperature, nanoparticle size, nanolayer, thermal conductivity of the base fluid, pH of the nanofluid and the thermal conductivity of the nanoparticles. However, there is a distinct lack of investigation and reported research on the nanolayer and its properties.
In this study, the effect of uncertainty of the nanolayer properties on the effective thermal conductivity and viscosity of nanofluids, and heat transfer are discussed in detail. The results show that the uncertainties can cause 20% error in the calculation of the Nusselt number and 24% for the Reynolds number. Therefore, more research needs to be conducted on nanolayer properties in order to identify them accurately.
The density of some nanofluids, such as SiO2-water, SiOx-EG-water, CuO-glycerol and MgO-glycerol, has also been investigated experimentally. Therefore, the effects of nanolayer thickness and density on nanofluid properties are discussed in detail. The results show that nanolayer density and thickness have a significant effect on nanofluid density, and nanolayer density is found to be between void and base fluid density.
Consequently, by analysing experimental results and performing a theoretical analysis, a model has been derived to calculate the density of nanofluids.
Specific heat capacity is the other nanofluid property that is discussed in this study. Experimental data from literature, available formulae and the presented model for nanofluid density have been used to identify nanofluid-specific heat capacity, while nanofluid density is one of the parameters in calculating specific heat capacity. This investigation was performed using a model – used by different authors – that also considers the nanolayer. The specific heat capacity of nanofluids that resulted from two methods of calculation has been compared with available experimental data. This investigation shows that the proposed model for the density of nanofluids provides better agreement for specific heat capacity in comparison to experimental data.
Supervisor: Dr Mohsen Sharifpur
Co-Supervisor Prof Josua P Meyer
A. Jami, 2016. "Impeller Fault Detection under Fluctuating Flow Conditions using Artificial Neural Networks"
Maintenance of equipment at the required condition to ensure a reliable performance, as well as improvement of safety, are major concerns in the field of asset integrity management. Condition monitoring is a procedure that allows one to identify early signs of failures and implement efficient maintenance plans to eliminate the uncertainties in machine operation. In addition, vibration monitoring is known as a detection tool for early detection of degradation from the expected performance. It is often superior to other condition monitoring techniques, due to its high sensitivity and simplicity of implementation. Vibration analysis provides substantial information regarding the operating condition of components and aids to remedy problems. Therefore, it can be used to detect a wide range of fault conditions in rotating machinery, such as imbalance, misalignment of internal shafts, looseness, cracked shaft, gear failures, rolling element bearing damages, motor faults and impeller issues.
The primary intention of the research reported in this dissertation is to investigate the applicability of a neural network methodology for the detection and diagnosis of mechanical defects of impellers in centrifugal pumps. The study focuses on extracting appropriate features from vibration signals associated with pump impellers and the performance of artificial neural networks (ANNs) using these features. The second intention is to enhance maintenance decisions regarding the actual impeller condition. This leads to a transition from time based preventive maintenance to condition based maintenance, and also improving the safety and reliability of pumping systems, as well as reducing unexpected and catastrophic failures. Hence, vibration analysis techniques are used as a principal tool to characterise the impeller conditions under flow variation, with the requirements of data collection, data processing, transformation and selection of essential features corresponding to the running condition.
This dissertation presents a study of current vibration analysis techniques to extract the required features, namely time based features, frequency based features and wavelet based features. An experimental setup is developed to measure the impeller vibration. The experiment is performed using seven impeller fault conditions such as crack and imbalance under fluctuating
flow conditions to simulate non-stationary conditions in the system. Also, the evolution of features over varying flow rates are evaluated in order to identify features that contain fundamental information corresponding the fault characteristics. Moreover, the collected features form non-dimensional training data sets are used to train ANNs. Comparisons of different training algorithms, network hidden nodes and effectiveness of different transfer functions are performed to select the most appropriate parameters of networks.
Validation of the results prove that the accuracy of ANN prediction improves considerably by using decomposed vibration signals and energy based features. Comparison of the network accuracy based on wavelet packet transform (WPT) features with time analysis and frequency analysis based features, indicate that WPT-ANN lead to lower mean square errors and higher correlation coefficients, as well as shorter training times. The WPT-ANN model can save computational time and provides better diagnostic information, which can be effectively used for classification of impeller defects under non-stationary conditions.
Supervisor:Prof Stephan Heyns
DA Ramatlo, 2016."Optimal Design of a Guided Wave Rail Web Transducer using Numerical Modelling"
Ultrasonic Guided Waves can propagate over long distances, and are thus suitable
for the interrogation of long structural members such as rails. A recently developed
Ultrasonic Broken Rail Detection (UBRD) system for monitoring continuously welded
train rail tracks, primarily detects complete breaks. This system uses a guided wave
mode with energy concentrated in the head of the rail, which propagates large distances and which is suitable for detecting defects in the rail head. Exploiting a second mode, with energy concentrated in the web section, would allow us to e_ectively detect defects in the web of the rail.
The objective of this study is to develop an ultrasonic piezoelectric transducer that can excite a guided wave mode with energy concentrated in the web of the rail. It is required that the transducer must strongly excite such a mode at the operational frequency of the UBRD system. The objective is thus to obtain a design with optimal performance.
A recently developed numerical modelling technique is used to model the interaction
of the transducer with the rail structure. The technique employs a 2D Semi-Analytical
Finite Element (SAFE) mesh of the rail cross-section and a 3D _nite element mesh of the transducer; and is thus referred to as SAFE-3D. The accuracy of the SAFE-3D method was validated though experimental measurements performed on a previously developed transducer.
A design objective function representative of the energy transmitted by the transducer to the web mode was selected. The identi_ed design variables were the dimensions of the transducer components. The performance of the transducer was optimized using a response surface-based optimization approach with a Latin Hypercube sampled design of experiments (DoE) that required SAFE-3D analyses at the sampled points. A Nelder- Mead optimization algorithm was then used to an optimal transducer design on the response surface.
The performance of the optimal transducer predicted by the response surface was found to be in good agreement with that computed from SAFE-3D. The optimum transducer was manufactured and experimental measurements veri_ed that the transducer model was exceptionally good. The design method adopted in this study could be used to automate the design of transducers for other sections of the rail or other frequencies of operation.
KEYWORDS: Ultrasonic guided wave; Piezoelectric transducer; SAFE-3D; Optimization
Supervisor: Dr. D.N. Wilke
Co-supervisor: Dr. P.W Loveday
Co-supervisor: Dr. C.S Long
Kyoung-Yeoll (John) Lee, 2016 "Cavity natural convection of zinc oxide-water nanofluid experimental work"
Nanofluids are recognized to have great potential for conventional heat transfer fluids that could benefit industries. However, many in-depth numerical studies and fewer experimental studies have been conducted on natural convection of nanofluid and their results are inconsistent. In this study, natural convection heat transfer characteristics of zinc oxide (ZnO)-water nanofluid is investigated in rectangular enclosure through cavity flow experimental measurements. The ZnO-water nanofluids are prepared with different volume fractions of 0.09, 0.18, 0.36, 0.5 and 1 (vol.%) (0.5, 1, 2, 3 and 5.67 weight percentage) and for Rayleigh number (Ra) varies from 7.9E+7 to 8.9E+8. The stability of the ZnO nanofluid is verified using a spectrophotometer and zeta potential measurement at various temperatures and concentrations. Zeta potential values are measured within the stable range, and no sedimentation of nanoparticles is indicated within 24 hours. The viscosity of ZnO-water nanofluid is also measured experimentally, which is 20% higher than the use of the traditional Einstein viscosity model at 1 vol.%. Consequently, the suspension of ZnO nanoparticles in water does not enhance the natural convection heat transfer coefficient. The average Nu increases as the Ra increases, but the average Nu decreases as the volume fraction of the nanofluid increases. The systematic deterioration of the natural convection heat transfer coefficient is observed as increasing in the concentration of nanoparticles.
Supervisor: Dr M Sharifpur
Co-supervisor: Prof JP Meyer
C. Grobler, 2016 "Multi-Objective Parallelization of Efficient Global Optimization"
Design optimization is a subject field where mathematical algorithms are used to improve designs. Analyses of designs using computational techniques often require significant com- puting resources, and for these problems, an efficient optimization method is needed. Efficient Global Optimization (EGO), first proposed by Jones et al.  is an optimization method which aims to use few function evaluations when optimizing a design problem. In this study, we use a multi-objective strategy to parallelize EGO.
EGO is part of a set of algorithms called surrogate optimization methods. A set of initial designs are analyzed and then a response surface is fitted to the evaluated designs. In each iteration, EGO selects the set of design variables for which the next analysis will be performed. It makes this decision based on two opposing criteria. EGO will either decide to sample where the predicted objective function value is low, an exploitation approach, or where there is high uncertainty, an exploration approach.
In each iteration, the classical EGO only selects one design per iteration. This selected design vector is either a result of exploitation or exploration based on a measure referred to as maximum Expected Improvement (EI). However, the modern day computing envi- ronment is capable of running multiple different analyses in parallel. Thus, it would be advantageous if EGO would be able to select multiple designs to evaluate in each iteration.
In this research, we treat EGO’s inherent selection criteria to either exploit or explore as
a multi-objective optimization problem, since each criterion can be defined by a separate objective function. In general multi-objective optimization problems don’t only have one solution, but a set of solutions called a Pareto optimal set. In our proposed strategy multiple designs from this Pareto optimal set are selected by EGO to be analyzed in the subsequent iteration. This proposed strategy is referred to as Simple Intuitive Multi- objective ParalLElization of Efficient Global Optimization (SIMPLE-EGO).
We start our study by investigating the behaviour of classical EGO. During each iteration of EGO, a new design is selected to be evaluated. This is performed by finding the maximum of the Expected Improvement (EI) function. Maximizing this function initially proved challenging. However, by exploiting information regarding the nature of the EI function, the maximization problem is simplified significantly, and the robustness of finding the maximum is enhanced. More importantly, solving this maximization problem robustly, dramatically improves the convergence behaviour once a local basin has been found.
We compare our SIMPLE-EGO method to a multi-objective optimization algorithm (EGO- MO) published by Feng et al. . We first investigate the behaviour of EGO, EGO-MO, and SIMPLE-EGO. Thereafter the convergence performance of these methods is quanti- fied.
As expected the parallelization of both SIMPLE-EGO and EGO-MO lead to faster con- vergence on a range of test functions compared to classical EGO, which only sampled one point per iteration. The convergence characteristics of SIMPLE-EGO and EGO-MO are also markedly different. We conclude with a discussion on the advantages and disadvan- tages of the investigated methods.
 Z. Feng, Q. Zhang, Q. Zhang, Q. Tang, T. Yang, and Y. Ma. A multiobjective opti- mization based framework to balance the global exploration and local exploitation in expensive optimization. Journal of Global Optimization, 61:677–694, 2015.
 D. R. Jones, M. Schonlau, and W. J Welch. Efficient global optimization of expensive black-box functions. Journal of Global Optimization, 13:455–492, 1998.
Supervisor:Prof. S. Kok
Co-Supervisor: Dr D. N. Wilke
H. J. Theron, 2016. "Modelling and characterization of a modified 3-DoF pneumatic Gough-Stewart platform"
Stabilised line of sight optical payloads for maritime vessels require variable platform conditions during the development, test and evaluation phases. A ship deck motion simulator is one means of generating such conditions in a controlled laboratory environment. This dissertation describes the aspects of the modelling, identification and validation of a ship motion simulator, in the form of a pneumatically actuated 3-DOF modified Gough-Stewart manipulator, to generate a realistic simulation environment for controller design. The simulation environment is a Matlab supervised MSC ADAMS/Matlab Simulink co-simulation in which Simulink houses the pneumatic model, the friction model, and the controller, and ADAMS runs the dynamic model of the physical hardware. A similar simulator cannot be found in published literature forcing a development of the model from the ground up, using published information as a foundation. The simulator model is broken up at the subsystem level which comprises the valve mass flow model, the piston chamber and force model, the complete actuator model and finally the complete ship simulator model. Each of these is derived, identified, and validated. The requirements of the simulator as well as the simulation environment is derived from real-life measurements done on seafaring vessels. An inverse kinematic solution is presented as a set of lookup tables which are generated from the outputs of MSC ADAMS by manipulating the simulator platform over the whole range of movements through Matlab. The reverse of the process is then used to ensure that actuator extensions generate the correct platform attitude — the attitude errors as shown to be infinitely small. Two valve mass flow models are proposed, a classical model and an ISO model, the first derived from thermodynamic principles and the second based on the ISO-6358 standard. The parameters of the two models are identified through experimental charging and discharging of a constant volume pressure chamber and sampling the temporal pressure and temperature outputs. The mass flow is calculated from the measured data through parameter estimation. Validation is done by comparing the temporal pressure outputs of the models with the actual measured pressure signals. The mean absolute error for the best fit ISO model is less than half of the Classic model at 0.4 MPa (MAE < 2 kPa) and the temporal pressure relationships in the closed-loop and open-loop tests shows a 93% correlation against measured pressure signals. The combination of the derived actuator chamber model and the valve mass flow model produces a realistic actuator model. The force equation of each of the actuators makes provision for a nonlinear friction component. The actuator friction model is based on a simple stick-slip relation with an acceleration dependent Stribeck function and an exponential viscous friction component. This model is also identified with data from the actual hardware. The complete ship motion simulator model is validated through open-loop as well as closed-loop tests. The open-loop tests are performed with chirp or sinusoidal signal excitation from a stable elevated offset starting condition. The ratio of the measured and simulated extension amplitudes in the open-loop is larger than 0.95 while the ratio of the rise times (tm/ts) is approximately 0.85. The closed-loop validation tests are conducted with both heave and roll inputs and compared well with the real system. A 14% difference in the actuator position amplitude (between the simulated and measured systems), and a 20% slower extension rate at 0.05 Hz that increases at 1 Hz to match the measured rate are observed. The maximum large signal bandwidth is 0.617 Hz, and is only limited by the mass flow. A simplified plant model is derived and compared with the high performance model and is subsequently used for a state feedback controller design and evaluation. The final controller gains deliver a stable system with the same 0.617 Hz bandwidth limitation and a controller that is insensitive to loop gain changes from 0.5 to 15.
Supervisor: Prof. N. J. Theron
G Stephens ,2016. "Characterisation of Filling Stage Models for Vacuum Infusion"
The development and proposal of models to simulate the vacuum assisted infusion processes have received significant attention over the last decade. In this study four permeability and four compaction models are considered in a standard resin infusion simulation. This results in 16 models that follow from the combination of the four permeability and four compaction models. All 16 models are characterised in this study, with the aim to investigate the extent to which the models are able to represent an experimentally measured pressure time response for the filling stage. This study therefore investigates the innate ability of the models to represent the experimental response. Each model is characterised using an exhaustive robust inverse analysis approach.
The need for the proposed robust inverse strategy was in response to the multi-modal nature of the inverse problem. Brute force inverse approaches were found to be too computationally expensive. The robust strategy calculates the model response for a Latin hypercube sampled set of parameters in the domain. A partial least squares regression then uses the parameters and response sets to generate an initial starting point for an interior point optimisation algorithm to solve the conventional least squares fit. This method was first shown to give reliable results on two example problems before using it on the vacuum infusion problem.
Using this robust strategy it was found that all 16 models have the same ability to represent the experimentally measured response. The response corresponds well to the simulated response of a more sophisticated resin infusion simulation that predicts the response from experimentally estimated parameters.
This study will hopefully stimulate future researchers to also characterise newly proposed models to investigate the innate ability to represent a desired response in addition to the predictive capability using physically justifiable parameters.
supervisor: Dr DN Wilke
M.A. Rahiman, 2016. "Title: An Exploration of the Relationship between Maintenance Performance and Resource Productivity"
As a consequence of their relative magnitude with respect to overall organisational expenditure, potential sources for significant cost savings involve maintenance costs, raw material costs and energy consumption. Previously conducted but inconclusive research indicates that there may be a relationship between maintenance activities and resource productivity. If this is the case, knowledge of such a relationship may unveil opportunities for direct productivity enhancement. Moreover, it may also serve as an aid in making improved measurements of the true value of the maintenance function. This in turn may enable practitioners to recognise when resource reallocation may be required to achieve greater levels of productivity.
The objective of this research is to explore the relationship between maintenance activities and resource productivity. It aims in part to assess if opportunities for productivity enhancement exist as a result of such a relationship. It also aims to establish if resource productivity can serve as a representative measure of maintenance performance. This study is based on rigorously proven theoretical propositions which are tested empirically on data procured from a metallurgical plant in South Africa.
The conclusion of this study is that the maintenance function enables equipment to process resources productively. Resource productivity may thus have the propensity to serve as an encompassing and cost effective measure of maintenance performance. In terms of its potential in this regard, decreases in resource productivity may offer valuable signals which indicate that corrective action is warranted.
In terms of productivity enhancement, this study elucidates the fact that machinery should always be kept in the best operating condition possible. When machinery malfunctions are discovered, it should be repaired in a timely manner to prevent unnecessary wastage from occurring.
Supervisor: Prof JL Coetzee.
Hans-Rudolf Björn Bosch, 2016. "FTire model parameterization and validation of an all-terrain SUV tyre"
Tyre modelling has been a focal point of vehicle dynamics modelling since the beginning of vehicle dynamics research. Many tyre models are based on single point contact models which utilize some form of the Pacejka Magic Formula curve fit. The Pacejka Magic Formula approach was formulated in the 1980s and has certain advantages such as high computational efficiency and easily obtainable parameterization data. However, the Pacejka Magic Formula is limited to function on smooth roads and a finite number of well defined, long wavelength discrete obstacles.
A high fidelity approach in the form of Cosin’s FTire tyre model was developed, in which the tyre is modelled as a three dimensional object populated with bending, tangential, lateral and radial stiffnesses as well as damping. The tyre is numerically approximated with a predetermined number of elements. The disadvantages of using FTire include its low computational efficiency and the large number of parameters prescribed to parameterize the tyre model. However, FTire is claimed to be capable of accurately predicting the forces and moments generated by the tyre on smooth as well as uneven road surfaces for on-road tyres.
The focus of this study lies on parameterizing and validating an FTire model of an all-terrain SUV tyre. The aim is to verify whether a parameterized FTire model is able to predict the tyre behaviour of an all-terrain SUV tyre for lateral and longitudinal forces on smooth road surfaces and vertical forces on uneven but hard terrain.
Static laboratory and dynamic field tests are conducted to acquire parameterization and validation test data to parameterize the FTire model. An Adams model of the tyre testing equipment is implemented to simulate the FTire model and validate it against dynamic validation test results.
It is found that the FTire model is able to predict the lateral tyre behaviour well on a smooth road surface. The longitudinal tyre behaviour on a smooth road surface and vertical tyre behaviour on an uneven road surface are predicted very well by the parameterized FTire model.
Supervisor: Prof P. S. Els
Marc Robert Greenland , 2016. "Analysis of Conjugate Heat Transfer and Pressure Drop in Microchannels for Different "
In this study the heat transfer and hydrodynamic parameters were experimentally investigated for a single microchannel housed in a stainless steel solid base material for different aspect ratios in the laminar regime with water as the working fluid. The stainless steel base material had a low thermal conductivity (15.1 W / mK) which magnified the conjugative effects in order to better understand the heat transfer. Rectangular microchannels with a height and width of 0.64 mm x 0.41 mm for Test Section 1, 0.5 mm x 0.5 mm for Test Section 2 and 0.43 mm x 0.58 mm for Test Section 3 were considered. The overall width of the solid substrate was 1.5 mm and the length was 50 mm for all of the test sections. The aspect ratio of the channel and the solid substrate was kept equal. A constant heat flux of 10 W / cm2 was applied to the bottom outer wall of the test section. A sudden contraction inlet and a sudden expansion outlet manifold contained pressure ports, to measure the pressure drop across the test sections, and thermocouples measured the mean inlet and outlet fluid temperatures. Thermocouples were used to measure the outer top and side wall temperatures at four equally spaced positions along the axial direction. The amount of axial heat conduction was below 0.6 % for all of the test sections and therefore warranted the use of a two-dimensional conduction model to determine the heat transfer parameters at the fluid to solid interface based on the outer measured wall temperatures. The local Nusselt number decreased, along the axial direction but increased towards the exit for all of the test sections. The average Nusselt number increased with the flow rate and the critical Reynolds number for fully turbulent flow Test Section 1 was 1950, for Test Section 2 was 2250 and for Test Section 3 was 1650. The average Nusselt number was directly related to the perimeter of the microchannels’ two side walls and the bottom wall (not the top wall), and thus decreased as the aspect ratio of the channel increased. The experimentally determined Nusselt numbers were larger for all three test sections when compared to common acceptable correlations. The friction factor decreased with the flow rate and was smaller in magnitude when compared to conventional theories. The diabatic friction factor magnitudes were smaller than the adiabatic friction factors. The friction factor decreased as the aspect ratio decreased, where the aspect ratio was calculated by taking the maximum of the microchannels width or height, divided by the minimum of the two. The possibility of a relationship could exist between the Colburn j-factor and the friction factor when considering the results for Test Section 1 and Test Section 2 but the results for Test Section 3 were significantly different.
Keywords: microchannel, heat transfer, pressure drop, single phase, laminar, stainless steel, water
Supervisors: Dr J. Dirker and Prof J.P. Meyer
Thomas Christian Montgomery, 2016. "Optimal distribution of discrete heat sources in a two-dimensional
In the study, the optimal distribution of discrete heat sources in a two-dimensional data centre was
investigated. The optimal placement of the cool supply air inlet and outlet was also investigated.
The governing equations were solved by using the finite volume method. The computational fluid
dynamics code Fluent was used to solve the governing equations. Optimisation was achieved using a
goal-driven optimisation approach and a response surface methodology.
The numerical model was validated using past experimental work and the results were in good
agreement with each other, showing an error of less than 6%. The realisable k-ε turbulence model
was used as closure equations to solve the Reynolds-averaged Navier-Stokes equations. Additionally,
the viscosity affected near-wall regions were modelled using a wall treatment method.
The optimum distribution of constant height (42 U) server racks was established for three different
configurations of inlet and outlet locations. After these optimal placements were established, the
effect of varying the height of the server racks was investigated for the same inlet and outlet
placements and the optimum locations were determined. By means of a sensitivity analysis, it was
found that the placement of the first and last servers as well as their respective heights had the most
influence on the heat transfer between the server panels and the ambient surroundings.
It was concluded that the inlet and outlet should be placed on opposing walls of the data centre and
variable server rack heights should be used in order to achieve maximum heat transfer.
Keywords: two-dimensional, discrete heat source, optimisation, response surface, data centre
Supervisors: Prof T. Bello-Ochende and Prof J.P. Meyer
E Miles, 2016. "Optimal Control Surface Mixing of a Rhomboid-Wing UAV"
This thesis describes the development of an open-loop control allocation function – also
known as a ‘mixing function’ – for aircraft with an unconventional control surface setup (i.e.
not consisting of a conventional elevator, rudder and ailerons) by using mathematical
optimisation. The techniques used to design the control allocation and mixing used on the
unconventional configuration when flying it without artificial stability or control
augmentation is provided. A typical application of this control mixing would be to enable a
pilot to operate an unconventional unmanned aerial vehicle (UAV) as if it was a conventional
model aircraft during flight testing or as a backup mode should any sensor failures occur
during a typical flight test program. The allocation can also be used to simplify the inner
control structure of a UAV autopilot or stability augmentation system. Although this type of
mixing would be straightforward on a conventional airframe, an unconventional
configuration has several unique characteristics that complicate the modelling and design
A custom six degree of freedom (6DOF) formulation for flight simulation was made
available to model the aircraft and run various scripts to evaluate the aircraft response when
the control allocation function is implemented. The simulation model was used to develop the
mixing function that maps conventional input commands to the unconventionally situated
control surfaces in the most optimal way.
The design process was formulated as a multi-objective optimisation problem, which was
solved using a custom sequential quadratic programming and custom leapfrog programming
method. A methodology was proposed to define the constraints, which can be customised for
a particular aircraft or application.
The control allocation function was implemented in two different simulation environments to
investigate the suitability of candidate designs. A robustness study was performed to evaluate
the impact of actuator failures on the aircraft control response using the designed control
allocation system. The proposed control allocation design methodology can also be used to
design the inner control loops of more sophisticated control systems such as stability
augmentation and automatic flight control, which is also briefly discussed in this thesis.
Supervisor: Dr B.A. Broughton
Co-supervisor: Prof J.P. Meyer
Alan Glenn Guthrie, 2016. "3D COMPUTER VISION CONTACT PATCH MEASUREMENTS INSIDE
OFF-ROAD VEHICLE TYRES"
The interaction between tyres and terrain is one of the most studied areas in the vehicle
dynamics and terramechanics research communities because it is the only region where excitation
forces act on the vehicle if aerodynamics is not considered. The tyre area which
deforms against the ground is called the contact patch. Measuring the contact patch has
been accomplished statically in the past; however, measurement while a wheel rotates has
A number of attempts to measure carcass deformation from inside the tyre have succeeded in
measuring small areas or single points but full field measurement has never been attempted.
This study describes the design and testing of a system which uses stereo cameras and
image correspondence to measure the deformation of the inside of the tyre carcass in the
contact region completely. The system includes a stabilisation mechanism which prevents
the cameras from rotating, ensuring that the cameras view the inside of the contact region
at all times.
Software to capture and process the images captured is developed and tested to ensure measurement
accuracy. The 3D results produced by the software are compared to one another
where possible and any trends or problems are discussed. Results indicate that the full 3D
displacement field in the contact region can be measured accurately. The information produced
is expected to be extremely valuable for development and validation of tyre and vehicle
Supervisor: Prof P. S. Els
Ibrahim Garbadeen, 2016. "Natural convection of multi-walled carbon nanotubes with water mixtures in a square enclosure".
The use of nanofluids in buoyancy-driven heat transfer can be very useful in enhancing the performance of various heat transfer applications. In this thesis, natural convection by multi-walled-carbon nanotubes (MWCNT) was studied in a square enclosure with differential heating by two opposite walls. Low particle concentrations of 0 – 1% based on volume were considered at Rayleigh numbers of 104 – 108. Thermal conductivities and viscosities of the nanofluids were experimentally determined. It was found that thermal conductivity and viscosity increased with increasing concentration by 6% and 58%, respectively. Models based on these experimental results were obtained and subsequently used in a numerical study of a two-dimensional simulation of natural convection in a square cavity using a commercial code. Results revealed an initial enhancement in the Nusselt numbers to a maximum of 22% which occurs at 0.14 % particle concentration and a Rayleigh number of 108. Beyond the maximum, the Nusselt number deteriorated. This was true for the different Rayleigh numbers studied with percentage enhancement in the Nusselt number increasing with increasing Rayleigh numbers. Further analysis was done to predict heat transfer performance of higher particle concentrations up to 8% which showed a general decline in the Nusselt numbers by increasing particle concentration. An experimental setup was subsequently used to study natural convection in an insulated square cavity with different temperature differences between the two opposite sides for particle concentrations of 0–1% at Rayleigh numbers between 2.1x108 and 6x108. Results from the experimental and numerical studies were subsequently compared and the validity of projected results for higher particle concentration was therefore assessed. The experimental results supported the overall behaviours of the nanofluids obtained from the numerical analysis. However, the experimental results of maximum enhancement in the Nusselt number was 42% at particle concentration 0.1% and a Rayleigh number of 6x108. Nevertheless, both results indicated the existence of an optimum particle concentration at which heat transfer in MWCNT nanofluids is maximised. The variation in the performance nanofluid was attributed to the counteracting, non-linear effects of thermal conductivity and viscosity both of which increases by increasing particle concentration. The thermal conductivity effect which improves heat transfer performance was observed to be more dominant for a very narrow range of low particle concentration up to 0.1 % while the viscous effect which diminishes heat transfer performance was found to be more dominant at higher particle concentration.
Keywords: Nanofluids, MWCNT, natural convection, cavity flow, volume fraction, viscosity.
Dr Mohsen Sharifpur; Prof Johan Slabber; Prof Josua Meyer
Pieter C. B. Luyt, 2016. "A leak tight design methodology for large diameter flanges based on non-linear
modelling and analysis"
There is currently a need for large diameter flanges for the supply of water in South Africa. These large
diameter pipe flanges are required to accommodate pipes with nominal bores of up to 4 m and should
successfully withstand internal pressures of up to 8 MPa. No current relevant standard / code contains
prescribed design values for flanges which either operate at such high pressures or have such large
diameters. Due to this an alternative method of design, by means of non-linear finite element modelling,
is proposed. Three types of integral flange designs are considered, namely: flat face, raised face, and
a modified raised face with an O-ring groove. The effects of creep-relaxation, flange rotation, and the
bolting sequence are considered.
For each of these designs a finite element model was created and compared to a small scale experiment
which included strain and contact pressure measurements. The proposed non-linear finite element
models were capable of accurately predicting the strains in the flanges as well as the contact pressures
between the faces of the flange and the surfaces of the packing material. Finally, a comparison between
the ASME design method and the proposed non-linear finite element modelling design method was
done for the large diameter flanges. It was found that the ASME design code did not have the ability to
accurately predict the stresses in the flanges. It was also found that by using the maximum equivalent
Von Mises stress as failure criteria for the flanges and fasteners, and contact pressure for the sealing
ability, circular bolted flange connections which are lighter, safer, and leak tight could be designed by
means of the proposed non-linear finite element models.
Keywords: flat face flange; raised face flange; raised face flange with an O-ring groove; flange rotation;
creep-relaxation; contact pressure
Supervisor: Prof. Nico J. Theron
Co-supervisor: Mr Francesco Pietra
C.B. Church, 2016. 'Turbomachine Internal Pressure and Blade Response Modelling"
Blades are critical components of turbomachines, failure of a single blade may result in catastrophic
failure of the entire machine. One study found that blade failure was the third largest cause of power
generation unit unavailability. Their condition during operation is therefore of interest to monitor.
Various intrusive and non-intrusive blade vibration measurement (BVM) techniques have been
developed for this purpose. Intrusive techniques such as strain gauge approaches and the frequency
modulated grid method require expensive and complex alteration of the actual blades and/or casing.
Further, they are prone to failure due to operation in harsh working environments. Therefore the use
of intrusive techniques has been predominantly limited to design verification, testing and research.
Blade tip timing approaches are currently at the forefront of BVM. The practicality, accuracy and ease
of implementation of these approaches have limited their commercial roll out. An alternative nonintrusive
source of blade vibration information was found in the internal casing pressure signal (CPS).
As the machine operates the blade movement excites the fluid in the casing, producing a measureable
response. Unlike BTT approaches which deal with a scarcity of information, CPS based methods must
identify blade vibration from a complex signal which contains multiple other sources of information.
The issue of how to model the blades’ response and fluid interaction is the topic of this investigation.
An available single stage turbomachine mock setup was modified for internal pressure and direct
blade vibration measurements. Pressure measurements were taken in line with a redesigned hub and
rotor blade assembly. Strain gauges (SG) were applied to blades in order to capture their response.
The blades’ response was modelled as the combination of a forcing function and a multiple degree of
freedom transfer function. Repurposed experimental modal analysis frequency response
reconstruction techniques were used to model the blades’ transfer function. It was found that this
technique was able to capture the blades’ underlying behaviour to a high degree. The forcing function
was modelled in the time domain as a series of Gaussian shaped force distributions. It was found that
the model was able to capture many important aspects of the forcing behaviour. Both the forcing
function and blade transfer function were explored using constrained optimisation techniques.
The blade-fluid interaction was modelled as a Fourier series. It was shown that the blade behaviour
cannot be extracted from a pressure signal using standard frequency analysis techniques. The viability
of an inverse problem solution methodology, for the purpose of blade behaviour extraction, was
investigated. This was achieved by solving reduced components of the model with SG measurements
and observations from pressure measurements. Further the need to isolate the pressure field about
individual blades was motivated and a novel time domain windowing technique provided.
Keywords: Turbomachine, blade vibration, casing pressure, signal processing, optimisation.
Supervisor: Prof. P.S. Heyns
Mulock-Houwer, 2016. "The effect of adjacent tubes on the diabatic friction factors in the transitional flow regime"
Heat exchangers are used throughout the world in important processes such as the generation of electrical energy. Modern heat exchangers are often forced to operate in the transitional flow regime, where flow can be unpredictable. Most of the research that has been done on the transitional flow regime has focussed on the influence of heat transfer and the inlet effects. However, all these studies made use of only a single tube, while most heat exchangers would typically have a bundle of tubes such as in shell-and-tube type heat exchangers. The purpose of this study was to investigate the effect of adjacent tubes on the transitional flow regime during diabatic conditions. An experimental set-up was purposefully built for this investigation and two test sections were investigated. A single-tube test section was built for validation purposes, since similar work has been done. A triple-tube test section was built with three tubes spaced at a pitch distance of 1.4 outer diameters. The mass flow rate, as well as the pressure drops over the fully-developed section was measured for each tube. From the pressure drop data the friction factors were calculated. Furthermore, a heat flux of 3 kW/m2 was applied to each tube and the inlet, outlet and wall temperatures were measured, to ensure that specifically the diabatic friction factors were determined. Water was used as the working fluid and tests were run over a Reynolds number range of 1 000 - 6 500. An uncertainty analysis showed the maximum uncertainty of the friction factors to be 8.3%. The laminar, transitional and turbulent flow regimes could be identified from the friction factor data. The results from the single-tube test section correlated well to the literature with transition starting at a Reynolds number of 2 380 and ending at 3 050. The results from the triple-tube test section showed the start of transition to be initiated by the presence of adjacent tubes, with the Centre-tube entering transition at a Reynolds number of 1 970. The outer tubes experience a delayed start in transition at Reynolds numbers of 3 000 and 2 800 for the Left-tube and Right-tube respectively. The end of transition occurred at approximately the same Reynolds number (3 100) for all three tubes of the triple-tube test section. Since the Centre-tube entered transition earlier than the outer tubes, maldistribution was evident, with the water taking the path of least resistance. The flow rate in the Centre-tube showed an average difference of 2.8% in the Reynolds number range of 1 970 to 3 150. Maldistribution proved to be negligible when all three tubes were in the laminar or turbulent flow regimes.
Supervisor: Professor J. P. Meyer
Ntumba Tshimanga, 2016. "Experimental investigation and model development for thermal conductivity of glycerol-based nanofluids"
Glycerol has historically been used as an antifreeze fluid to facilitate heat transfer in the automotive and air conditioning and refrigeration industries. It has also been used as a lubricant in the processing of food and the production of pharmaceuticals and cosmetics. Although a lot of work has been done recently to evaluate the potential to enhance heat transfer using nanoparticles mixed with a base fluid to form a nanofluid, no work has been done on using glycerol as a base fluid. Therefore the purpose of this study was to investigate the effect of nanoparticle volume fraction, nanoparticle size and temperature on the thermal conductivity of stable glycerol-based nanofluids. Two types of metal oxide nanoparticles were considered namely MgO and α-Al2O3. The particle sizes of the MgO ranged from 21 nm to 119 nm and for the α-Al2O3 it ranged from 31 nm to 134 nm. The thermal conductivities were determined by experimental measurements and with analytical and empirical models. The thermal conductivity measurements were taken at temperatures ranging from 20˚C to 45˚C, for nanofluids prepared at volume fractions ranging from 0.5% to 4%. The nanofluids were prepared with a two-step method that included ultrasound mixing to ensure the nanoparticles were fully dispersed and deagglomerated in the glycerol. The experimental results showed that both the α-Al2O3-glycerol and MgO-glycerol nanofluids had substantially higher thermal conductivity than the base fluid. It was also found that at room temperature, the effective thermal conductivity remains almost constant for at least 50 hours. The maximum thermal conductivity enhancement for the α-Al2O3-glycerol nanofluids was observed for a 4% volume fraction to be 19.5% for a nanoparticle size of 31 nm. For the MgO-glycerol nanofluids the maximum thermal conductivity enhancements were also for a volume fraction of 4%, however, the enhancement was 18% for a particle size of 21 nm. Furthermore, the thermal conductivities as function of nanoparticle size, volume fraction and temperature, of the two nanofluids were investigated. It was found that the thermal conductivities of the α‑Al2O3-glycerol nanofluids were significantly more dependent on particle size than the MgO-glycerol nanofluids. Furthermore, it was found that no equations exist at present that can accurately predict the thermal conductivity of glycerol based nanofluids and therefore new empirical equations correlations were developed.
Keywords: Nanofluids, thermal conductivity, glycerol, nanoparticle size, volume fraction, temperature, stability.
Supervisors: Dr Mohsen Sharifpur and Prof Josua P Meyer
Wietsche Clement William Penny, 2016. "Anti-lock Braking System performance on rough terrain
The safety of motor vehicles is of primary concern in the modern age as the death rate of road users are still at unacceptably high numbers and is the second largest cause for unnatural death worldwide. Consumers often expect unrealistic performance and comfort levels from their vehicles regardless of terrain or conditions, and the Sport Utility Vehicle class is often under the most pressure to meet these high expectations.
Literature reveals that the performance of Anti-lock Braking Systems (ABS) deteriorates on rough off-road terrains due to a number of factors such as axle oscillations, wheel speed fluctuations and deficiencies in the algorithms. This leads to complications such as loss of vertical contact between the tyres and the terrain and poor contact patch generation that eventually results in reduced longitudinal force generation.
In this study, an ABS modulator is retrofitted on a test vehicle to perform brake pressure control. The hydraulic modulator is controlled by an embedded computer, running the Linux operating system, onto which a slightly modified version of the Bosch ABS algorithm is coded in C-language. Brake tests are conducted with the vehicle on hard concrete terrains for both smooth roads and rough Belgian paving. The algorithm is also implemented in Matlab/Simulink using co-simulation with a validated non-linear full vehicle ADAMS model employing a validated FTire tyre model. The co-simulation model was validated with the test data on both flat and rough terrains and experimental results correlate well with simulation results when the recorded brake pressures from the test data are given as input to the simulation model.
Test data and simulation results indicate that wheel speed fluctuations can cause inaccuracies in the estimation of vehicle velocity and excessive noise on the derived rotational acceleration values. This leads to inaccurate longitudinal slip calculation and poor control decisions respectively. Although possible solutions to the identified problem are not explored in detail, the developed simulation model and test vehicle can be used to test improved ABS algorithms and suspension control strategies to solve the deterioration of ABS performance on rough terrain.
Supervisor : PS Els
Erin Sarah Vause, 2016. "THE INLET EFFECTS OF MULTIPLE TUBES ON THE ADIABATIC PRESSURE DROP OF SMOOTH, HORIZONTAL TUBES, IN THE TRANSITIONAL FLOW REGIME"
There exists many investigations in the field of pressure drop inside smooth tubes; however, there is a research gap where this flow is in the transitional flow regime. All work in the transitional flow regime thus far has been concerned with a single tube and in limited cases the effect of different types of inlets were investigated. Many heat exchangers such as shell-and-tube heat exchangers consist of a number of closely packed tubes leading from a common header and in some cases may operate in or close to the transitional flow regime. However, it is not known what effect adjacent tubes will have on pressure drop and the flow distribution in the transitional flow regime. The purpose of this study is therefore to investigate the effect that the presence of adjacent tubes will have on the pressure drop in the transitional flow regime. The study is limited to smooth and circular, horizontal tubes in the fully developed, transitional flow regime and adiabatic pressure drops. The transition effects were investigated experimentally by developing and building an experimental set-up on which, firstly, the pressure drops could be measured of one tube to be used as a reference and then secondly, the pressure drop of three tubes in parallel, equally spaced, 1.4 diameters apart. The internal tube diameters of all tubes were 3.97 mm and the tube lengths were 6 m. The pressure drops were measured over a length of 1.97 m, at the end of the tubes where the flow was fully developed. The pressure drops were measured with pressure transducers while the inlet and outlet temperatures of the water were measured with PT100 probes. All the tubes were connected to a calming section to ensure a square-edge inlet. Experiments were conducted with water at Reynolds numbers from 700 to 5 100 to ensure that the pressure drops and thus friction factors could be determined for fully developed flow throughout the laminar, transitional and turbulent flow regimes. The uncertainty of the friction factors were all less than 1%. It was found that the centre tube experienced an earlier onset of transition than that previously seen in single tube tests, at a Reynolds number of 1 840. In the outer tubes, transitional flow was delayed well beyond that previously seen in literature. The flow in all three tubes underwent transition into fully developed turbulent flow by the maximum Reynolds number of 3 340. The effect of having multiple, adjacent inlets caused a maldistribution in the mass flow rate, with a 5.8% difference in the flow rates of the outer tubes in the transitional flow regime. New correlations were developed to predict the friction factor for transitional flow in each of three adjacent tubes at an inlet pitch distance of 1.4 times the inner tube diameter. Overall, it can be concluded that multiple tube entrances have an effect on the transitional flow in all of the tubes and should be further investigated for other pitch distance and tube arrangements.
Supervisor: Prof J.P. Meyer
Nicole Coetzee, 2015. "Heat Transfer Coefficients of Smooth Tubes in the Turbulent Flow Regime"
Several well-known equations are available in literature that can be used to determine the heat transfer coefficients of smooth tubes in the turbulent flow regime. When comparing the results of these equations they vary over a considerable range. Although in many cases it is assumed that the Gnielinski equation is one of the most accurate as it is the most recently developed correlation, the uncertainty of this equation has not be quantified as yet. Many of the equations have been developed using the data that was obtained from experimental testing as far back as 80 years ago. As more accurate instrumentation is available, it should be possible to collect more accurate data during experimental testing. The purpose of this study was threefold: to take accurate heat transfer measurements and to quantify the uncertainties of the Nusselt numbers as a function of Reynolds number; to compare the measured data with existing correlations and to develop an accurate Nusselt number correlation from the data. Experiments were conducted on two smooth circular tubes with a heat transfer length 3.75 m with inner tube diameters of 8.3 mm and 14.2 mm using water over a Reynolds number range of 10 000 to 220 000 and a Prandtl number range of 3.2 to 4. Surface roughness analysis was also performed to ensure the tubes can be considered as smooth tubes. Pressure drop measurements were also taken over tube lengths of 4.1 m as a function of Reynolds number. The experiments were conducted using a tube-in-tube heat exchanger with the hot water in the inner tube and the cold water in the annulus operating in a counter flow configuration. The friction factor values were determined from pressure drop measurements and the heat transfer coefficients were determined from the Wilson Plot method using temperature and mass flow rate measurements. The average uncertainties of the friction factors and Nusselt numbers were both less than 3%. The results were compared to the existing literature and it was found that at lower Reynolds numbers the Nusselt numbers deviated slightly to those of Gnielinski with the results comparing more closely with increasing Reynolds number. The friction factor results correlated well with that of Blasius. A new Nusselt number correlation, which is a function of Reynolds number, Prandtl number and friction factor, was developed. The equation predicted all the measured values within 3%. The equation is, however, limited in not taking into consideration large variations in fluid properties.
Supervisor: Prof J.P. Meyer
Torsten Peter Löwe,2015. "Modelling of a Passive Hydraulic Steering for Locomotives"
During the past few decades, substantial improvements were made to rail infrastructure worldwide. This was necessary to accommodate the ever increasing transportation demand and requirements. Nowadays, trains are required to transport heavier loads and to travel at higher speeds.
One of the major improvements was achieved by the development of the off-flange curving bogie designs to reduce wheel and rail wear. Off-flange designs include passive steering and actively controlled steering. The development and implementation of self-steering bogies on locomotives was promoted in the early 1980’s by two major locomotive manufacturers. Up to date, thousands of these locomotives, with built-in self-steering bogies, have been manufactured and taken into service (Swenson, 1999).
Most self-steering bogies have mechanical linkage systems to steer the wheel sets. As an alternative to the mechanical linkage system, the DCD Group (a South African manufacturer of rail and mining equipment) initiated the development of a Passive Hydraulic Steering (PHS) system.
First PHS prototype systems, developed by DCD, have proven that huge wear reduction possibilities exist on both, rails and wheels. In addition the prototype systems also significantly decreased noise and vibration levels when negotiating tight corners (Swenson & Scott, 1996 and DCD Rolling Stock, 2012). However, existing prototype solutions require further improvements and development for optimisation. To be able to identify and implement improvements, the need exists to perform modelling and testing of the systems to obtain a better understanding of the operation and suitability of a complete unit.
The aim of this research project is thus, to mathematically model an existing prototype PHS system and validate the model with data from experiments and tests. This model can then be used in order to improve and optimise performance, cost and reliability of the system, before mass production is considered.
A literature survey was conducted, focusing on general wheel and rail wear mechanisms, techniques to improve wheel and rail life and on existing techniques for modelling the hydraulics and multi-body dynamics of locomotive systems.
The literature survey was followed by extensive laboratory tests on component basis, a quarter PHS system and on the full PHS system. From these tests all parameters needed for the characterisation of the PHS and the mathematical model were determined. These tests also provided data for the validation of the PHS model.
Finally, a mathematical model of the PHS system was successfully generated and validated. This model can now be used in a multi-body dynamic locomotive simulation to evaluate its effectiveness.
The results and findings of the literature survey, experiments and modelling are reported on and discussed in this report.
Supervisor: Prof P.S. Els
Co-Supervisor: Dr C. Kat
Mangolo Masenya, 2015. "Enhancing the maintenance system for critical pumps in a power station"
Maintenance is a significant aspect of asset management. Recent findings on Eskom’s asset management system in 2013 and the 2014 publication of the new asset management standard (ISO 55000) has highlighted the need to evaluate maintenance within the power utility to ascertain the degree of alignment to the minimum requirements stipulated in the standard. This research project derives requirements for a maintenance system, evaluates current maintenance processes and activities at a power station, and offers recommendations for improvement where gaps are identified.
The principal objective of this research project is to enhance maintenance of critical assets in the Eskom power production process, using the Eskom Matimba power station as the basis for the investigation. This objective is achieved by first establishing the current maintenance system and practices at the power station through collection and review of documented maintenance-related information, policies and procedures from the station. Additional maintenance-related information is also sourced through interaction and interviews with staff employed by the power station.
The research then gathers the requirements for an effective and efficient maintenance organization from literature, as well as asset management standards, and systematically evaluates the existing maintenance system for selected assets (critical pumps in electricity generation process) against these requirements with the aim of identifying improvement opportunities to the current maintenance system.
In addition to frameworks for effective maintenance, the research also investigates quantitative tools for optimizing maintenance and develops a reliability engineering model to optimise asset maintenance costs for selected critical pumps. The research also highlights factors that influence the success of improvement initiatives such as adoption of ISO standards.
Findings from evaluating the maintenance system for critical pumps at the power station against asset management requirements are that fundamental requirements for a maintenance system such as maintenance resource allocation, documented maintenance policies, computerised maintenance management systems and commitment to continuous improvement are met and in place. Areas of improvement identified and recommendations include increasing proactive maintenance; enhancing comprehensive asset information management, increasing asset management awareness and increasing functional or departmental integration.
The findings from the research are applicable to the power station studied and offer a starting point for a maintenance improvement initiative for this station. However, the findings and recommendations are seen to be applicable to maintenance of other critical assets in similar power plants and are potentially beneficial in establishing a maintenance system for new power plants.
supervisor: Prof PS Heyns
Benjamin Gwashavanhu, 2015. "Evaluation of optical techniques applied to online turbomachinery blade vibration measurements".
Understanding the dynamic characteristics of blades is important in the online condition monitoring of turbomachinery. Conventionally contact methods are used for this purpose. However improvements in technology now allow for the use of non-contact methods. Contact measurement techniques for turbomachinery blade vibration analysis typically involve the use of strain gauges and accelerometers. These present some complications when analyzing rotating machinery. Being contact in nature, mass loading can affect the integrity of measurements captured. Turbomachines typically operate under the adverse conditions of high temperatures, high flow rates and sometimes wet environments. This significantly reduces the life of contact transducers installed to capture the blade dynamics. Installation of telemetry systems for signal transmission is also necessary. In addition to being invasive and expensive, telemetry systems can introduce electrical noise.
These factors make it desirable to explore the applicability of various optical non-contact methods for analyzing turbomachine blade vibrations, such as Laser Doppler Vibrometry (LDV) and photogrammetry. Both techniques have been successfully used to analyze vibrations of structures. Photogrammetry is a full-field measurement technique which allows for non-intrusive simultaneous measurement of vibrations at different locations on a blade. This is particularly important for the updating of numerically developed models of structures, investigation of structural global dynamics, and more effective localization of damage. Accelerometers have been used to validate a variation of photogrammetry, three dimensional point tracking (3DPT), for rotational applications and discrepancies attributed to the contact nature of accelerometers were observed. To build confidence in the use of 3DPT as a non-contact method for analyzing rotating machines, it is necessary to investigate how well it correlates with various non-contact methods. Through such an investigation aspects that need to be addressed when using 3DPT to analyse turbomachines can be identified. If reliable measurements can be obtained using this technique, further investigations such as online damage detection and characterization in rotating structures can be conducted.
In this study Tracking Laser Doppler Vibrometry (TLDV) and 3DPT are used as non-contact methods to investigate the online vibrations of a turbomachine test rotor. To employ TLDV on the test rotor, the dynamics of the scanning mirrors of a Polytec Scanning Vibrometer (PSV) are characterized using a frequency response approach. Look-up tables are constructed to provide the necessary phase angle compensation for the two signals supplied to the mirrors, to obtain a circular scanning path. Photogrammetric 3DPT is then validated by tracking the TLDV laser spot focused on one of the test rotor blade using high speed cameras, and comparing the 3DPT measurements to TLDV blade out-of-plane vibration measurements. The correlation between the two non-contact measurement techniques is presented. This establishes the validity of the employed scanning system, and also serves to show how well the two non-contact methods correlate with each other, when investigating dynamics of turbomachinery blades. 3DPT is then used to analyze the responses of the test rotor blades under excitation. Various Operational Deflection Shapes (ODSs) of the blades are identified and the results obtained are presented.
The use of ODSs obtained from 3DPT to investigate irregularities along turbomachinery blades is also presented. This information is used to show that ODSs captured using 3DPT can be used to online detect and localize blade damage in turbomachines.
supervisor: Prof PS Heyns
Jan-Sjoerd van den Bergh, 2015. "Effects of Friction and Gas Modelling on Vehicle Dynamics Simulation".
Validated simulation models have become ever more important in the current technological and economic environment, where simulation is an integral part of the design process. In the field of vehicle dynamics, it is no different, where vehicle manufacturers and researchers are relying more heavily on simulation than ever before. In the competitive field of research and development, the phrase “as accurate as possible, as complex as is necessary” rings true for vehicle models. Due to the “as complex as necessary” approach, many complex phenomena such as suspension kinematics and suspension friction remain un-modelled, as the assumption is made that the effects are negligible. The seemingly negligible effects negatively affect the validity of simulation models, especially when deviating from the specific manoeuvre for which the model was originally created.
In this study, focussed on a vehicle with a hydropneumatic suspension system, the effect of gas modelling methodology, friction, and friction modelling strategy on the validity the suspension unit characteristics, and a full non-linear vehicle dynamics model is presented. The approach to gas modelling included three permutations of the ideal gas formulation, namely isothermal, adiabatic, and a heat transfer dependent thermal time-constant approach. The effects of friction were accounted for using a rudimentary lookup table approach, a LuGre, and a Modified LuGre friction model, while using the case where friction is neglected as reference.
The results showed that the gas modelling approach, and the effects of friction, each have a significant effect on model accuracy and validity when compared to physical test results. The improvement is witnessed on both the single suspension unit characteristic as well as on the full non-linear simulation model. This effectively proves that seemingly negligible effects may have a significant effect on model validity.
Study Leader: Prof. N.J. Theron
Co-Study Leader: Prof. P.S. Els
Henriette Christine Nolte, 2015. "Analysis and Optimisation of a Receiver Tube for Direct Steam Generation in a Solar Parabolic Trough Collector"
This study focused on a numerical second law analysis and optimisation of a receiver tube operating
in a parabolic trough solar collector for small-scale application. The receiver functioned in a Rankine cycle. The focus was on entropy generation minimisation in the receiver due to the high quality exergy losses in this component. Water functioned as the working fluid and was heated from ambient conditions (liquid) to a superheated state (vapour), consequently, the receiver tube was subject to both single phase as well as two-phase flow.
Entropy generation in the receiver tube was mainly due to finite temperature differences as well as fluid friction. The contribution of each of these components was investigated. Geometrical as well as operating conditions were investigated to obtain good guidelines for receiver tube and plant design. An operating pressure in the range of 1 MPa (Tsat = 180◦C) to 10 MPa (Tsat = 311◦C) was considered. Furthermore a mass flow range of 0.15 kg/s to 0.4 kg/s was investigated.
Results showed that beyond a diameter of 20 mm, the main contributor to the entropy generation was the finite temperature differences for most conditions. Generally, operating pressures below 3 MPa showed bad performance since the fluid friction component was too large for small operating pressures. This phenomenon was due to long two-phase lengths and high pressure drops in this region. The finite temperature difference component increased linearly when the tube diameter was increased (due to the increase in exposed area) if the focused heat flux was kept constant. However, the fluid friction component increased quadratically when the diameter was reduced.
In general when the concentration ratio was increased, the entropy generation was decreased.
This was due to more focused heat on each section of the receiver pipe and, in general, resulted in shorter receiver lengths. Unfortunately, there is a limit to the highest concentration ratio that can be achieved and in this study, it was assumed to be 45 for two-dimensional trough technology.
A Simulated Annealing (SA) optimisation algorithm was implemented to obtain certain optimum parameters. The optimisation showed that increasing the diameter could result in a decrease in entropy generation, provided that the concentration ratio is kept constant. However, beyond a certain point gains in minimising the entropy generation became negligible. Optimal operating pressure would generally increase if the mass flow rate was increased. Finally, it was seen that the highest operating pressure under consideration (10 MPa) showed the best performance when considering the minimisation of entropy in conjunction with the maximisation of the thermodynamic work output.
Supervisors: Prof. T. Bello-Ochende, Prof. J.P. Meyer
M.Everts, 2015. "Heat Transfer and Pressure Drop of Developing Flow in Smooth Tubes in the Transitional Flow Regime"
Heat exchangers have a wide range of applications and engineers need accurate correlations to optimise the design of these heat exchangers. During the design process, the best compromise between high heat transfer coefficients and relatively low pressure drops is usually in the transitional flow regime. Limited research has been done on tube flow in the transitional flow regime. These studies considered either fully developed flow, or average measurements of developing flow across a tube length. No research has been done with the focus on developing flow in smooth tubes in the transitional flow regime. Therefore, the purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics of developing flow in the transitional flow regime. An experimental set-up was designed, built and validated against literature. Heat transfer and pressure drop measurements were taken at Reynolds numbers between 500 and 10 000 at three different heat fluxes (6.5, 8.0 and 9.5 kW/m2). A total of 398 mass flow rate measurements, 19 158 temperature measurements and 370 pressure drop measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. The test section was a smooth circular tube and had an inner diameter and length of 11.52 mm and 2.03 m, respectively. An uncertainty analysis showed that the uncertainties of the Nusselt numbers and Colburn j-factors varied between 4% and 5% while the friction factor uncertainties varied between 1% and 17%. Five different flow regimes (laminar, developing laminar, transitional, low-Reynolds-number-end and turbulent) were identified in the first part of the tube during the experiments and nomenclature was developed to more clearly identify the boundaries of the different flow regimes. The developing laminar regime was unique to developing flow and decreased along the tube length. Both the start and end of transition were delayed along the tube length and the width of the transition region decreased slightly. This is in contrast with the results obtained in literature where the effect of the non-dimensional distance from the inlet on fully developed flow in the transition region was investigated. Transition was also slightly delayed with increasing heat flux, but secondary flow effects had no significant influence on the width of the transition region. The relationship between heat transfer and pressure drop was investigated and correlations were developed to predict the Nusselt number as a function of friction factor, Reynolds number and Prandtl number in the laminar, transitional, low-Reynolds-number-end and turbulent flow regimes. Overall, it can be concluded that the heat transfer characteristics of developing and fully developed flow differ significantly and more work needs to be done to fully understand the fundamentals before the heat transfer and pressure drop characteristics are fully understood.
Supervisor: Prof J.P. Meyer
Johannes Marthinus Koorts, 2015. "Entropy Minimisation and Structural Design for Industrial Heat Exchanger Optimisation"
The mass flow rate for shell-and-tube, tube-fin and tube-in-tube industrial-type heat exchangers can be optimised by minimising entropy generation over a finite temperature difference. The purpose of the work was to apply the principles of entropy generation minimisation based on the second law of thermodynamics to determine whether the intercept between entropy generation due to heat transfer and fluid friction is a good approximation for the global minimum entropy generation, as well as to optimise a number of variables. Optimisation was achieved by applying numerical methods.
In order to yield meaningful results, the optimisation was done by setting a number of boundary conditions such as the maximum inlet steam temperature. The heat exchanger optimisation was based on a case study of the heat exchangers used at Columbus Stainless’ cold products division.
The mentioned case study consisted of 27 industrial-types of heat exchangers with power ratings ranging between 100 and 800 kW. The original specification included lengths varying between 1 and 2.42 m, shell-side diameters from 0.4 to 2 m and number of tubes varying from 1 to 20. The medium mass flow rates ranged between 0.048 and 0.855 kg/s while the steam mass flow rates ranged between 0.029 and 0.236 kg/s. The medium output temperatures were between 303 and 403K
The effect of various conditions was taken into account. Apart from the conditions already mentioned, the specific material properties at their specific pressures and temperatures (such as density, viscosity, conductivity, heat transfer coefficient) were also taken into account in the calculations.
Through numeric optimisation it was possible to conclude that the main variables that affected entropy generation were the steam inlet temperature, followed by the tube-side diameter for the given sample set. These variables were thus used in all further graphs as the main variables to be changed.
These variables were manipulated and the entropy generation due to fluid friction and heat transfer were independently plotted to determine if the intercept of these two lines could be used as a good approximation of the global minimum entropy generation. It was found that the approximation is not that good, unless one uses the entropy generation due to operational heat transfer, which yielded deviations between the intercept values and the global minimum ranged between 0.21 % and 21.88 % with an average deviation of 6.52 %, which is considered to be a good approximation. The constructal design theory does thus hold well under the current operating conditions.
It was clear that the main mechanism contributing to entropy generation was the effect of fluid friction, although this was only the case at smaller tube diameters. Of the entropy generated due to fluid friction the majority was contributed due to the tube side (steam), with almost no entropy generated due to fluid friction in the shell-side (medium) of the heat exchanger.
The effect of a number of variables on entropy generation were discussed and plotted. It was seen that an increased inlet temperature resulted in most cases in less entropy generated and that larger tube diameters have a similar effect due to less entropy generated due to fluid friction.
By using the principles of entropy generation minimization the entropy generated of each heat exchanger could be reduced by between 2% and 64%, with the tube-fin heat exchangers having the largest scope for improvement in the sample set used.
By using the principals of construal design, the intercepts of the entropy generation due to heat transfer and due to fluid friction was used to determine the optimal diameter. This correlation yielded very good results (Within 1% of the global minimum entropy generation) to predict the global minimum entropy generation.
Supervisors:Tunde Bello-Ochende, Josua P. Meyer
Keywords: Entropy, shell-and-tube, tube-fin, tube-in-tube, heat exchanger, heat transfer, irreversibility, model, thermodynamics, numerical, optimisation, geometric, constructal design
Jacob E. van der Westhuizen, 2015. "INVESTIGATION INTO USING LIQUID CRYSTAL THERMOGRAPHY AS PRIMARY TEMPERATURE MEASUREMENT TECHNIQUE FOR OBTAINING LOCAL WALL TEMPERATURES AND HEAT TRANSFER COEFFICIENTS IN TUBE-IN-TUBE HEAT EXCHANGERS."
The feasibility of obtaining local wall temperatures by using liquid crystal thermography (LCT) in a counter‑flow tube-in-tube heat exchanger was investigated. Local annulus-side heat transfer coefficients at the inlet and thermodynamically and hydrodynamically underdeveloped regions were also obtained while operating at steady conditions.
The heat transfer coefficients of the tube‑in‑tube heat exchanger are, however, disputed in the literature, as conflicting sources are easily found. In most literature sources the problem is simplified by assuming constant heat transfer coefficients throughout the length of the heat exchanger and the boundary layer growth is generally ignored at inlet regions.
Thermocouples pose practical problems when measuring temperatures in heat exchangers. LCT is investigated as alternative surface temperature measurement technique. This study aims to develop a methodology for directly measuring wall temperatures inside a tube‑in‑tube heat exchanger. These temperatures were further used to calculate local heat transfer coefficients.
In this study, a 1m long tube-in-tube test section with an annulus diameter ratio of 0.54 (ratio of the inner wall of the annulus to its outer wall) was constructed, in which liquid crystal thermography was employed as an alternative wall temperature measurement technique to thermocouples. Temperature-sensitive paint was applied to the inner wall of the annulus in order to measure the wall temperatures non-intrusively. Complete temperature maps could be constructed for different thermal conditions which indicated differences of up to 10 °C in wall temperature at the inlet regions, which would have been difficult to capture with thermocouples. This study covered a total of nine different annular flow and thermal conditions for cooled and heated cases. The annular flow conditions ranged from laminar (Re = 1000) flow to fully turbulent flow (Re = 13 800).
In general, the heat transfer coefficients were found not to be constant along the length of the heat exchanger. The averaged heat transfer coefficients at the inlet were compared with existing correlations in the literature for full-length heat exchangers and were found to be higher by an average of 44 % over the data presented. Uncertainties on the local heat transfer coefficient were found to be approximately 80% for the cooled annulus cases and 45% for a heated annulus. This was mostly due to the practical laboratory restrictions imposed by fluid temperature limits.
It was found that liquid crystal thermography could be used successfully for directly measuring the wall temperatures of tube-in-tube heat exchangers with very low surface temperature uncertainties (0.03 °C). With the approach developed in this study, a method was found for determining local heat transfer coefficients without introducing wall thermocouples or any other disturbances in the passage of the annular fluid.
Supervisors: Dr Jaco Dirker and Prof Josua P. Meyer
JM Conradie, 2014. "Finite Element Modelling of Off-Road Tyres"
Most tyre models developed to date require a fair amount of data before an accurate representation
of the tyre can be obtained. This study entails the development of a simplified, yet accurate, nonlinear
Finite Element (FE) model of an “off-road” tyre to study the behaviour of the tyre due to radial
loading conditions. The study aims to develop a FE tyre model that can solve fast and be accurate
enough to be used in multibody dynamic vehicle simulations. A model that is less complex than
conventional detailed FE models is developed.
The work explores the use of superimposed finite elements to model the varying stiffness in the
respective orthogonal directions of the sidewall and tread of the tyre. Non-linear elements defined
by Neo-Hookean or Ogden models and elements with different linear orthogonal stiffnesses are
superimposed onto each other to simulate the global material properties of the tread and the
sidewall of the tyre investigated.
The geometry of the tyre studied was measured experimentally using laser displacement transducers
and digital image correlation techniques. Material properties of segments of the tyre were obtained
by performing tensile tests on samples. Since the rubber slipped against the clamps during the
experiment, deformation of the segments was also measured using digital image correlation. These
geometrical and material properties were used as input to develop a finite element model of an “offroad”
Measurements were conducted using laser displacement transducers, load cells mounted to
actuators, etc. to obtain accurate sidewall deformation profiles and global radial load vs.
displacement curves for different radial loading conditions. The data obtained from the results was
used to validate the tyre model developed.
Numerous analyses are performed with different combinations of moduli of elasticity in the
respective orthogonal directions of the sidewall stiffness and the tread to investigate its influence on
the global behaviour of the tyre model.
The main focus of the project was to develop a tyre model from data obtained from laser and
photogrammetry measurements in a laboratory that accurately represents tyre behaviour due to
radial forces. A finite element model that can simulate the effect of radial forced and obstacles on a
tyre was developed. The use of two subsets of elements, superimposed onto each other to simulate
global material properties of the rubbers, steel wires, polyester and nylon threads, was investigated.
The combination of material properties that gave the best fit for all the load cases investigated were
determined. The finite element model correlated well with the load vs. displacement graphs and
sidewall displacement profiles determined experimentally.
The solving time is still fairly high and is still not quite suitable for real-time dynamic simulation.
However, it solves faster than more complex tyre models where details of steel wires, etc. are
included in the model.
For future studies it is recommended that different element types be investigated in the tyre model.
The study proves that equivalent material properties can be used to simulate the composite
properties of the materials in tyres. Most tyres can be divided into a few regions that each has its
own material structure right through the region. These regions can be characterized by simple tests
and the input can be used as a first estimation of the tyre’s material properties for the model.
Accurate validation criteria should be used to validate the tyre model if time does not allow for
excessive testing of the material properties of all the rubber, steel wires, polyester threads, etc.
Geometric displacement data at various loading conditions can be used for validation of the tyre
The model developed can be used to investigate the effect of different stiffnesses and other material
changes in the sidewall or tread of a tyre. Useful insight can be obtained from the finite element
model developed for dynamic simulation where the force vs. global displacement data is important.
Keywords: finite element analysis, finite element modelling, model updating, experimental testing,
non-linear modelling, non-linear materials, tyres, multibody, ride simulation
Supervisors: Professors P.S. Heyns & P.S. Els
C Booysen, 2014. "Fatigue Life Prediction of Steam Turbine Blades during Start-up Operation Using Probabilistic Concepts"
Fatigue in low pressure turbine blades has been recognised to be one of the primary causes of steam turbine blade failures worldwide. As a result various methods for predicting the fatigue life of the blades have been proposed. Application of these methods has traditionally been performed using deterministic models which often require overly conservative assumptions. However, given the range of uncertainty in key variables such as material properties, loading and damping; the question is then raised about the subjectivity in the selection of these parameters. An alternative approach is to incorporate probabilistic modelling which can eliminate overly conservative assumptions and allow for uncertainty in key variables to be accounted for. This necessitates the need for development of a probabilistic model which can be used in fatigue life calculations.
The objective of this work is to develop a methodology which can be used to assess the high cycle fatigue life of low pressure steam turbine blades during resonance conditions encountered during a turbine start-up by making use of probabilistic principles. Material fatigue properties are determined through experimental testing of used blade material X22CrMoV12-1 along with statistical modelling using regression analysis to interpret the stress-life diagram. A finite element model of a free-standing LP blade is developed using the principle of sub-structuring which enables the vibration characteristics and transient stress response of the blade to be determined for variations in blade damping. Rainflow cycle counting and a linear damage accumulation rule enabled fatigue damage accumulation during start-up condition to be quantified. Random curve fitting routines are performed on the fatigue and FEM stress data to ensure that the selection of the random variables used in fatigue life calculations is stochastic in nature. The random vectors are selected from a multivariate normal distribution.
Forced response analysis revealed that increased levels of blade damping significantly decreased the peak dynamic stresses which had a significant effect on the overall fatigue life. The use of confidence intervals in the probabilistic fatigue life model worked effectively in being able to account for uncertainty in the material fatigue strength parameters and varying stress in the blade root. Probabilistic modelling was shown to be successful in being able to predict the fatigue life of the blade as results are shown to reinforce those of the discrete life model.
Supervisor: Prof PS Heyns
E.Jansen, 2014. "Thermodynamic optimisation of an open-air solar thermal Brayton cycle"
This project mainly focused on the implementation of the second law of thermodynamics
relating to the design of heat-exchanging components in an open-air solar thermal Brayton cycle.
These components include one or more regenerators (in the form of cross-flow heat exchangers)
and the receiver of the parabolic dish where the system heat was absorbed. The generation of
entropy is under close investigation since the generation of entropy goes hand in hand with the
destruction of exergy, or available work. This phenomenon is caused by two factors, namely the
transfer of heat across a finite temperature difference and also the friction that is caused by the flow
of a working fluid in a system consisting of components and ducts. The dimensions of some
components were used to optimise the cycles under investigation. Entropy Generation Minimisation
(EGM) was employed to optimise the system parameters by considering their influence on the total
generation of entropy. Various assumptions and constraints were considered and discussed to aid in
the solution process, making it simpler in some cases and more feasible in others. The total entropy
generation rate and irreversibilities were determined by considering all of the individual components
and ducts of the system, and their respective inlet and outlet conditions such as temperature and
pressure. The major system parameters were evaluated as functions of the mass flow rate to allow
for proper discussion of the system performance. Ultimately, conclusions and recommendations
were made, which state the optimum system to be used in this type of solar application, where the
amount of net power output is the main driving factor.
Supervisors: Dr T Bello-Ochende and Prof JP Meyer
MC Beckley, 2014. "Comparison of Sampling Methods for Kriging Models"
This study aims to generate from a three-dimensional data set of carbon dioxide flux in the
Southern Ocean, a sample set for use with Kriging in order to generate estimated carbon
dioxide flux values across the complete three-dimensional data set. In order to determine
which sampling strategies are to be used with the three-dimensional data set, a number of
a-priori and a-posteriori sampling methods are tested on a two-dimensional subset. These
various sampling methods are used to determine whether or not the estimated error variance
generated by Kriging is a good substitute for the true error as a measure of error.
Carbon dioxide is a well known ”greenhouse gas” and is partially responsible for climate
change. However, some anthropogenic carbon dioxide is absorbed by the oceans and as such,
the oceans currently play a mitigating role in climate change by acting as a sink for carbon
dioxide. It has been suggested that if the production of carbon dioxide continues unabated
that the oceans may become a source rather than a sink for carbon dioxide. This would mean
that the oceanic carbon dioxide flux (exchange of carbon dioxide between the atmosphere
and the surface of the ocean) would invert. As such, modelling of the carbon dioxide flux is
of clear importance. Additionally as the Southern Ocean is highly undersampled, a sampling
strategy for this ocean which would allow for high levels of accuracy with small sample sizes
would be ideal.
Kriging is a geostatistical weighted interpolation technique. The weights are based on the covariance
structure of the data and the distances between points. In addition to an estimate at
a point, Kriging also produces an estimated error variance which can be used as an indication
of uncertainty. This study made use of model data for carbon dioxide flux in the Southern
Ocean. This data covers a year by making use of averaged data for 5 day intervals. This
results in a three-dimensional data set covering latitude, longitude and time. This study used
this data to generate a covariance structure for the data after the removal of trend and using
this covariance structure, tested various sampling strategies in two dimensions, sampling
approximately 10% of the two-dimensional data subset. These sampling strategies made use
of either the estimated error variance or the true error and included two simple heuristics,
genetic algorithms, the Updated Kriging Variance Algorithm and Random Sampling. Two
of the genetic algorithms tested were selected to maximise the error measure of interest, in
order to determine the full range of errors that could be generated. The percentage absolute
errors obtained across these methods ranged from 2.1% to 64.4%.
Based on these strategies, the estimated error variance was determined to not be an accurate
surrogate for true error and that in cases where absolute error is available, such as data
minimisation, absolute error should be used as the measure of error. However, if no data is
available then it does provide an easy to calculate measure of error. This study also concluded
that Addition of a Point at Point of Maximum Absolute Error does provide a good validation
sampling method to which other methods may be compared.
Additionally, based on true errors and computational requirements, three methods were selected
to be implemented on a three-dimensional subset of the data. These methods were
Random Sampling, Addition of a Point at Point of Maximum Absolute Error and Addition of
a Point at Point of Maximum Estimated Error Variance. Each of these methods for sampling
were performed twice on the data, sampling up to approximately 5% of the data. Random
Sampling produced percentage absolute errors of 21.02% and 20.98%, Addition of a Point at
Point of Maximum Estimated Error Variance produced errors of 18.54% and 18.55% while
Addition of a Point at Point of Maximum Absolute Error was able to produce percentage
absolute errors of 14.33% and 14.32%.
Supervisor: Prof. S Kok
J. Crous, 2014. " The influence of a coupled formulation on the fluid dynamics in a large scale journal bearing"
In the pursuit of more accurate diagnostics of turbo machinery sophisticated rotor and bearing models are to be developed in order to better understand the dynamics of the rotor-bearing system. This study is concerned with such bearing models.
Four distinct fluid models are developed: The first two have a viscous fluid formulation, where fluid dependencies enter the momentum equations primarily through the viscosity of the fluid. The last two have a viscoelastic fluid formulation where dependencies enter the equations through an additional differential constitutive relation. This constitutive relation is strongly coupled with the momentum equation.
The dependencies included in the formulation of the fluid are: pressure, shear rate and temperature. The coupling of the fluid models is subject to the dependencies present in the formulation. Uncoupled, weakly coupled and strongly coupled formulations are compared in this work.
The formulated models are solved numerically using the Finite Volume Method in the open source program OpenFOAM. These models were newly implemented in OpenFOAM as part of this study. The models are validated by comparing results with various known analytical solutions.
A region of the bearing is subsequently analysed, where the dependencies of the lubricant are most prominent. In this region the influence of a weak and strong coupled formulation of the fluid dynamics in the oil film was considered.
In this study it is shown that both weak and strong couplings influenced the fluid behaviour significantly. It is shown that when these dependencies are no longer isolated in the mere adjustment of fluid properties is inadequate to account for the influence of dependencies.
The weak coupled formulation shows the difference between the coupled and uncoupled formulations. The weak coupling influence the fluid dynamics to the same extent as the pressure dependency in the region considered. The departure from the classical formulation is however observed to be uniform in the case of a weak coupling.
The difference between the uncoupled and strongly coupled formulation was not as great as in the weakly coupled case. Although the difference was less, it was seen that the presence of the strong coupling was about 40% of that of the temperature dependency in the region considered. The change in flow, for the strong coupled formulation, was non-homogenous compared to the classical formulation.
The influence of the coupling is therefore different in nature. The weak coupling changes the flow more than the strong coupling compared to the classical formulation. The strong coupling introduce a new characteristic to the fluid behaviour not seen with the weak coupled formulation.
Lastly it is shown that in order to model the bearing adequately, the fluid model and the coupling of the governing equations are not trivial. Great care must be taken in both the fluid model used as well as the formulation of the coupled equations.
Supervisors: Prof PS Heyns, Dr J. Dirker
Natasha Botha, 2014. "Effect of numerical modelling assumptions on the simulated corneal response during Goldmann applanation tonometry"
It is widely known that Central Corneal Thickness (CCT) and Radius of Curvature (RoC) influence the estimated IntraOcular Pressure (IOP) obtained from Goldmann Applanation Tonometry (GAT). However, not much is known about the influence of corneal material properties, especially in a clinical setting.
Several numerical studies have been conducted in an attempt to quantify the influence of corneal material properties on the IOP. These studies agree that corneal material properties do influence the estimated IOP, which contradict the initial premise on which GAT was designed, namely that material properties do not influence the obtained GAT readings. Also, there is no consensus among these studies with respect to corneal material properties, thus a wide range of proposed properties exist.
A possible explanation for this range of available corneal properties is the numerical modelling assumptions used, which seem to be quite different. Different sets of experimental inflation test data were used to calibrate the constitutive models and different limbal boundary conditions were applied to simulate the experimental setup as well as in vivo conditions during GAT simulations. Therefore the purpose of this study is to determine whether these modelling assumptions influence the obtained IOP and ultimately the overall conclusions.
A Finite Element (FE) model of the human cornea is developed, implementing a constitutive model to represent the complex corneal structure and two limbal boundary conditions. This model is then calibrated using two different sets of experimental inflation test data. During calibration of the fibre reinforced elastic constitutive model it is found that independent of the assumptions made regarding the material coefficients, that the numerical inflation data compare well with the experimental data for all cases.
Using this model a GAT simulation is conducted to estimate the IOP and the influence of the modelling assumptions, cornea geometry and material properties are then investigated. The results indicate that the modelling assumptions, cornea geometry and material properties do influence the estimated IOP. However, when assuming the cornea ground substance stiffness to be constant, it is found that the influence on IOP due to material properties is not as significant. A correction equation is also proposed to account for the corneal geometric properties by calibrating the numerical model for a numerically normal cornea. This is done by utilising the various data sets which are obtained during the calibration of the constitutive model with the experimental inflation test data.
It is concluded that using only inflation data to calibrate the constitutive model is not sufficient to uniquely describe the corneal material. This is evident as different material data sets are obtained, even though the experimental inflation data is matched well for a variety of considered cases. Each of these material data sets, in conjunction with geometric properties, yield different estimates for IOP during GAT simulations.
This study therefore recommends the use of additional experimental data, such as strip extensometry, along with inflation test data to adequately calibrate a numerical model. It should also be noted that when modelling GAT care should be taken when considering the choice of limbal boundary condition, experimental data for calibration and assumptions made with regards to material coefficients, as these choices could potentially influence the outcomes and conclusions of a study.
Supervised by Prof. Schalk Kok and Ms. Helen M. Inglis
FH Burger, 2014. "Three-dimensional conductive heat-spreading layouts obtained using topology optimisation for passive internal electronic cooling"
In this study, topology optimisation for heat-conducting paths in a three-dimensional domain was investigated. The governing equations for the temperature distribution were solved using the finite volume method, the sensitivities of the objective function (average temperature) were solved using the adjoint method, and finally, the optimal architecture was found with the method of moving asymptotes (MMA) using a self‑programmed code. A two-dimensional domain was evaluated first as a validation for the code and to compare with other papers before considering a three-dimensional cubic domain.
For a partial Dirichlet boundary, it was found that the converged architecture in three dimensions closely resembled the converged architectures from two dimensions, with the main branches extending to the outer corners of the domain. However, the partial Dirichlet boundary condition was not realistic, and to represent a more realistic case, a full Dirichlet boundary was also considered.
Supervisors: Dr J. Dirker and Prof J.P. Meyer
R.Scheepers, 2014."A COMPARATIVE STUDY OF FINITE ELEMENT METHODOLOGIES FOR TORSIONAL VIBRATION RESPONSE CALCULATIONS OF BLADED ROTORS"
Turbo-generator trains are susceptible to torsional vibration which can lead to fatigue cracking
and failure. Methods are available for the measurement and calculation of the torsional natural
frequencies of these systems for the purpose of design, monitoring and life prediction.
Calculation methods are conventionally based on one dimensional (1D) finite element (FE)
methodologies which require the simplification of a number of aspects including the participation
of flexible blades in torsional vibration modes.
The accuracy of 1D, three dimensional (3D) and three dimensional cyclic symmetric (3DCS) FE
methods was investigated by the application thereof on a small test rotor. Experimental
measurements of static and dynamic vibration responses were conducted with rotation and
torsional forcing accomplished through the use of a DC motor and a digital control system
optimised for fast transient and stable steady state response. Blade stagger angle was
demonstrated to have a significant effect on torsional frequencies although no stress stiffening
effects were noted in the speed range considered. Similarly, damping was measured to decrease
with blade stagger angle but not with rotational speed. Step changes in torsional frequencies due
to the activation of the motor field and armature currents required optimisation of the motor
models for static and dynamic conditions. Shaft torsional vibration responses were found not to
include all blade modes and vice versa.
Supervisor: Professor P.S. Heyns
EF Williams, 2014. "Design and Analysis of a Practical Large-Force Piezoelectric Inchworm Motor with a Novel Force Duplicator"
The work presented in this dissertation on piezoelectric inchworm motors (IWM) is part of a process to gain an understanding of the design, analysis and testing of this smart actuator technology. This work will form the foundation of what will hopefully lead to the realisation of a production-ready IWM design to be used in energy-scarce, battery-operated Unmanned Aerial Vehicles (UAVs), and forms part of a larger national drive to expand the UAV industry in South Africa. Although the principles used in the design of IWMs are well known, a new innovation is employed. A novel way to increase the force capacity of IWMs without compromising on the speed or displacement when compared to conventional methods is shown to be effective, and was used for the first time on IWMs. The use of a simple design equation is demonstrated to be useful in predicting the load limits and step displacements. Challenges of finding a correlation between predicted and measured performance values are discussed and solutions are presented. The history of IWMs and some background on piezoelectricity are given for the reader not familiar with these. The use of micro ridges on the clamp mechanisms is explored. The effects of the control signals on the mechanism of the motor are discussed in detail and some important comments on electrical controllers are made. The emphasis is on designing a strong motor that capitalises on the high-force density of piezoelectric material.
Supervisor: Prof NJ Theron
Co-supervisor: Dr Philip Loveday
Darshik Garach,2014. "Heat Transfer and Pressure Drop in Microchannels with Different Inlet Geometries for Laminar and Transitional Flow of Water"
An experimental investigation of heat transfer and pressure drop in rectangular microchannels was conducted for water in the laminar and transitional regimes for three different inlet configurations. The inlet types under consideration were the sudden contraction, bellmouth, and swirl inlet types, and hydraulic diameters of 0.57 mm, 0.85 mm, and 1.05 mm were covered. The sudden contraction inlet type was investigated for both adiabatic, as well as diabatic cases. For this inlet type, adiabatic friction factors were predicted well by the laminar Shah and London correlation, while for diabatic cases this correlation resulted in over predictions of up to 15%. The critical Reynolds numbers were found to be between 1 800 and 2 000 for adiabatic cases, while for diabatic cases the transition regime commenced at a Reynolds number of about 2 000. The bellmouth inlet type was investigated for diabatic cases only. Here, laminar friction factors were also over-predicted by up to 15% by the Shah and London correlation and an early onset of transition was observed at a Reynolds number of 1 600. Correcting for the wall to bulk fluid temperature viscosity ratio improved on friction factor prediction accuracy. The swirl inlet type, which was only investigated for a hydraulic diameter of 1.05 mm and with a diabatic wall, exhibited the highest friction factors. Unlike the trends of conventional theory, Nusselt numbers in the laminar flow regime exhibited an increase with the Reynolds number. The axial conduction effect was found to affect results as the value for M was greater than 0.01as described by the literature. During transition, the bellmouth inlet type exhibited an increase in the friction factor and Nusselt number of up to 30% and 70% respectively, compared to the sudden contraction inlet type. For the swirl inlet type, increases of up to 72% and 120% for friction factor and Nusselt numbers respectively were achieved compared to the sudden contraction inlet type. After the onset of transition, Nusselt numbers approached the Gnielinski correlation predictions for turbulent flow. Based on the experimental data obtained in this study, a correlation was developed which describes the relation between the friction factor and Colburn j-factor. The correlation has a mean absolute error of 6% and predicts 92% of the results for all inlet types to within 15%.
Supervisors: Dr. J. Dirker, Prof. J.P. Meyer
Christian M. Kangaj,2014. "Pull-out of hooked end steel fibres from epoxy matrix: Experimental and Numerical study"
The reinforcement of concrete with steel fibres changes the failure of the composite material from catastrophic brittle failure to pseudo-ductile behaviour as a result of crack-bridging by the fibres, and the additional work which is absorbed by fibre pull-out. A good understanding of the properties of fibre-reinforced concrete depends on an understanding of fibre pull-out process. The main aim of the current study is to investigate, both experimentally and numerically, the pull-out behaviour of a single hooked end steel fibre from epoxy matrix, where epoxy was chosen to replace concrete in order to enable visualisation of the pull-out process. The experimental and numerical results both contribute to the development of a physical understanding of the mechanism of pull-out.
Experimental studies included the evaluation of the mechanical properties of hooked end steel fibre and epoxy matrix by means of tensile tests, the manufacturing of pull-out specimens consisting of a single hooked end steel fibre embedded in epoxy matrix, and the experimental characterisation of the fibre pull-out. The significant features (peaks and minima) of the load vs. displacement graph were correlated to stills taken from a video of the pull-out process, in which the plastic deformation of the fibre is evident. Small deformations (spalling) were also observed in the matrix. A model is proposed for the mechanisms which interact during the pull-out process.
Numerical analysis has been performed using a 3D finite element analysis implemented in ANSYS, incorporating nonlinear contact between the fibre and the matrix and plastic deformation of the fibre. Sensitivity studies investigated the effects of material and interface properties on the resultant load vs. displacement curve of the pull-out. Results of these studies demonstrated the importance of accurately modelling the plastic behaviour of the steel fibre in predicting the pull-out response of the system.
The numerical and experimental pull-out behaviours capture the same fundamental behaviour. The fibre displacement at critical intervals is well-correlated, and the numerically-predicted load is within the range of the experimental results at these intervals. The finite element analysis allows investigation of the stress and strain distributions. The plastic strain accumulated in different regions of the fibre, and the maximum stress experienced in the matrix are studied, and correlated with the global response, and further understanding is gained into the mechanisms of fibre pull-out.
Supervisor: Mrs Helen M. Inglis
Co-supervisor: Prof Schalk Kok
Mj Stallmann, 2014. " Tyre model verification over off-road terrain"
Vehicle dynamic simulations form a significant part of the design and development process of vehicles. These simulations are used to study and improve the vehicle’s durability, ride comfort and handling capabilities. All forces acting on the vehicle are either generated in the tyre-road interface or are due to aerodynamic effects, where at low speeds the latter one can be ignored. The accuracy of the tyre model describing the forces on the tyre-road interface is thus of exceptional importance. It ensures that the simulation model is an accurate representation of the actual vehicle.
Various approaches are adopted when developing mathematical tyre models. Many of these models are developed to study the handling capabilities of passenger cars over a smooth road. Passenger car tyres are the focal point as larger tyres introduce some difficulties due to their size and load rating. Off-road truck tyres also differ in their construction which will influence force and moment generation of the tyre. Research efforts are increasing to meet the need of tyre models that can describe the behaviour of the tyre over uneven terrain with sufficient accuracy. This thesis addresses the question of whether existing mathematical tyre models can accurately describe the forces and moments generated by a large off-road tyre while driving over rough terrain.
The complexity of different mathematical tyre models varies greatly, as does the parameterisation efforts required to obtain the model parameters. The parameterization of most tyre models relies on some experimental test data that is used to extract the necessary information to fit model parameters. The selection of a suitable tyre model for a simulation is often dependent on the availability of such experimental data and the effort to identify the required parameters. In this study the parameterisation process for four different tyre models, are discussed in detail to highlight the difficulties in acquiring the test data and the effort to parameterize the model. The models considered are the One Point Contact, 3D Equivalent Volume contact, 3D Enveloping Contact and FTire model.
Experimental measurements are conducted on a 16.00R20 Michelin XZL tyre. Laboratory tests, as well as field tests, over discrete obstacles and uneven hard surfaces are used for parameterisation and validation purposes. Simulation results are compared to experimental test data to determine whether the models could be used to describe the tyre road interactions with sufficient accuracy. Recommendations are made for tyre model selection and model accuracy for simulations over rough off-road surfaces.
Supervisor: Prof. P.S. Els
A Strydom, 2013. "Controllable suspension design using Magnetorheological Fluid"
The purpose of this study is to mitigate the compromise between ride comfort and handling of a small single seat off-road vehicle known as a Baja. This has been achieved by semi-active control of the suspension system containing controllable magnetorheological (MR) dampers and passive hydro-pneumatic spring-damper units.
MR fluid is a viscous fluid whose rheological properties depend on the strength of the magnetic field surrounding the fluid, and typically consists of iron particles suspended in silicone oil. When a magnetic field is applied to the fluid, the iron particles become aligned and change the effective viscosity of the fluid. The use of MR fluid in dampers provides variable damping that can be changed quickly by controlling the intensity of the magnetic field around the fluid. Various benefits associated with the use of MR dampers have led to their widespread implementation in automotive engineering.
Many studies on conventional vehicles in the existing literature have demonstrated the conflicting suspension requirements for favourable ride comfort and handling. Generally, soft springs with low damping are ideal for improved ride comfort, while stiff springs with high damping are required for enhanced handling. This has resulted in the development of passive suspension systems that provide either an enhanced ride quality or good drivability, often targeting one at the expense of the other.
The test vehicle used for this study is distinct in many ways with multiple characteristics that are not commonly observed in the existing literature. For instance, the absence of a differential in the test vehicle driveline causes drivability issues that are aggravated by increased damping.
The majority of existing MR damper models in the literature are developed for uniform excitation and re-characterisation of model parameters is required for changes in input conditions. Although recursive models are more accurate and applicable to a wider range of input conditions, these models require measured force feedback which may not always be available due to limitations such as packaging constraints. These constraints required the development of alternative MR damper models that can be used to prescribe the current input to the damper.
In this study parametric, nonparametric and recursive MR damper models have been developed and evaluated in terms of accuracy, invertibility and applicability to random excitation. The MR damper is used in parallel with passive damping as a certain amount of passive damping is always present in suspension systems due to friction and elastomeric parts.
Most of the existing studies on suspension systems have been performed using linear two degree of freedom vehicle models that are constrained to specific conditions. Usually these models are implemented without an indication of the ability of these models to accurately represent the vehicles that these studies are intended for.
For this study, a nonlinear, three-dimensional, 12 degrees of freedom vehicle model has been developed to represent the test vehicle. This model is validated against experimental results for ride comfort and handling. The MR damper models are combined with the model of the test vehicle, and used in ride comfort and handling simulations at various levels of passive damping and control gains in order to assess the potential impact of suspension control on the ride quality and drivability of the test vehicle.
Simulation results show that lower passive damping levels can significantly improve the ride comfort as well as the handling characteristics of the test vehicle. Furthermore, it is observed that additional improvements that may be obtained by the implementation of continuous damping control may not be justifiable due to the associated cost and complexity.
Supervisor: Prof. P.S. Els
Co-Supervisor: Dr. S. Kaul
H A Hamersma, 2013."Longitudinal vehicle dynamics control for improved vehicle safety"
An autonomous vehicle is a vehicle that is capable of navigating and driving with no human intervention whatsoever through the utilization of various sensors and positioning systems. The possible applications of autonomous vehicles are widespread, ranging from the aerospace industry to the mining and military sectors where the exposure of human operators to the operating conditions is hazardous to their health and safety. Automobile accidents have become the leading cause of death in certain segments of the world population. Removing the human driver from the decision-making process through automation may result in significantly safer highways. Although full autonomy may be the ultimate goal, there is huge scope for systems that aid the driver in decision making or systems that take over from the driver under conditions where the human driver fails.
The aim of the longitudinal control system to be implemented on the Land Rover test vehicle in this study is to improve the vehicle’s safety by controlling the vehicle’s longitudinal behaviour. A common problem with sports-utility-vehicles is the low rollover threshold, due to a high centre of gravity. Rather than modifying the vehicle to increase the rollover threshold, the aim of the control system presented here is to prevent the vehicle from exceeding speeds that would cause the vehicle to reach its rollover threshold.
In order to develop a control system that autonomously controls the longitudinal degree of freedom, a model of the test vehicle (a 1997 Land Rover Defender 110 Wagon) was developed in MSC.ADAMS/View and validated experimentally. The model accurately captures the response of the test vehicle to supply forces as generated by the engine and demand forces applied through drag, braking and engine braking. Furthermore, the model has been validated experimentally to provide reliable simulation results for lateral and vertical dynamics.
The control system was developed by generating a reference speed that the vehicle must track. This reference speed was formulated by taking into account the vehicle’s limits due to lateral acceleration, combined lateral and longitudinal acceleration and the vehicle’s performance capabilities. The control system generates the desired throttle pedal position, hydraulic pressure in the brake lines, clutch position and gear selection as output. The MSC.ADAMS\View model of the test vehicle was used to evaluate the performance of the control system on various racetracks of which the GPS coordinates were available. The simulation results indicate that the control system performs as expected.
Finally, the control system was implemented on the test vehicle and the performance was evaluated by conducting field tests in the form of a severe double lane change manoeuvre. The results of the field tests indicated that the control system limited the acceleration vector of the vehicle’s centre of gravity to prescribed limits, as predicted by the simulation results.
Supervisor: Prof P.S. Els
MD Stocks, 2013. "Geometric optimisation of heat transfer in complex geometries using Newtonian and non-Newtonian fluids"
The continual advance in manufacturing processes has resulted in significantly more compact, high performance, devices. Consequently, heat extraction has become the limiting factor, and of primary concern. Therefore, a substantial amount of research has been done regarding high efficiency micro heat exchangers, employing novel working fluids.
This dissertation numerically investigated the thermal behaviour of microchannel elements cooled by Newtonian and non-Newtonian fluids, with the objective of maximising thermal conductance subject to constraints. This was done, firstly, for a two-dimensional simple microchannel, and secondly, for a three-dimensional complex microchannel. A numerical model was used to solve the governing equations relating to the flow and temperature fields for both cases. The geometric configuration of each cooling channel was optimised for Newtonian and non-Newtonian fluids, at a fixed inlet velocity and heat transfer rate. In addition, the effect of porosity on thermal conductance was investigated.
Geometric optimisation was employed to the simple and complex microchannels, whereby an optimal geometric ratio (height versus length) was found to maximise thermal conductance. Moreover, analysis indicated that the bifurcation point of the complex microchannel could be manipulated to achieve a higher thermal conductance.
In both cases, it was found that the non-Newtonian fluid characteristics resulted in a significant variation in thermal conductance as inlet velocity was increased. The characteristics of a dilatant fluid greatly reduced thermal conductance on account of shear-thickening on the boundary surface. In contrast, a pseudoplastic fluid showed increased thermal conductance.
A comparison of the simple and complex microchannel showed an improved thermal conductance resulting from greater flow access to the conductive area, achieved by the complex microchannel.
Therefore, it could be concluded that a complex microchannel, in combination with a pseudoplastic working fluid, substantially increased the thermal conductance and efficiency, as opposed to a conventional methodology.
Keywords: Non-Newtonian fluid; Thermal conductance; Geometric optimisation; Microchannel; Complex geometry
Supervisors: Prof T Bello-Ochende and Prof JP Meyer
PENI Junior YEKOLADIO, 2013."Thermodynamic Optimization of Sustainable Energy System: Application to the optimal design of heat exchangers for geothermal power systems"
The present work addresses the thermodynamic optimization of small binary-cycle geothermal power plants. The optimization process and entropy generation minimization analysis were performed to minimize the overall exergy loss of the power plant, and the irreversibilities associated with heat transfer and fluid friction caused by the system components. The effect of the geothermal resource temperature to impact on the cycle power output was studied, and it was found that the maximum cycle power output increases exponentially with the geothermal resource temperature. In addition, an optimal turbine inlet temperature was determined, and observed to increase almost linearly with the increase in the geothermal heat source. Furthermore, a coaxial geothermal heat exchanger was modeled and sized for minimum pumping power and maximum extracted heat energy. The geofluid circulation flow rate was also optimized, subject to a nearly linear increase in geothermal gradient. In both limits of the fully turbulent and laminar fully-developed flows, a nearly identical diameter ratio of the coaxial pipes was determined irrespective of the flow regime, whereas the optimal geofluid mass flow rate increased exponentially with the Reynolds number. Several organic Rankine Cycles were also considered as part of the study. The basic types of the ORCs were observed to yield maximum cycle power output. The addition of an IHE and/or an OFOH improved significantly the effectiveness of the conversion of the available geothermal energy into useful work, and increased the thermal efficiency of the geothermal power plant. Therefore, the regenerative ORCs were preferred for high-grade geothermal heat. In addition, a performance analysis of several organic fluids was conducted under saturation temperature and subcritical pressure operating conditions of the turbine. Organic fluids with higher boiling point temperature, such as n-pentane, were recommended for the basic type of ORCs, whereas those with lower vapour specific heat capacity, such as butane, were more suitable for the regenerative ORCs.
Keywords: Geothermal energy, Organic Rankine Cycles, Optimization, Exergy analysis, Entropy Generation Minimization analysis, binary cycle, Enhanced Geothermal System.
Supervisor: Prof T.Bello-Ochende
Co-supervisor: Prof J P Meyer
Warren Van Zyl,2013. "SINGLE-PHASE CONVECTIVE HEAT TRANSFER AND PRESSURE DROP COEFFICIENTS IN CONCENTRIC ANNULI"
Varying diameter ratios associated with smooth concentric tube-in-tube heat exchangers are known to have an effect on their convective heat transfer capabilities. Linear and non-linear regression models exist for determining the heat transfer coefficients, however, these are complex and time consuming, and require much experimental data in order to obtain accurate solutions. A large dataset of experimental measurements on heat exchangers with annular diameter ratios of 0.483, 0.579, 0.593 and 0.712 with respective hydraulic diameters of 17.01 mm, 13.84 mm, 10.88 mm and 7.71 mm was gathered. Mean Nusselt numbers were determined using the modified Wilson plot method, a non-linear regression scheme and the logarithmic mean temperature difference method. These three methods presented disagreements with existing correlations based on local wall temperatures. The local Nusselt numbers were determined using the logarithmic mean temperature difference method. Local wall temperature measurements were made using a novel method which minimized obstructions within the annulus. Friction factors were calculated directly from measured pressure drops across the annuli. Both heated and cooled horizontal annuli in fully turbulent flow with Reynolds numbers based on the hydraulic diameter varying from 10 000 to 45 000 with water as the working medium were investigated.
Supervisor: Dr J Dirker
Co-supervisor: Prof J P Meyer
Timothy Prinsloo, 2013. "Damage Detection Methodology for Composite UAV Wings using Modal Analysis and Probabilistic Concepts"
Monitoring of structural integrity is critical in manyfields today, and particularly so in the civil, mechanical and aerospaceengineering industries. In the aerospace industry, appreciably sized and almostexclusively composite UAVs share the airspace with other aircraft. Such composite structuresalso pose numerous uncertainties to structural health monitoring and analysistechniques. This necessitates research into a methodology for practicaland effective structural health monitoring techniques.
Thiswork presents a methodology for structural health monitoring and particularlydelamination detection in composite wing structures. The approach usesexperimental modal analysis with due consideration for the probabilisticeffects of random variations in material and geometrical properties, for thepurpose of a general and non wing-specific damage detection technique.
Alarge number of composite material coupons were tested to determine statisticaldistributions of 2D orthotropic material properties, using an optical imagecorrelation system to reduce the expense of testing. Uncertainties in the winggeometry arising from manufacturing variances were taken into consideration. Thematerial properties of the foam spar and resin beadings were considered isotropicand deterministic. A finite element model of the wing was subsequently improvedusing a scanning laser vibrometer to conduct detailed experimental modalanalyses of five wings, and a multi-model updating approach based on frequencyand mode shape information was used to update selected sensitive materialproperties. Significant improvement was accomplished.
Usingthe probabilistic material property database, a confidence region wasestablished for wing mode shapes through a Monte Carlo procedure. It was shownthat delamination effects are capable of perturbing the dynamic mode shapesbeyond the confidence regions implied by the material uncertainties. Thisprovides a basis for further development of a structural health monitoringmethodology for composite structures, taking due account of the manyuncertainties in the structure.
Supervisor: Professor P.S. Heyns
Patrick M.K. Sambayi, 2013. "DRILL WEAR MONITORING USING INSTANTANEOUS ANGULAR SPEED"
Various researchers have investigated the use of conventional measurements (vibration, acoustic emission, force, and torque measurements) to assess drill tool condition on drilling machines. Although it has been successfully established that such measurements could in principle be used, it however not yet found its way into industry as standard practice, and drill bits are still often replaced non-optimally. The major problems are related to the lack of system-made sensors for drilling, the poor signal-to-noise ratio resulting in sophisticated signal processing and the inconvenience of using drill based strain or conventional sensors on production machines. To overcome the above problems, some researchers recommend advanced and sophisticated signal processing, while others advise that effort should be put into the intelligent and/or innovative use of available sensor technology by developing some form of instrumented tool for drill operations.
This dissertation considers the use of these conventional technologies monitoring parameters against the use of the instantaneous angular speed (IAS) of the spindle for monitoring the drill condition. It is shown that an angular speed approach can generate diagnostic information similar to the conventional measurements, without some of the instrumented sensor complications. No advanced signal processing techniques are required as the delay associated with the processing is often not beneficial in automated drilling monitoring. Satisfactory results were obtained using common time domain signal processing methods. The curve trends have illustrated that the drill bits wear rapidly at the end of its life except for small drills which in general fail by fracture. A diagnosis of the drill wear based on a simple regression analysis was done and the results are promising.
Keywords: Wear monitoring; Drilling; Vibration; Torque; Angular speed; Regression analysis
Supervisor: Professor P.S. Heyns
Vishaal Bhana, 2013."Online damage detection on shafts using torsional and undersampling measurement techniques"
The presence of cracks in rotors is one of the most dangerous defects of rotating machinery. This can lead to catastrophic failure of the shaft and long out-of-service periods. The occurrence of a crack in a rotating shaft introduces changes in flexibilities which alters the dynamics during operation. This research deals with detecting damage in rotors by means of constantly monitoring the variation in the rotor’s dynamics during normal operating conditions. This project entails a computer finite element section as well as an experimental investigation.
The flexibility in the region of the crack is different from an uncracked section. A finite element model of a shaft is built and investigated. The damaged model is the same except that the nodes in the location of the crack are not equivalenced in order to represent the crack. A simple constant cross-sectional shaft with semi-circular transverse surface cracks varying in size have been modelled on the Patran finite element software and a normal modes analysis was done using the Nastran solver. The results revealed a change in the natural frequencies due to the variation in the size of the crack.
The experimental investigation involved creating sample shafts with damage positioned in them that would closely resemble what one may find in actual real-life situations and the dynamics during rotation with various torsional loadings are investigated and monitored using three methods. A fibre-optical sensor, Digital image correlation system and telemetry strain gauges were used. Undersampling techniques were used for the DIC system. Results showed that the fibre-optic sensor is by far the most favourable as it is able to detect damage under constant operation. The finite element model was updated by re-modelling the geometry, damage and material properties. The solution of the analysis matched the experimental results closely and model verification was achieved.
Supervisor: Professor P.S Heyns
Sarel Francois van der Westhuizen, 2013."Slow active suspension control for rollover prevention"
Rollover prevention in Sports Utility Vehicles (SUV’s) offers a great challenge in vehicle safety. By reducing the body roll angle of the vehicle the load transfer will increase and thus decrease the lateral force that can be generated by the tires. This decrease in the lateral force can cause the vehicle to slide rather than to roll over. This study presents the possibility of using slow active suspension control to reduce the body roll and thus reduce the rollover propensity of a vehicle fitted with a hydro-pneumatic suspension system. The slow active control is obtained by pumping oil into and draining oil out of each hydro-pneumatic suspension unit individually.
A real gas model for the suspension units as well as for the accumulator that supplies the oil is incorporated in a validated full vehicle Adams model. This model is then used to simulate a double lane change manoeuvre performed by a SUV at 60 km/h and it is shown that a significant improvement in body roll can be obtained with relatively low energy requirements.
The proposed control is successfully implemented on a Land Rover Defender test vehicle. A Proportional-Derivative (PD) controller is used to control on-off solenoid operated valves and the flow is adjusted using the lateral acceleration as a parameter. Experimental results confirm that a significant improvement in body roll is possible.
Supervisor: Prof. Pieter Schalk Els
Mr Kersten Grote, 2013."The inﬂuence of multi-walled carbon nanotubes on single-phase heat transfer and pressure drop characteristics in the transitional ﬂow regime of smooth tubes."
There are in general two diﬀerent types of studies concerning nanoﬂuids. The ﬁrst one concerns itself with the study of the eﬀective thermal conductivity and the other with the study of convective heat transfer enhancement. The study on convective heat transfer enhancement generally incorporates the study on the thermal conductivity. Not many papers have been written on the convective heat transfer enhancement and even fewer concerning the study on multi-walled carbon nanotubes in the transitional ﬂow regime. In this paper the thermal conductivity and viscosity was determined experimentally in order to study the convective heat transfer enhancement of the nanoﬂuids. Multi-walled carbon nanotubes suspended in distilled water ﬂowing through a straight, horizontal tube was investigated experimentally for a Reynolds number range of a 1 000 - 8 000, which included the transitional ﬂow regime. The tube was made out of copper and has an internal diameter of 5.16 mm. Results on the thermal conductivity and viscosity indicated that they increase with nanoparticle concentration. Convective heat transfer experiments were conducted at a constant heat ﬂux of 13 kW/m2 with 0.33%, 75% and 1.0% volume concentrations of multi-walled carbon nanotubes. The nanotubes had an outside diameter of 10 - 20 nm, an inside diameter of 3 - 5 nm and a length of 10 - 30µm. Temperature and pressure drop measurements were taken from which the heat transfer coeﬃcients and friction factors were determined as a function of Reynolds number. The thermal conductivities and viscosities of the nanoﬂuids were also determined experimentally so that the Reynolds and Nusselt numbers could be determined accurately. It was found that heat transfer was enhanced when comparing the data on a Nusselt number as a function of Reynolds number graph but comparing the results on a heat transfer coeﬃcient as a function of average velocity graph the opposite eﬀect was observed. Performance evaluation of the nanoﬂuids showed that the increase in viscosity was four times the increase in the thermal conductivity which resulted in an ineﬃcient nanoﬂuid. However, a study on the performance evaluation criterion showed that operating nanoﬂuids in the transition and turbulent ﬂow regime due to the energy budget being better than that of the distilled water.
Keywords: Nanoﬂuids, multi-walled carbon nanotubes, transition, convective heat transfer, performance evaluation
Supervisor: Prof J P Meyer
Co-supervisor: Dr TJ McKrell (MIT)
Mr Logan Page 2012 "Geometric optimization for the maximum heat transfer density rate from cylinders rotating in natural convection"
In this study we investigate the thermal behaviour of an assembly of consecutive cylinders in a counter-rotating configuration cooled by natural convection with the objective of maximizing the heat transfer density rate (heat transfer rate per unit volume). A numerical model was used to solve the governing equations that describe the temperature and flow fields and an optimisation algorithm was used to find the optimal structure for flow configurations with two or more degrees of freedom. The geometric structure of the consecutive cylinders was optimized for each flow regime (Rayleigh number) and cylinder rotation speed for one and two degrees of freedom. Smaller cylinders were placed at the entrance to the assembly, in the wedge-shaped flow regions occupied by fluid that had not yet been used for heat transfer, to create additional length scales to the flow configuration.
It was found that the optimized spacing decreases and the heat transfer density rate increases as the Rayleigh number increases, for the optimized structure. It was also found that the optimized spacing decreases and the maximum heat transfer density rate increases, as the cylinder rotation speed was increased for the single scale configuration at each Rayleigh number. Results further showed that there was an increase in the heat transfer density rate of the rotating cylinders over stationary cylinders for a single scale configuration.
For a multi scale configuration it was found that there was almost no effect of cylinder rotation on the maximum heat transfer density rate, when compared to stationary cylinders, at each Rayleigh number; with the exception of high cylinder rotation speeds, which serve to suppress the heat transfer density rate. It was, however, found that the optimized spacing decreases as the cylinder rotation speed was increased at each Rayleigh number. Results further showed that the maximum heat transfer density rate for a multi scale configuration (with stationary cylinders) was higher than a single scale configuration (with rotating cylinders) with an exception at very low Rayleigh numbers.
Supervisors: Prof T Bello-Ochende, Prof J P Meyer
Ms Melissa Hallquist 2012"Heat Transfer And Pressure Drop Characteristics Of Smooth Tubes At A Constant Heat Flux In The Transitional Flow Regime"
Due to constraints and changes in operating conditions, heat exchangers are often forced to operate under conditions of transitional flow. However, the heat transfer and flow behaviour in this regime is relatively unknown. By describing the transitional characteristics it would be possible to design heat exchangers to operate under these conditions and improve the efficiency of the system.
The purpose of this study was to experimentally measure the heat transfer and pressure drop characteristics of smooth tubes at a constant heat flux in the transitional flow regime. The measurements were used to describe the flow behaviour of this regime and attempt to develop a correlation that can be used in the design of a heat exchanger.
An experimental set-up was developed, consisting of an overall set-up, a removable test section as well as a controller, which ensured a uniform heat flux boundary. The test section allowed for the measurement of the temperature along the length of the test section, the pressure drop across the test section, the heat flux input and the flow rate. The measurements were used to determine the heat transfer coefficients and friction factor of the system.
Three test sections were developed with outer diameters of 6, 8 and 10 mm in order to investigate the influence of heat exchanger size. Each test section was subject to four different heat flux cases of approximately 1 500, 3 000, 4 500 and 6 000 W/m2. The experiments covered a Reynolds number range of 450 to 10 300, a Prandtl number range of 4 to 7, a Nusselt number range of 2.3 to 67, and a Grashoff number range of 60 to 23 000.
Good comparison was found between the measurements of this experiment and currently available literature. The experiments showed a smooth transition from laminar to turbulent flow with the onset of transition dependent on the heat flux of the system and with further data capturing, a correlation can be found to describe the Nusselt number in the transitional flow regime.
Supervisor: Prof J P Meyer
Mr Ridhwaan Suliman 2012. " Development of parallel strongly couples hybrid fluid-streucture interaction technology involving thin geometrically non-linear structures"
This work details the development of a computational tool that can accurately model strongly-coupled fluid-structure-interaction (FSI) problems, with a particular focus on thin-walled structures undergoing large, geometrically non-linear deformations, which has a major interest in, amongst others, the aerospace and biomedical industries.
The first part of this work investigates improving the efficiency with which a stable and robust in-house code, Elemental, models thin structures undergoing dynamic fluid-induced bending deformations. Variations of the existing finite volume formulation as well as linear and higher-order finite element Formulations are implemented. The governing equations for the solid domain are formulated in a total Lagrangian or undeformed conguration and large geometrically non-linear deformations are accounted for. The set of equations is solved via a single-step Jacobi iterative scheme which is implemented such as to ensure a matrix-free and robust solution. Second-order accurate temporal discretisation is achieved via dual-timestepping, with both consistent and lumped mass matrices and with a Jacobi pseudo-time iteration method employed for solution purposes. The matrix-free approach makes the scheme particularly well-suited for distributed memory parallel hardware architectures. Three key outcomes, not well documented in literature, are highlighted: the issue of shear locking or sensitivity to element aspect ratio, which is a common problem with the linear Q4 finite element formulation when subjected to bending, is evaluated on the finite volume formulations; a rigorous comparison of finite element vs. finite volume methods on geometrically non-linear structures is done; a higher-order finite volume solid mechanics procedure is developed and
The second part of this work is concerned with fluid-structure interaction (FSI) modelling. It considers the implementation and coupling of a higher-order finite element structural solver with the existing finite volume fluid-flow solver in Elemental. To the author’s knowledge, this is the first instance in which a strongly-coupled hybrid finite element–finite volume FSI formulation is developed. The coupling between the fluid and structural components with non-matching nodes is rigorously assessed. A new partitioned fluid-solid interface coupling methodology is also developed, which ensures stable partitioned solution for strongly-coupled problems without any additional computational overhead. The solver is parallelised for distributed memory parallel hardware architectures. The developed technology is successfully validated through rigorous temporal and mesh independent studies of representative two-dimensional strongly-coupled large-displacement FSI test problems for which analytical or benchmark solutions exist.
Supervisors: Dr S Kok/Dr A G Malan/Prof J P Meyer
Mr I.Mathebula 2011. " Friction Factor Correlations for Perforated Tubes at Low Injection Rates"
Perforated tubes are widely used in industry for various applications. They can be found in commercial buildings or homes with stylish perforation patterns for aesthetic purposes while some perforation patterns are designed for suppressing specific frequencies in exhaust mufflers. Common pipes are sometimes customised into perforated pipes by simply drilling multiple holes along the length of the pipe for a cost effective irrigation system. An interesting application for perforated tubes arises when fluid injection is introduced through the perforations. Such a case arises when perforated tubes are used for horizontal oil well drilling, French drains or other draining applications. The pressure losses experienced under these conditions has led to the development of friction factor correlations, which consider the effects of the perforations and fluid injection. However, most of these correlations do not quantify the influence of fluid acceleration caused by injection in the familiar friction factor form. The present study reports friction factor correlations measured at low injection rate when the additional kinetic energy from the fluid injection process is considered. The friction factor measurements were conducted in copper tubes with an internal diameter of 20 mm and a wall thickness of 1 mm at three non-dimensional pitches of 0.375, 0.75 and 1.5. A perforated length-to-diameter ratio of 40:1 was used and each perforation row contained seven small perforation holes with a diameter of 1.5 mm spaced evenly around the perimeter of the tube. The perforations were staggered row to row, resulting in triangular perforation patterns. Water was used as a test medium and the Reynolds numbers at the tube outlet ranged from 20 000 to 60 000. Injection ratios were varied from 0 to 5% in increments of 1% to obtain a total of 135 unique combinations of perforated tube friction factor data at different injection ratios, Reynolds numbers and non-dimensional perforation pitches. The experiments were condensed into simple friction factor correlations, which can be used to predict the pressure losses expected across perforated tubes when fluid injection is present.
Supervisor: Prof J.P. Meyer
Miss L.Smith 2011."An Interactive Boundary Layer Modelling Methodology for Aerodynamic flows"
Computational fluid dynamics (CFD) simulation is a computational tool for exploring flow applications in science and technology. Of central importance in many flow scenarios is the accurate modelling of the boundary layer phenomenon. This is particularly true in the aerospace industry, where it is central to the prediction of drag.
Modern CFD codes as applied to modelling aerodynamic flows have to be fast and efficient in order to model complex realistic geometries. When considering viscous flows, the boundary layer typically requires the largest part of computational resources. To simulate boundary layer flow with most current CFD codes, requires extremely fine mesh spacing normal to the wall and is consequently computationally very expensive. Boundary layer modelling approaches offer considerable computational cost savings.
One boundary layer method which proved to be very accurate is the two-integral method of Drela (1985). Coupling the boundary layer solution to inviscid external flow, however, is a challenge due to the Goldstein singularity, which occurs as separation is approached.
This research proposed to develop a new method to couple Drela’s two-integral equations to a generic outer flow solver in an iterative fashion. The study introduced an auxiliary equation, which was solved along with the displacement thickness to overcome the Goldstein singularity without the need to solve the entire flow domain simultaneously. In this work, the incompressible Navier-Stokes equations were used for the outer flow.
In the majority of previous studies, the boundary layer thickness was simulated using a wall transpiration boundary condition at the interface between viscous and inviscid flows. This boundary condition was inherently non-physical since it added extra mass into the system to simulate the effects of the boundary layer. Here, this drawback was circumvented by the use of a mesh movement algorithm to shift the surface of the body outward without regridding the entire mesh. This replaced the transpiration boundary condition.
The results obtained show that accurate modelling is possible for laminar incompressible flow. The predicted solutions obtained compare well with similarity solutions in the case of flat and inclined plates, and with the results of a NACA0012 airfoil produced by the validated XFOIL code (Drela and Youngren, 2001).
Supervisor: Prof J.P. Meyer, Dr O.F. Oxtoby and Dr A.G. Malan
Mr A.G.B. Mowat 2011 "Modelling of non-linear aeroelastic systems using a strongly coupled fluid-structure-interaction methodology"
The purpose of this study was to develop a robust fluid-structure-interaction (FSI) technology that can accurately model non-linear flutter responses for sub- and transonic fluid flow. The Euler equation set governs the fluid domain, which was spatially discretised by a vertex-centred edge-based finite volume method.
A dual-timestepping method was employed for the purpose of temporal discretisation. Three upwind schemes were compared in terms of accuracy, effciency and robustness, viz. Roe, HLLC (Harten-Lax-Van Leer with contact) and AUSM+ -up (Advection Up-stream Splitting Method). For this purpose, a second order unstructured MUSCL (Monotonic Upstream-centred Scheme for Conservation Laws) scheme, with van Albada limiter, was employed. The non-linear solid domain was resolved by a quadratic modal reduced order model (ROM), which was compared to a semi-analytical and linear modal ROM. The ROM equations were solved by a fourth order Runge-Kutta method. The fluid and solid were strongly coupled in a partitioned fashion with the information being passed at solver sub-iteration level. The developed FSI technology was verified and validated by applying it to test cases found in literature.
It was demonstrated that accurate results may be obtained, with the HLLC upwind scheme offering the best balance between accuracy and robustness. Further, the quadratic ROM offered significantly improved accuracy when compared to the linear method.
Supervisors: Prof. A.G. Malan and Prof. J.P. Meyer
Mr L Prinsloo 2011 "A critical evaluation of the design of removable cover-plate header boxes for air-cooled heat exchangers "
Large air-cooled heat exchangers (ACHEs) are most popularly implemented in the petrochemical and power industries at arid locations. They operate on a simple concept of convective heat transfer, whereby air in the surrounding atmosphere is caused to flow across a tube bundle, which in turn transports a process fluid. The distribution and direction of the process fluid flow may furthermore be guided via a set of appropriately located header boxes, which essentially consist of a collection of welded flat plates and nozzle attachments. Perforations on one of the faces of these boxes serve as an interface to the tube bundle.
The overall design and construction of an ACHE is commonly regulated by an American Petroleum Institute (API) standard, which is required to be used in conjunction with acceptable design codes. In spite of this, the design of certain header box configurations remains of prominent concern. It is the focus of the present study to investigate the approach adopted for a header box variant labelled as the removable cover type. In this configuration, one of the plates used to construct the header box is left removable. This plate is fastened and sealed by a collection of bolted joints and gasket.
One appropriate design code for the header box equipment is the ASME (American Society of Mechanical Engineers) boiler and pressure vessel code, although it provides no specific approach pertaining to the removable cover design. Instead it has been commonplace in industry for a number of aspects from this code to be synthesized, together with a collection of assumptions surrounding the header box behaviour, into an all encompassing design by rule approach. In this approach, the header box behaviour is accepted as being planar, whilst circumstances such as nozzle attachments and associated loading would suggest that a more comprehensive approach should be undertaken.
The aim of the present study is therefore to critically evaluate the current practice, and establish its adequacy. To do so, a finite element model (FEM) of an example header box design is developed. Subsequent comparisons with the stress distribution predicted via current practice are performed, and problems identified. An alternative analytical approach developed from rigid frame theory is further shown to provide improved results. According to Norris et al. , a rigid frame may be defined as ‘... a structure composed of a number of members all lying in one plane and connected so as to form a rigid configuration by joints, some or all of which are moment-resisting (rigid) instead of hinged ...’.
The linear elastic design by analysis approach, presented in the ASME code, is also utilised as a method for establishing design adequacy, whereby the finite element method is adopted. This approach is implemented in a more detailed investigation of nozzle placement and external loading. Design results obtained via the respective design by rule, and design by analysis methods are compared throughout the study.
Supervisors: Ms H.M. Inglis and Dr S. Kok
Mr J van den Bergh 2011 "An Algebraic Multigrid Solution Strategy for Efficient Solution of Free-Surface Flows "
Free-surface modelling (FSM) is a highly relevant and computationally intensive area of study in modern computational ﬂuid dynamics. The Elemental software suite currently under development offers FSM capability, and employs a preconditioned GMRES solver in an attempt to effect fast solution times. In terms of potential solver performance however, multigrid methods can be considered state-of-the-art.
This work details the investigation into the use of Algebraic Multigrid (AMG) as a high performance solver tool for use as black box plug-in for Elemental FSM. Special attention was given to the development of novel and robust methods of addressing AMG setup costs in addition to transcribing the solver to efficient C++ object-oriented code. This led to the development of the so-called Freeze
extension of the basic algebraic multigrid method in an object-oriented C++ programming environment. The newly developed Freeze method reduces setup costs by periodically performing the setup procedure in an automatic and robust manner.
The developed technology was evaluated in terms of robustness, stability and speed by applying it to benchmark FSM problems on structured and unstructured meshes of various sizes. This evaluation yielded a number of conclusive ﬁndings. First, the developed Freeze method reduced setup times by an order of magnitude. Second, the developed AMG solver offered substantial performance increases over the preconditioned GMRES method. In this way, it is proposed that this work has furthered the state-of-the-art of algebraic multigrid methods applied in the context of free-surface modelling.
Supervisors: Prof. A.G. Malan and Dr. D.N. Wilke
Mr D.D. Ndenguma 2011 "Computational fluid dynamics model for controlling dust and methane in underground coalmines "
Airborne dust and methane are common problems in the underground coalmines. They pose health and safety risk to mining personnel, and a safety risk to mining equipment as well. In order to prevent these risks, air borne dust and methane concentrations must be reduced to within the acceptable levels. In South Africa, the dust and methane concentration in coalmines should not exceed 2.0 mg/m³ and 0.5% per volume, respectively.
Different ventilation systems have been designed since the history of underground coal mining. Unfortunately, none provides an ultimate solution to the dust and methane problem, especially in the most critical areas of the underground coalmine, like blind-end of the heading and last through road.
By changing airflow patterns and air velocity, it is possible to obtain an optimum ventilation design that can keep dust and methane within the acceptable levels. Mine ventilation is one of the popular ways of controlling both dust and methane.
Since it is very difficult to conduct experiments in the underground coalmine due to harsh environmental conditions and tight production schedules, the investigator made use of a Computational Fluid Dynamics (CFD) modelling technique. The models were then experimentally verified and validated using a scaled down model at University of Pretoria.
After verification, further numerical analysis was done to in order to obtain a method for determining optimum fan positions for different heading dimensions.
This study proves that CFD can be used to model ventilation system of a scaled down coalmine model. Therefore chances that this might be true for the actual mine are very high but it needs to be investigated. If this is found to be true, then CFD modelling will be a very useful tool in coalmine ventilation system research and development.
Supervisors: Dr J. Dirker and Prof N.D.L. Burger
Mr J Jansen van Rensburg 2011 "Selective feature preserved elastic surface registration in complex geometric morphology "
In the study of functional morphology, evolutionary biologists, paleontologists and anthropologists use numerical tools to enquire into the adaptation of organic form to accommodate the relevant physics. The effect of a chosen elastic registration procedure, uncertainty of the mapping and discretisation of geometries are inspected within a contextual study using two complex skull geometries. Non-rigid registration offers a researcher interested in aspects of functional morphology the opportunity to do direct comparisons and statistics when digital geometries of related subject shapes are available. A finite element analysis is done on the two skulls to simulate masticatory induced stress. These results obtained are elegantly compared during the post-processing stage using the non-rigid mapping between the relevant computational domains. As shown in the study, exact difference values are not necessarily obtained but a non-rigid map between subject shapes and numerical results could give a good indication on the location of differences.
Supervisors: Dr. S. Kok, Dr. D.N. Wilke
Mr WG le Roux 2011 "Maximum net power output from an integrated design of a small-scale open and direct solar thermal Brayton cycle "
The geometry of the receiver and recuperator in a small-scale open and direct recuperative solar thermal Brayton cycle can be optimised in such a way that the system produces maximum net power output. The purpose of this work was to apply the second law of thermodynamics and entropy generation minimisation to optimise these geometries using an optimisation method. The dynamic trajectory optimisation method was used and off-the-shelf micro-turbines and a range of parabolic dish concentrator diameters were considered. A modified cavity receiver was used in the analysis with an assumed cavity wall construction method of either a circular tube or a rectangular channel. A maximum temperature constraint of 1 200 K was set for the receiver surface temperature. A counterflow plate-type recuperator was considered and the recuperator length was constrained to the length of the radius of the concentrator. Systems producing a steady-state net power output of 2 – 100 kW were analysed. The effect of various conditions, such as wind, receiver inclination and concentrator rim angle on the maximum net power output, and optimum geometry of the system were investigated. Forty-five different micro-turbines and seven concentrator diameters between 6 and 18 metres were considered. Results show the optimum geometries, optimum operating conditions and minimum entropy generation as a function of the system mass flow rate. The optimum receiver tube diameter was relatively large when compared with the receiver size. The optimum counterflow plate-type recuperator channel aspect ratio is a linear function of the optimum system mass flow rate for a constant recuperator height. The optimum recuperator length and optimum NTU are small at small system mass flow rates but increase as the system mass flow rate increases until the length constraint is reached. For the optimised systems with maximum net power output, the solar receiver is the main contributor to the total rate of minimum entropy generation. The contributions from the recuperator, compressor and turbine are next in line. Results show that the irreversibilities were spread throughout the system in such a way that the minimum internal irreversibility rate was almost three times the minimum external irreversibility rate for all optimum system geometries and for different concentrator diameters. For a specific environment and parameters, there exists an optimum receiver and recuperator geometry so that the system can produce maximum net power output.
Supervisors: Dr T. Bello-Ochende and Prof J. P. Meyer
Mr T Botha 2011 "High Speed Autonomous Off-Road Vehicle Steering "
High speed cornering of an off-road vehicle poses considerable challenges to the development of an autonomous vehicle due to the non-linear dynamics of the tyre road interface as well as those of the vehicle as a whole during high lateral accelerations. Most driver models are developed for low speed applications using linear control methods under the assumption of linear vehicle dynamics. The dynamics of a vehicle however become highly non-linear as the lateral acceleration increases, thus rendering these linear models unusable during high speed manoeuvres.
In this study, two robust driver models for use in an autonomous vehicle capable of path following at both low and high speeds are presented. Both models make use of the relationship between the yaw acceleration and steering rate to control the yaw angle of the vehicle. The first driver model is derived from the simulation of a full non-linear vehicle model in ADAMS. The Magic Tyre Formula is used to model the relationship between the vehicle's yaw acceleration and steer rate as a function of vehicle speed. The second driver model is a mathematical model which incorporates a form of sliding control. The model includes the lateral tyre dynamics as modelled by the Pacejka '89 tyre model.
Both driver models are coupled with a gain scheduling proportional derivative controller to reduce the cross-track error.
The two driver models were implemented on a Land Rover Defender and experimentally validated by performing a double lane change manoeuvre at speeds up to 80km/h. The vehicle remained stable even though the lateral accelerations experienced were 80% of the vehicle limits.
The result is a robust controller capable of path following at various speeds and at high lateral accelerations.
Supervisor: Prof S Els
Mr AC Hohls 2011 "Investigation into Phase Transformation of Yttria Stabilized Zirconia Femoral Heads."
27 Retrieved Yttria Stabilised Tetragonal Zirconia (Y-TZP) femoral heads were studied for the occurrence of tetragonal to monoclinic phase transformation and the effects that such transformation has on the bearing surface. The mean monoclinic percentage found is 53.6% with 25 of the samples having transformed more than 20%. This finding nullifies earlier predictions that it would take 25 to 30 years to transform to a monoclinic content of 30 to 40% inside the human body (Chevalier, Drouin & Calés 1997). It was however shown that Hot Isostatic Pressed (HIP’ed) Y-TZP femoral heads have a better, though still not adequate, resistance to phase transformation in the human body than non-HIP’ed femoral heads.
Results of various investigations show that this transformation degrades the surface condition of the femoral heads, which in turn increases wear and subsequently decreases the survival rate of the prosthesis due to a greater risk of aseptic loosening.
It is postulated that a great contributing factor to the phase transformation is increased temperatures inside the bearing couple, due to inadequate lubrication between the two bearing surfaces.
Tetragonal to monoclinic phase transformation and its associated effects renders Y-TZP femoral heads less attractive for hip replacements.
Supervisor: Dr NDL Burger
Mr ASD Dymond 2011 "Multiple objective optimization of an airfoil shape."
An airfoil shape optimization problem with conflicting objectives is handled using two different multi-objective approaches. These are a an 'a priori' scalarization approach where the conflicting objectives are assigned weights and summed together to form a single objective, and the direct multi-objective approach.
The optimization formulations for both approaches contain challenging numerical characteristics which include noise, multi-modality and undefined regions. Gradient-, surrogate- and population-based single objective optimization methods are applied to the `a priori' formulations. The gradient methods are modified to improve their performance on noisy problems as well as to handle undefined regions in the design space. The modifications are successful but the modified methods are outperformed by the surrogate methods and population based methods.
Population-based techniques are used for the direct multi-objective approach. Two established optimization algorithms and two custom algorithms are implemented. The custom algorithms use fitted unrotated hyper ellipses and linear aggregating functions to search the design space for non-dominated designs. Various multi-objective formulations are posed to investigate different aspects of the airfoil design problem. The non-dominated designs found by the direct multi-objective optimization algorithms are then presented.
Supervisor: Dr Schalk Kok
Co-Supervisor: Dr Bennie Broughton
Mr F U Ighalo 2011 "Optimisation of Microchannels and Micropin-fin Heat Sinks with Computational Fluid Dynamics in Combination with a Mathematical Optimisation Algorithm."
In recent times, high power density trends and temperature constraints in integrated circuits have led to conventional cooling techniques not being sufficient to meet the thermal requirements. The ever-increasing desire to overcome this problem has led to worldwide interest in micro heat sink design of electronic components. It has been found that geometric configurations of micro heat sinks play a vital role in heat transfer performance. Therefore, an effective means of optimally designing these heat sinks is required. Experimentation has extensively been used in the past to understand the behaviour of these heat extraction devices. Computational fluid dynamics (CFD) has more recently provided a more cost-effective and less time-consuming means of achieving the same objective. However, in order to achieve optimal designs of micro heat sinks using CFD, the designer has to be well experienced and carry out a number of trial-and-error simulations. Unfortunately, this will still not always guarantee an accurate optimal design. In this dissertation, a design methodology which combines CFD with a mathematical optimisation algorithm (a leapfrog optimisation program and DYNAMIC-Q algorithm) is proposed. This automated process is applied to three design cases. In the first design case, the peak wall temperature of a microchannel embedded in a highly conductive solid is minimised. The second case involves the optimisation of a double row micropin-fin heat sink. In this case, the objective is to maximise the total rate of heat transfer with the effect of the thermal conductivity also being investigated. The third case extends the micropin-fin optimisation to a heat sink with three rows. In all three cases, fixed volume constraint and manufacturing restraints are enforced to ensure industrial applicability. Lastly, the trends of the three cases are compared. It is concluded that optimal design can be achieved with a combination of CFD and mathematical optimisation.
Supervisors: Dr T Bello-Ochende and Prof Josua Meyer
Mr S Smitt 2011 "Contraction heat transfer coefficient correlation for rectangular pin fin heat"
The demand for smaller but more powerful electronic components is ever increasing. This demand puts a strain on engineers to produce optimal cooling designs for these electronic components. One method for cooling these electronic components is with heat sinks which effectively increase the surface area available for extracting the heat from the electronic components. Computational Fluid Dynamics (CFD) software is sometimes used to aid in the design process, but CFD simulations are computationally expensive and take long to complete. This causes the design engineer to test only a few proposed designs based on his/her experience and select the design that performs the best out of the tested designs, which might not be the optimum.
The temperature distribution inside the heat sink can be solved relatively quickly with the diffusion equation, but the flow around the heat sink complicates the CFD simulation and increases the solving time significantly. Therefore, applications have been developed where the interaction between the heat sink and the flow around the heat sink is replaced by heat transfer coefficients. These coefficients are calculated from correlated equations which contain the flow properties. The flow properties are extracted from a flow network solver, which solves the flow around the heat sink. This procedure results in less expensive simulations, which can be used together with an optimisation procedure to develop an optimum cooling design.
In this dissertation, a correlation for the contraction heat transfer coefficients of rectangular pin fin heat sinks was developed. A methodology was developed where consecutive regression lines were fitted to a large set of data extracted from numerous CFD simulations. The combination of these regression lines formed the basis of the correlation, which was divided into two correlations; one for laminar flow and another for turbulent flow. The correlations were tested against CFD simulations as well as experimental data. The results indicate that these correlations can be effectively used to calculate the contraction heat transfer coefficients on pin fin heat sinks.
Supervisors: dr DJ de Kock and Prof Josua Meyer
The flexibility in the region of the crack is different from an uncracked section. A finite element model of a shaft is built and investigated. The damaged model is the same except that the nodes in the location of the crack are not equivalenced in order to represent the crack. A simple constant cross-sectional shaft with semi-circular transverse surface cracks varying in size have been modelled on the Patran finite element software and a normal modes analysis was done using the Nastran solver. The results revealed a change in the natural frequencies due to the variation in the size of the crack.
The experimental investigation involved creating sample shafts with damage positioned in them that would closely resemble what one may find in actual real-life situations and the dynamics during rotation with various torsional loadings are investigated and monitored using three methods. A fibre-optical sensor, Digital image correlation system and telemetry strain gauges were used. Undersampling techniques were used for the DIC system. Results showed that the fibre-optic sensor is by far the most favourable as it is able to detect damage under constant operation. The finite element model was updated by re-modelling the geometry, damage and material properties. The solution of the analysis matched the experimental results closely and model verification was achieved.
Supervisor: Professor P.S Heyns
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