K. Arnachellan, 2017, "Aerodynamic loss reduction in a vane cascade with leading-edge fillet and upstream endwall film-cooling."
Secondary flow structures account for nearly 50% of aerodynamic losses experienced in the turbine blade passages. The adverse effects of these vortex structures transport the hot mainstream fluid towards the endwall blade surfaces, which enhances thermal stresses and leads to blade failure. The effects of leading-edge fillets and film-cooling with flush slots located upstream near the leading-edge region were investigated experimentally in the study in a large-scale linear vane cascade in which the aerodynamic flow field was considered. The introduction of slot film flow and fillet aimed to reduce the effects of the secondary flow structures from the leading edge through the passage towards the exit in an effort to decrease the pressure losses, improve film-cooling coverage and flow field uniformity for the next blade row. The two-dimensional vane profile was obtained from the hub-side airfoil of the GE-E3 engine nozzle guide vane. The slots were configured for two experimental cases to evaluate the influence of coolant flow rate and momentum; first, the effects of slot film injection from all four slots were observed and then compared with the second case injecting coolant only through the two central slots. Further effects were investigated by combining slot film-cooling with the leading-edge fillets employed on the endwall blade junction. The flow field measurements were quantified with spatial distributions of axial vorticity, total pressure loss, endwall static pressure and flow angle deviations taken across the cascade passage. The measurements were obtained at a Reynolds number of 2.0E+05 based on the cascade inlet velocity and vane chord length. Film-cooling inlet blowing ratios between 1.1 and 2.3 were investigated with the supply of coolant provided by a secondary channel. Film-cooling results were compared with the baseline case without slot film flow and fillet. The results indicated substantial improvement in the passage and exit planes with high inlet blowing ratios. The introduction of high momentum coolant flow from the central slots was seen to create laterally reversed axial vorticity, thereby counteracting the cross-flow tendency in the passage. The effects at the passage exit showed suppressed vortex structures with slot film injection from the two central slots only, with further improvements in the flow angle deviations. The leading-edge slots were seen to contribute positive axial vorticity, which enhanced the passage vortex that was pushed away from the endwall at the exit. When the fillet was introduced, it had favourable effects in reducing the pitchwise pressure gradients along the endwall. Filleted film-cooling then resulted in a faint passage vortex system (50-80% size and 20-50% strength reduction) with a restored endwall boundary layer at high film flow rates. The leading-edge fillet was highly effective at the inlet of the blade passage because it weakened the horseshoe vortex formation. Thus, upstream slot film-cooling has great potential to decrease the aerodynamic losses and is further compounded with the leading-edge fillet.
Supervisor: Dr. G.I. Mahmood (Supervisor)
Co-Supervisor: Prof. J.P. Meyer
L.M.J. Pallent, 2017, "The influence of a multiple tube inlet condition on heat transfer in the transitional flow regime."
In the industrial design of heat exchangers, engineers have long followed the general rule of avoiding transitional flow, and have rather designed a system operating in the turbulent flow regime. Whilst the turbulent regime is better for heat transfer, the higher friction inside the tube results in a much higher pressure drop which inevitably results in the system requiring a more powerful pump than if the system were to operate in the laminar regime. Designing a heat exchanger that operates in the turbulent flow regime is often the safer option, since little published design data is available for flow in the transitional flow regime, giving rise to numerous unwanted uncertainties during the design phase. Recent research into the transitional flow regime has resulted in promising experimental data that shows the regime is not as unstable as previously suspected. The regime allows for higher heat transfer than flows in the laminar regime, yet lower pressure drops than flows in the turbulent regime. Numerous investigations have previously been performed on a single uniformly heated tube operating in the transitional flow regime, however, there exists no data on the influence of a multiple tube inlet condition, as typically found in shell and tube heat exchangers, on the heat transfer characteristics. The purpose of this study was thus to determine the influence of varying tube pitch ratios on the fully developed heat transfer characteristics of three smooth circular horizontal tubes. An experimental set up was designed and built to accommodate a single tube heat exchanger used for validation purposes, and a multiple tube heat exchanger comprising of three identical and equally spaced tubes. Using a DC power supply, the tubes were uniformly heated at 2, 3 and 4 kW/m2 along the length of the test section. The heat transfer characteristics were determined experimentally for outer diameter tube pitch ratios of 1.25 and 1.5 of three 4 mm inner diameter tubes, each 6 m in length for a range of Reynolds numbers of 1 000 to 7 000. Water was used as the test fluid. Using PT100 probes and thermocouples at the inlet, outlet and outer surface of the test section, it was found that the presence of multiple tubes at the inlet of the heat exchanger for a pitch ratio of 1.25 promoted the onset of transition for the centre tube, and sharpened the transition gradient of the outer tubes. This effect noticeably increased with increasing heat flux and was absent at the higher pitch ratio of 1.5.
Supervisor: Prof J.P. Meyer
K.R.S. WRight, 2017, "The Effects of Age and Wear on the Stiffness Properties of an SUV tyre"
With an increasing need for accurate full vehicle models, a sensitivity analysis of the modelling of tyres depending on their age and wear was conducted. This included a sensitivity analysis into the accuracy of acquiring the tyre stiffnesses on a static test setup.
An FTire model is developed with the aim to update this model with basic tests to give a more accurate representation of the aged or worn tyre. A well-researched and documented method is used to artificially age the tyres. During the aging process the tyre was statically tested to monitor the potential changes in characteristics. Tyres were also worn on a dynamic test setup and periodically tested to monitor the property changes. These tests included both static and dynamic measurements.
The results indicate that the vertical and longitudinal stiffnesses of the tyre have convincing dependencies on the age and wear of the tyre. While the aging process was a trustworthy method, the wear process created irregular wear across and around the tyre subsequently skewing the results. Simple methods of updating the FTire tyre model without re-parameterising the model completely, was found to be effective in accounting for age and wear.
Supervisor: Professor PS Els
M. Joubert, 2017, "The influence of a multiple tube inlet condition on fully-developed friction factors in the transitional flow regime"
Concentrated solar power systems, such as parabolic trough heat exchangers, are being used more frequently in the energy sector. Engineers require accurate design information to optimise the effectiveness of these heat exchangers. Recent studies have shown that the transitional flow regime could potentially be the optimum region of operation, as it provides the best possible compromise between high heat transfer and low pressure drop. However, designers often choose to avoid the transitional flow region entirely, to evade the various uncertainties associated with this flow regime. Avoiding the transitional flow regime is not always possible, as changes in operating conditions, design constraints, and various heat transfer augmentation methods often result in the heat exchanger operating in the transitional flow regime. Although some research has been conducted on the effect of different inlet configurations on the pressure drop characteristics in the transitional flow regime, most of the work focused on single tubes, whereas the bulk of the heat exchangers available in industry would typically have bundles of tubes. The aim of this investigation is to experimentally obtain fully-developed pressure drop characteristics in the transitional flow regime, for different tube pitch ratios, under isothermal and diabatic conditions. An experimental setup was designed and built to house three tubes, spaced apart at different pitch ratios. A single tube heat exchanger was used for validation purposes, since reliable pressure drop correlations are readily available in various published journals. The multi-tube test section consisted of smooth stainless steel tubes, each with an inner diameter and length of 4 mm and 6 m, respectively. The study was performed for Reynolds numbers ranging from 1 000 to 7 000, at three different heat fluxes. Transition effects were investigated for two tube pitch ratios, namely 1.25 and 1.5, based on the outer diameter of the tubes. It can be concluded that the presence of adjacent tubes had a significant effect under isothermal conditions, causing delayed and more abrupt transition from laminar to turbulent flow. This effect was more significant for the pitch spacing of 1.25 than for the pitch of 1.5. However, the disturbance caused at the inlet due to the presence of adjacent tubes seems to be damped out by buoyancy induced secondary flow effects with the addition of heat. This dampening effect becomes more significant for increasing heat fluxes.
Supervisor: Prof J.P. Meyer
E. Huisamen, 2017, "A thermohydraulic model that represents the current configuration of the SAFARI-1 secondary cooling system"
This document focuses on the procedure and results of creating a thermohydraulic model of the secondary cooling system of the SAFARI-1 research reactor at the Pelindaba facility of the South African Nuclear Energy Corporation (Necsa) to the west of Pretoria, South Africa.
The secondary cooling system is an open recirculating cooling system that comprises an array of parallel-coupled heat exchangers between the primary systems and the main heat sink system, which consists of multiple counterflow-induced draught cooling towers.
The original construction of the reactor was a turnkey installation, with no theoretical/technical support or verifiability. The design baseline is therefore not available and it is necessary to reverse-engineer a system that could be modelled and characterised.
For the nuclear operator, it is essential to be able to make predictions and systematically implement modifications to improve system performance, such as to understand and modify the control system. Another objective is to identify the critical performance areas of the thermo-hydraulic system or to determine whether the cooling capacity of the secondary system meets the optimum original design characteristics.
The approach was to perform a comprehensive one-dimensional modelling of all the available physical components, which was followed by using existing performance data to verify the accuracy and validity of the developed model. Where performance data is not available, separate analysis through computational fluid dynamics (CFD) modelling is performed to generate the required inputs.
The results yielded a model that is accurate within 10%. This is acceptable when compared to the variation within the supplied data, generated and assumed alternatives, and when considering the compounding effect of the large amount of interdependent components, each with their own characteristics and associated performance uncertainties.
The model pointed to potential problems within the current system, which comprised either an obstruction in a certain component or faulty measuring equipment. Furthermore, it was found that the current spray nozzles in the cooling towers are underutilised. It should be possible to use the current cooling tower arrangement to support a similar second reactor, although slight modifications would be required to ensure that the current system is not operated beyond its current limits. The interdependent nature of two parallel systems and the variability of the conditions that currently exist would require a similar analysis as the current model to determine the viability of using the existing cooling towers for an additional reactor.
Supervisors: Prof JFM Slabber supervisor
Prof JP Meyer co-supervisor
C. Sanama, 2017, "Mathematical modelling of flow downstream of an orifice under flow-accelerated corrosion"
The main objective of this work is to establish an analytical model to evaluate the rate of corrosion in a horizontal pipe downstream of an orifice under flow-accelerated corrosion (FAC). FAC is a serious issue in nuclear and fossil power plants. In this work, an experimental setup was built to observe the effect of the flow on corrosion inside a tube. The experiments confirmed that the flow inside the tube caused more corrosion. However, accurate experimental data from literature has been selected and correlated by dimensional analysis, the modelling method of repeating variables and the Buckingham Pi theorem. It was found that the Sh number and the relative distance from the orifice are the main dimensionless parameters influencing FAC downstream of an orifice. The maximum value of the FAC rate could be well-predicted for the OR of 0.25, while the location of the maximum FAC rate could be well predicted for the OR of 0.5. The maximum FAC rate occurs between 2D to 4D downstream of the orifice and increases with a decreasing OR. This work could be useful for professionals in industry and researchers in the field and could be the starting point for a new way of evaluating the FAC rate downstream of a flow’s singularity.
Supervisor: Prof Mohsen Sharifpur
Co-Supervisor: Prof Josua Meyer
M.A.C Alfama, 2017, "Theoretical and Experimental Investigation of the Heat Transfer and Pressure Drop Optimisation on Textured Heat Transfer Surfaces"
Modern nuclear reactors still use Zirconium-4 Alloy (Zircaloy®) as the cladding material for fuel elements. A substantial amount of research has been done to investigate the boiling heat transfer behind the cooling mechanism of the reactor. Boiling heat transfer is notoriously difficult to quantify in an acceptable manner and many empirical correlations have been derived in order to achieve some semblance of a mathematical model. It is well known that the surface conditions on the heat transfer surface plays a role in the formulation of the heat transfer coefficient but on the other hand it also has an effect on the pressure drop alongside the surface. It is therefore necessary to see whether there might be an optimum surface roughness that maximises heat transfer and still provides acceptably low pressure drop.
The purpose of this study was to experimentally measure pressure drop and heat transfer associated with vertical heated tubes surrounded by flowing water in order to produce flow boiling heat transfer. The boiling heat transfer data was used to ascertain what surface roughness range would be best for everyday functioning of nuclear reactors.
An experimental set-up was designed and built, which included a removable panel that could be used to secure a variety of rods with different surface roughnesses. The pressure drop, surface temperature, flow rate and heat input measurements were taken and captured in order to analyse the heat transfer and friction factors.
Four rods were manufactured with different roughnesses along with a fifth rod, which remained standard. These rods were tested in the flow loop with water in the upward flow direction. Three different system mass flow rates were used: 0kg/s, 3.2kg/s and 6.4kg/s. Six repetitions were done on each rod for the tests, the first repetition was not used in the results since it served the purpose to deaerate the water in the flow loop. The full range of the power input was used for each repetition in the tests.
For the heat transfer coefficient at a system mass flow rate of 3.2kg/s, satisfactory comparisons were made between the test results and those found in literature with an average deviation of 14.53%. At 6.4kg/s system mass flow rate the comparisons deviated on average 55.45%. The velocity of the fluid in the test section was calculated from the pressure drop and was validated using separate tests. The plain rod, with no added roughness, was found to be the optimal surface roughness which is what is used in industry today.
The flow loop was in need of a couple of redesigns in order to produce more accurate results. Future work suggestions include adding more rods in the test section in order to investigate the nature of heat transfer in a rod bundle array as well as implementing all the suggested changes listed in the conclusion.
Supervisor: Prof J.F.M. Slabber
Co-Supervisor: Prof J.P. Meyer
G.F. Knijnenburg, 2017, "Development of a vibration isolation system for a rotary wing unmanned aerial vehicle"
Antiresonance vibration isolation has long been a well-known, studied and applied method for alleviating vibrations in stiff structures where small static deflection and a low transmissibility is needed, making it ideal for use in the rotor-craft industry. Most prior arts focus on passive single frequency antiresonance vibration isolation, while some, most notably liquid inertia vibration isolators, are adapted to actively isolate vibrations at more than one frequency. Very little literature is found on the adaptation of mechanical pendulum antiresonance vibration isolators for in-flight tunable multiple frequency isolation, and although these systems predate the more modern liquid inertia type isolator, there is merit in their further development and use as low cost, robust and low maintenance isolators. A feasibility study on the performance of changing each fundamental design variable to achieve antiresonance tuning concludes, that for the antiresonance frequency shift range of interest in this dissertation, no specific design variable change quantifiably outperforms another with respect to tuning the antiresonance. Concept designs are created and investigated, finding the superior method of tuning the vibration isolator based on other criteria like overall weight, design simplicity, practicality, robustness and reliability. Shifting the tuning mass on the pendulum arm is deemed to be the superior concept, with respect to the helicopter being developed, and a tuneable multi-frequency pendulum antiresonance vibration isolation system with a sliding concentrated mass is developed with ADAMS multi-body dynamics software and SolidWorks. The isolation system along with a full scale dummy fuselage and transmission-rotor assembly is manufactured and experimentally tested. Initial experimental results show antiresonance frequencies 10Hz higher than the design targets, this phenomenon is later discovered to be related to friction in the pin joints of the pendulum hinges, increasing the system overall stiffness. Needle roller bearings are inserted to eliminate the friction, and experimental and ADAMS model results are again compared showing good correlation, with experimental results isolating close to the three target frequencies within 3% error. An astonishing level of vibration isolation is observed with the largest transmissibility obtained at the three frequencies being 0.5%. This dissertation proves the concept of a tuneable mechanical pendulum vibration isolator, and its design methodology, particularly with respect to shifting the position of the tuning mass. Suggestions for further work are: to implement this system with an actuation mechanism, further research on the effects of friction in isolators and the use of said phenomenon as a tuning method, development of isolators implementing the other concept of changing the design variables and a comparison between the effect of normal damping and friction damping on vibration isolation.
Supervisor: Professor NJ Theron
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