Master's degrees completed 2016

Master's Degrees Completed 2016


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. [2] 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. [1]. 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.

References

[1] 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.

 

[2] 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
data centre"

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
process.
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
proven difficult.
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
dynamics models.

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.

Supervisors

:

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-Al2Oit 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

Wietsche"

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

Published by Barbara Huyssen

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