U Spangenberg, 2017 " Reduction of rolling contact fatigue through the control of the wheel wear shape"
Heavy haul railway operations permit the transport of huge volumes at lower cost than other modes of transport. The low cost can only be sustained if the maintenance costs associated with such railway operations are minimised. The maintenance costs are mainly driven by wheel and rail damage in the form of wear and rolling contact fatigue (RCF). Low wear rates in the wheel-rail interface have resulted in an increase in the prevalence of rail RCF, thereby increasing rail maintenance costs.
The aim of this study is to develop an approach to reduce rail RCF on South Africa’s iron ore export line by managing the worn wheel shape. This approach is developed by evaluating wheel and rail profile shapes that contribute the most to RCF initiation, studying the influence of suspension stiffness and rail profile changes as well as a redesign of the wheel profile.
The influence of wheel and rail profile shape features on the initiation of rolling contact fatigue (RCF) cracks was evaluated based on the results of multibody vehicle dynamics simulations. The damage index and surface fatigue index were used as two damage parameters to assess the influence of the different features. The damage parameters showed good agreement to one another and to in-field observations. The wheel and rail profile shape features showed a correlation to the predicted RCF damage. The RCF damage proved to be most sensitive to the position of hollow wear and thus bogie tracking. RCF initiation and crack growth can be reduced by eliminating unwanted shape features through maintenance and design and by improving bogie tracking.
Two potential mitigation measures had been adapted from those published in literature to reduce RCF. The mitigation measures involved changes in suspension stiffness to spread wheel wear across the tread and the use of gauge corner relief rail profiles. These mitigation measures were evaluated by means of multibody dynamics and wear simulations to determine their efficiency as a RCF management strategy to minimise maintenance costs. These mitigation measures, however, did not prove to be successful in reducing RCF initiation while maintaining a low wheel wear rate. The current operating conditions on South Africa’s iron ore line, although still not optimal overall, were found to be better in terms of their wear and RCF performance than the two proposed RCF mitigation measures.
Based on the finding of the study on two RCF mitigation measures it was recommended that a conformal wheel profile be developed to spread the wheel wear across the tread to reduce the occurrence and propagation of RCF cracks, while still maintaining low wheel wear rates. A comparative study of this new wheel profile design and the current wheel profile design was therefore performed using multibody dynamics simulation together with numerical wheel wear and RCF predictions. The advantages of the conformal wheel profile design were illustrated by evaluating the worn shape and resulting kinematic behaviour of the conformal design. The conformal design had a steadier equivalent conicity progression and a smaller conicity range compared with the current wheel profile design over the wheel’s wear life. The combination of a conformal wheel profile design with 2 mm hollow wear and inadequate adherence to grinding tolerances often result in two-point contact, thereby increasing the probability of RCF initiation. The conformal wheel profile design was shown to have many wear and RCF benefits compared with the current wheel profile design. However, implementation of such a conformal wheel profile must be accompanied by improved rail grinding practices to ensure rail profile compliance.
Based on these findings an approach is proposed where the conformal wheel profile design together with improved compliance of the in-service rail profiles to the target rail profile are implemented. This has the potential to reduce RCF initiation on South Africa’s iron ore export line.
Keywords:
Wheel-rail interaction, wheel-rail profiles, rolling contact fatigue, wear modelling, anti-head check profiles, conformal wheel-rail profiles, wheel profile design
Supervisor:Prof P.S. Els
Co-Supervisor: Dr. R.D. Fröhling
H A Hamersma, 2017 "ABS braking on rough terrain"
This aim of this project may be condensed to the following question:
Is there an improvement achievable in the braking performance of a vehicle on a rough road?
Several follow-up questions arise from the above problem statement:
a) What are the causes of the unsatisfactory stopping time and distance when braking on a rough road and how can they be addressed?
b) Can the off-road braking of a vehicle be modelled mathematically?
c) What are the criteria used to evaluate the on-road braking performance of a vehicle and can the off-road braking performance of a vehicle be evaluated using the same criteria?
d) Can the off-road braking performance be improved without compromising the on-road performance?
An extensive literature survey is done on existing research addressing these four questions. It is found that, although the literature acknowledges that the braking performance of a vehicle deteriorates under off-road conditions, very little has been done to address it. Two main factors influencing the braking performance are identified, namely the ABS algorithm inputs and tyre force generation characteristics. An experimentally validated vehicle model is developed that serves as the basis from which the research question will be addressed. An FTire model is parameterised and used as the tyre model throughout this study. Three measured off-road terrain profiles are used.
The first step in addressing the research question is developing a performance evaluation technique that can easily, quantifiably and visually compare the braking performance of several ABS systems on any road surface, in any condition. The performance evaluation technique considers the stopping distance, longitudinal deceleration, lateral path offset error, and yaw rate error as metrics.
The second step is investigating one of the common assumptions found in ABS algorithms, namely that the roll radius is constant. This is investigated experimentally and it is concluded that the assumption is valid on smooth and rough road surfaces when using the kinematic definition of the roll radius, but invalid when using the kinetic definition of the roll radius.
Investigation of the influence of the tyre force generation characteristics on the braking performance is the third step. It is found that the tyre normal force variation and corresponding suspension force variation correlates closely with the braking performance. A higher suspension force variation is associated with longer stopping distances.
The final step is the development of a three step control strategy that aims to reduce the suspension force variation. This is done by estimating the wheel hop using easy to measure states, predicting the suspension force variation based on these estimates, and finally selecting the ideal suspension configuration.
The control strategy, called the WiSDoM algorithm, was evaluated by doing several simulations on the three off-road road profiles, with different braking points as the only changed variable. The WiSDoM algorithm’s performance was compared with the baseline vehicle performance and found to decrease the stopping distance on all three off-road road profiles, without negatively affecting the stability of the vehicle. The WiSDoM algorithm did not have a significant influence on the braking performance on a smooth road.
Supervisor:Prof P.S. Els
W Bornman, 2017 "Energy optimisation for mine cooling systems through flow control"
The mining sector in South Africa accounted for 14.3% of all electricity supplied by Eskom in 2016. Up to 25% of this energy was consumed by mine cooling systems, suggesting that more focus should be placed on the energy consumption of cooling systems. Notwithstanding previous significant reductions obtained through energy efficiency in mine cooling systems, it was found that the current initiatives were not necessarily optimised to achieve their full potential. Through a review, several variable-flow energy-saving strategies were identified in the literature with the objective of ultimate integrated optimisation.
In this semi-empirical investigation, an integrated simulation model was developed to fully quantify the overall cooling system’s integrated energy consumption. This model was subsequently coupled to a commercial optimisation platform to arrive at a globally optimised system. The novelty of the current study lies in the development of a mathematically optimised mine cooling system which is not currently found in the literature.
The optimisation-friendly simulation model was constructed and verified through suitable component models and a component-based calibration process as a case study. For these verifications, modelling accuracies with root mean square of relative error (RMSRE) values between 0.0114 and 0.0651 were obtained. The components were subsequently coupled to form the integrated cooling system simulation model that was validated through a baseline simulation conducted on various isolated weekly datasets to confirm its integrity for simulation during and outside of the system calibration period. For these validations, RMSRE values in the range of 0.0362 to 0.0704 were obtained with average absolute error values between 2.44% and 4.61%. The baseline simulation validation was concluded with an hourly annual simulation that obtained an RMSRE value of 0.860 with an average absolute error of 6.22%.
During the case study, it was found that the use of an integrated optimised mine cooling system has the potential to reduce the total average annual energy consumption by roughly 18%. This translates to an annual energy saving of more than 7.1 GWh for the particular mine. Because the majority of underground mines in South Africa utilise similar cooling systems, the adoption of integrated mine cooling optimisation systems could result in significant reductions of energy consumption in this sector.
Supervisor:Prof J Dirker
Co-Supervisors: J.P. Meyer, D. Arndt
M A Baseer, 2017 "Wind resource assessment and GIS-based site selection methodology for efficient wind power deployment"
An enormous and urgent energy demand is predicted due to the growing global population, increase in power intensive industries, higher living standards, electrification of remote areas, and globalisation (transportation). Moreover, the global consciousness about the harmful effects of traditional methods of power generation on the environment. That, in turn, has created a need to strategically plan and develop renewable and sustainable energy generation systems. This study presents a wind resource assessment of seven locations proximate to the largest industrial hub in the Middle East, Jubail Industrial City, Kingdom of Saudi Arabia, and a Geographic Information System, GIS based model considering a multi-criteria wind farm site suitability approach for the entire Kingdom of Saudi Arabia and elsewhere.
The hourly mean wind speed data at 10, 50 and 90 m above the ground level (AGL) over a period of five years was used for a meteorological station at the Industrial Area (Central) of Jubail. At the remaining six sites, the meteorological data were recorded at 10 m AGL only. Five years of wind data were used for five sites and three years of data were available for the remaining one site. At the Industrial Area (East), the mean wind speeds were found to be 3.34, 4.79 and 5.35 m/s at 10, 50 and 90 m AGL, respectively. At 50 and 90 m AGL, the availability of wind speed above 3.5 m/s was more than 75%. The local wind shear exponent, calculated using measured wind speed values at three heights, was found to be 0.217. The mean wind power density values at measurement heights were 50.92, 116.03 and 168.46 W/m2, respectively. After the assessment and comparison of wind characteristics of all seven sites, the highest annual mean wind speed of 4.52 m/s was observed at Industrial Area (East) and the lowest of 2.52 m/s at the Pearl Beach with standard deviations of 2.52 and 1.1 m/s, respectively.
In general, at all sites, the highest monthly mean wind speed was observed in February/June and the lowest in September/October. The period of higher wind availability coincides with a high power demand period in the region attributable to the air conditioning load. The wind rose plots show that the prevailing wind direction for all sites was from the north-west. Weibull parameters for all sites were estimated using maximum likelihood, least-squares regression method (LSRM), and WAsP algorithm. In general, at all sites, the Weibull parameter, c, was the highest in the months of February/June and the lowest in the month of October. The most probable and maximum energy carrying wind speed was determined by all three methods. The highest value of most probable wind speed was found to be in the range of 3.2 m/s to 3.6 m/s at Industrial Area (East) and the highest value of maximum energy carrying wind speed was found to be in the range 8.6 m/s to 9.0 m/s at Industrial Area 2 (South) by three estimation methods. The correlation coefficient (R2), root mean square error (RMSE), mean bias error (MBE), and mean bias absolute error (MAE) showed that all three methods represent wind data at all sites accurately. However, the maximum likelihood method is slightly better than LSRM, followed by WAsP algorithm. The wind power output at all seven sites, from five commercially available wind turbines of rated power ranging from 1.8 to 3.3 MW, showed that Industrial Area (East) is most promising for wind farm development. At all sites, based on percentage plant capacity factor, PCF, the 1.8 MW wind turbine was found to be the most efficient. At Industrial Area (East), this wind turbine was found to have a maximum PCF of 41.8%, producing 6,589 MWh/year energy output. The second best wind turbine was 3 MW at all locations except the Al-Bahar Desalination Plant and Pearl Beach. At both of these locations, 3.3 MW was the next best option. The energy output from the 3 MW wind turbine at Industrial Area (East) was found to be 11,136 MWh/year with a PCF of 41.3%. The maximum duration of rated power output from all selected wind turbines was observed to be between 8 to 16.6% at Industrial Area 2 (South). The minimum duration of rated power output, less than 0.3% for all wind turbines, was observed at Pearl Beach. The maximum duration of zero power output of between 35 to 60% was also observed at Pearl Beach. The minimum duration of zero power output of between 12 to 23% was obtained in Industrial Area (East). Even though the 1.8 MW wind turbine is found to be most efficient, installation of a higher rated power wind turbine such as the 3 MW is a smart option as it would occupy less of the scarce land in the Industrial City. The cost of electricity, COE per kWh was estimated at each of the seven locations in Jubail, based on present value cost, PVC method for five selected wind turbines, and annual power output at these locations. The minimum cost of 0.023 US$ per kWh is obtained for Industrial Area (East) for the wind turbine of capacity 2,000 kW.
This study also presents a multi-criteria wind farm site suitability analysis by developing a model based on a geographic information system (GIS). The site suitability analysis considered different parameters, such as climatic, economic, aesthetic and environmental conditions, and formulated a criterion based on wind resource, accessibility by roads/highways, proximity to the electrical grid, and optimum/safe distance from various settlements and airports. The developed model was then applied to the entire Kingdom of Saudi Arabia by using long-term historical wind speed data from 29 meteorological stations across the country. The wind speed used in the criterion was interpolated to 100 m from 10 m AGL by using traditional one-seventh power law. To predict the wind speed in locations for which data are not available, a spatial interpolation technique, inverse distance weighted, was used to convert wind speed point data to raster structure. The data and GIS shape files of other criteria mentioned above were obtained from governmental organisations. The GIS shape files for roads and highways were merged as identical constraints were applied to both criteria. Subsequently, the data of all the criteria were reclassified into suitability scores. Two different modelling approaches were adopted, one in which equal weightage was given to all the components of the criteria, and the other in which different weightage selected based on the literature, were given to the different components of the criteria of site selection. The resulting suitability maps were distributed into six classes, from the most suitable to least suitable. In the suitability map, based on Method 1, 1.03% of the total classed area fell under the most suitable wind farm area, whereas in Method 2, the percentage was 1.86%. The percentages of the next best areas were 29.13% and 14.65% in the maps based on Method 1 and 2, respectively.
The wind farm site suitability indexed map reveals that the most suitable sites for wind farms are (i) Ras Tanura and Safwa along the Arabian Gulf coast in the Eastern Province, (ii) Turaif, Kaf and Al-Isawiyah in the Al-Jawf region along the northern borders, and (iii) Al-Wajh and Yanbu along the Red Sea coast in the western region. These three regions are windy, adequately populated, and well connected by roads/highways and the national electricity grid. Some central and south-eastern regions failed to qualify to be considered for wind farm development mainly because of scarce wind resources, low population, and poor connectivity by roads and electrical grid.
Supervisor:Prof JP Meyer
Co-Supervisors:Dr. S. Rehman, Dr. M.M. Alam
M. Moghimi Ardekani, 2017 "OPTICAL, THERMAL AND ECONOMIC OPTIMISATION OF A LINEAR FRESNEL COLLECTOR"
Solar energy is one of a very few low-carbon energy technologies with the enormous potential to grow to a large scale. Currently, solar power is generated via the photovoltaic (PV) and concentrating solar power (CSP) technologies. The ability of CSPs to scale up renewable energy at the utility level, as well as to store energy for electrical power generation even under circumstances when the sun is not available (after sunset or on a cloudy day), makes this technology an attractive option for sustainable clean energy. The levelised electricity cost (LEC) of CSP with thermal storage was about 0.16-0.196 Euro/kWh in 2013 (Kost et al., 2013). However, lowering LEC and harvesting more solar energy from CSPs in future motivate researchers to work harder towards the optimisation of such plants. The situation tempts people and governments to invest more in this ultimate clean source of energy while shifting the energy consumption statistics of their societies from fossil fuels to solar energy.
Usually, researchers just concentrate on the optimisation of technical aspects of CSP plants (thermal and/or optical optimisation). However, the technical optimisation of a plant while disregarding economic goals cannot produce a fruitful design and in some cases may lead to an increase in the expenses of the plant, which could result in an increase in the generated electrical power price.
The study focused on a comprehensive optimisation of one of the main CSP technology types, the linear Fresnel collector (LFC). In the study, the entire LFC solar domain was considered in an optimisation process to maximise the harvested solar heat flux throughout an imaginary summer day (optical goal), and to minimise cavity receiver heat losses (thermal goal) as well as minimising the manufacturing cost of the plant (economic goal). To illustrate the optimisation process, an LFC was considered with 12 design parameters influencing three objectives, and a unique combination of the parameters was found, which optimised the performance. In this regard, different engineering tools and approaches were introduced in the study, e.g., for the calculation of thermal goals, Computational Fluid Dynamics (CFD) and view area approaches were suggested, and for tackling optical goals, CFD and Monte-Carlo based ray-tracing approaches were introduced. The applicability of the introduced methods for the optimisation process was discussed through case study simulations. The study showed that for the intensive optimisation process of an LFC plant, using the Monte Carlo-based ray-tracing as high fidelity approach for the optical optimisation objective, and view area as a low fidelity approach for the thermal optimisation objective, made more sense due to the saving in computational cost without sacrificing accuracy, in comparison with other combinations of the suggested approaches.
The study approaches can be developed for the optimisation of other CSP technologies after some modification and manipulation. The techniques provide alternative options for future researchers to choose the best approach in tackling the optimisation of a CSP plant regarding the nature of optimisation, computational cost and accuracy of the process.
Supervisor:Prof. KJ Craig
C0-Supervisor:Prof JP Meyer
M. Mahdavi, 2017 "Study of flow and heat transfer features of nanofluids by CFD models: Eulerian multiphase and discrete Lagrangian approaches"
Choosing correct boundary conditions, flow field characteristics and employing right thermal fluid properties can affect the simulation of convection heat transfer using nanofluids. Nanofluids have shown higher heat transfer performance in comparison with conventional heat transfer fluids. The suspension of the nanoparticles in nanofluids creates a larger interaction surface to the volume ratio. Therefore, they can be distributed uniformly to bring about the most effective enhancement of heat transfer without causing a considerable pressure drop. These advantages introduce nanofluids as a desirable heat transfer fluid in the cooling and heating industries. The thermal effects of nanofluids in both forced and free convection flows have interested researchers to a great extent in the last decade.
Investigating the interaction mechanisms happening between nanoparticles and base fluid is the main goal of the study. These mechanisms can be explained via different approaches through some theoretical and numerical methods. Two common approaches regarding particle-fluid interactions are Eulerian-Eulerian and Eulerian-Lagrangian. The dominant conceptions in each of them are slip velocity and interaction forces respectively. The mixture multiphase model as part of the Eulerian-Eulerian approach deals with slip mechanisms and somehow mass diffusion from the nanoparticle phase to the fluid phase. The slip velocity can be induced by a pressure gradient, buoyancy, virtual mass, attraction and repulsion between particles. Some of the diffusion processes can be caused by the gradient of temperature and concentration.
The discrete phase model (DPM) is a part of the Eulerian-Lagrangian approach. The interactions between solid and liquid phase were presented as forces such as drag, pressure gradient force, virtual mass force, gravity, electrostatic forces, thermophoretic and Brownian forces. The energy transfer from particle to continuous phase can be introduced through both convective and conduction terms on the surface of the particles.
A study of both approaches was conducted in the case of laminar and turbulent forced convections as well as cavity flow natural convection. The cases included horizontal and vertical pipes and a rectangular cavity. An experimental study was conducted for cavity flow to be compared with the simulation results. The results of the forced convections were evaluated with data from literature. Alumina and zinc oxide nanoparticles with different sizes were used in cavity experiments and the same for simulations. All the equations, slip mechanisms and forces were implemented in ANSYS-Fluent through some user-defined functions.
The comparison showed good agreement between experiments and numerical results. Nusselt number and pressure drops were the heat transfer and flow features of nanofluid and were found in the ranges of the accuracy of experimental measurements. The findings of the two approaches were somehow different, especially regarding the concentration distribution. The mixture model provided more uniform distribution in the domain than the DPM. Due to the Lagrangian frame of the DPM, the simulation time of this model was much longer. The method proposed in this research could also be a useful tool for other areas of particulate systems.
Supervisor:Prof. Mohsen Sharifpur
Supervisor:Prof JP Meyer
J. Baloyi, 2017 "Thermodynamic Analysis of a Circulating Fluidised Bed Combustor"
The focus of the world is on the reduction of greenhouse gases, such as carbon dioxide, which contribute to the global warming currently experienced. Because most of the carbon dioxide emitted into the atmosphere is from fossil fuel combustion, alternative energy sources were developed and others are currently under study to see whether they will be good alternatives. One of these alternative sources of energy is the combustion of wood instead of coal. The advantages of wood are that it is a neutral carbon fuel source and that currently installed infrastructure used to combust coal can be retrofitted to combust wood or a mixture of wood and coal in an attempt to reduce the carbon dioxide emissions.
Spent nuclear fuel has to be cooled so that the decay heat generated does not melt the containment system, which could lead to the unintentional release of radioactive material to the surroundings. The heat transfer mechanisms involved in the cooling have historically been analysed by assuming that the fluid and solid phases are at local thermal equilibrium (LTE) in order to simplify the analysis.
The exergy destruction of the combustion of pine wood in an adiabatic combustor was investigated in this thesis using analytical and computational methods. The exergy destruction of the combustion process was analysed by means of the second law efficiency, which is the ratio of the maximum work that can be achieved by a Carnot engine extracting heat from the combustor, and the optimum work of the combustor. This was done for theoretical air combustion and various excess air combustions, with varied inlet temperatures of the incoming air. It was found that the second law efficiency reached an expected maximum for theoretical air combustion, and this held true for all varying air inlet temperatures. However, it was found that as the air inlet temperature was increased more and more, the maximum second law efficiency was the same for all excess air combustions, including the theoretical air combustion. It was also found that the results of the analytical and commercial computational fluid dynamics code compared well.
Another analysis was conducted of irreversibilities generated due to combustion in an adiabatic combustor burning wood. This was done for a reactant mixture varying from a rich to a lean mixture. A non-adiabatic non-premixed combustion model of a numerical code was used to simulate the combustion process where the solid fuel was modelled by using the ultimate analysis data. The entropy generation rates due to the combustion and frictional pressure drop processes were computed to eventually arrive at the irreversibilities generated. It was found that the entropy generation rate due to frictional pressure drop was negligible when compared with that due to combustion. It was also found that a minimum in irreversibilities generated was achieved when the air-fuel mass ratio was 4.9, which corresponded to an equivalent ratio of 1.64, which was lower than the respective air-fuel mass ratio and equivalent ratio for complete combustion with theoretical
Supervisor:Prof T Bello-Ochende
Supervisor:Prof JP Meyer
D H Diamond, 2017 "A Probabilistic Approach to Blade Tip Timing Data Processing"
Rotor blades are important structural components of axial flow turbomachinery. They experience immense loads and stresses due to harsh operating conditions. Rotor blade resonance occurs when the frequency of aerodynamic excitation coincides with a rotor blade’s natural frequency, leading to harmful blade vibrations. It is imperative to ensure the integrity of the rotor blades while at the same time reducing unnecessary maintenance outages. Blade Tip Timing (BTT) is a non-intrusive technique for measuring individual rotor blade vibration that can be used for condition monitoring. The technique uses stationary proximity sensors mounted circumferentially around the rotor casing to measure blade vibration. BTT data is, however, notoriously difficult to process. Rotor torsional vibration, limitations in data acquisition hardware, high levels of inherent noise and a limited number of proximity sensors are all factors that complicate BTT data analysis. These difficulties are impeding the widespread adoption of BTT. This thesis presents a new approach to BTT data processing. The overarching novelty in this work lies in the fact that several aspects of BTT data processing are performed probabilistically. This stands in stark contrast to the current state-of-the art processing methods, which are deterministic in nature. Four novel signal processing techniques using a probabilistic approach are derived and validated in this thesis. Several important needs are identified in the current BTT literature and novel probabilistic algorithms are derived which address those needs. The four algorithms focus on different aspects of BTT signal processing. Firstly, a novel algorithm is derived that can be used to compensate for the geometry of an incremental shaft encoder for signals acquired during arbitrary shaft speeds. Secondly, an algorithm is derived to calculate the blade tip deflection based on the shaft Instantaneous Angular Speed (IAS) as opposed to a constant speed assumption, and then to instantaneously determine if the rotor blades are experiencing resonance. Thirdly, a novel technique is developed to determine the blade tip deflection from the raw proximity sensor signals using local phase information in the signal, as opposed to conventional triggering criteria. Finally, a method is derived to calculate a rotor blade’s accumulated fatigue damage as a probability distribution from BTT data. All four algorithms are validated using laboratory and/or simulated experiments.
Supervisor: Prof. P S Heyns
Co-supervisor: Dr. A J Oberholster
L Smith, 2017 "Investigation of a modified low-drag body for an alternative wing-body-tail configuration"
A wing-body-tail (WBT) configuration has been proposed without the conventional tailplane, allowing for new design objectives for the aerodynamic shape of the fuselage. A shorter, low-drag body (LDB) can be employed with a lower structural mass and lower drag per unit volume. There is an additional possibility that a carefully-designed deflector flap (Kutta edge or KE) can modify the flow around the body to allow for a more uniform circulation between the body and the wing. This basic idea has been tested and supported in wind tunnel tests but there have been no systematic attempts to quantify or explore the design space for these WBT geometries. All prior investigations were experimental on the full configuration at a low Reynolds number (Re) range where the aerodynamics of simple, rigid, fixed wings becomes extraordinarily sensitive to small changes in geometry and the environment. Therefore, the purpose of this study was to first investigate a simple case of the NACA0012 airfoil and wing, which becomes non-trivial when making baseline comparisons for experimentation and computation purposes. A diagnostic procedure then guides comparisons and predictions in subsequent more complex cases. In a preliminary attempt to bring specifically-designed low-drag bodies (LDB) into the design space, a numerical study was conducted to investigate the effect of adding a KE to two LDBs whose properties have been well established in the technical literature. Initial experimental tests on KE deflection were always accompanied by aftbody deflection, and the same procedure was adopted in the numerical studies, so tail deflection was achieved through deflection of the entire aftbody. It was further noted that such measures were quite sensitive to details of separation over the body and tail and that, paradoxically, a preferred arrangement would be to locate the KE entirely within the bounds of the viscous wake. Finally, the notion that the body-tail combination can be used in conjunction with the wing to improve the net downwash of the configuration was considered. The two bodies used previously with the KE were fitted with an NACA0012 wing at a fixed angle of attack of 6°. The overall purpose of this study was to provide some insight into the differences between the previous experimental and numerical studies and to investigate a potential design space of the initially tested WBT configurations. Then further exploration of the design space was conducted by using 10 discrete WBT configurations (five deflection angles with and without KE) for the original experimental WBT at chord-based, Rec = 105 and two LDBs at their specific design length-based, Rel (1.2 x 106 for F-57 LDB and 107 for Myring LDB). This work confirms that the KE can influence the WBT wake structure as initially estimated in experiment. The results here suggest that the original KE concept requires careful matching in the reality of viscous flows over bodies and wings at finite Re. In particular, if the KE is wholly or partially immersed in a wake that derives from earlier upstream separation or wing-induced wake effects, then the KE cannot operate effectively and the body termination conditions must already be judged to be sub-optimal. Adding the trailing-edge increased the total drag coefficient and the expected improvement of induced drag did not lead to a net benefit. If there is an optimal WBT configuration that leads to significant benefits in lift-to-drag, L/D, then it presumably would have to live in a domain where separation is almost completely avoided. The second modifying consideration is that if an entire system is designed for a certain lifting objective, then the option of providing that weight support through a modified geometry that includes a KE might not be well described by a single number such as L/D. When system benefits of reduced wing length, area and weight are included (and subsequently fed back into the new design set-point for the lift coefficient), the interlocked design benefits of each component might be difficult to isolate. The properties of the idealized WBT project with real bodies at finite Re are not easy to predict, this work offers useful guidelines and a helpful start, with recommendations for further numerical and experimental investigations.
Supervisor:Prof JP Meyer
Co-Supervisors: Prof KJ Craig, Prof GR Spedding
O.O. Adewumi,2016 "Constructal design and optimisation of combined microchannels and micro-pin fins for microelectronic cooling"
Microchannels and micro pin fins have been employed for almost four decades in the cooling of microelectronic devices and research is still being done in this field to improve the thermal performance of these micro heat sinks. In this research, the constructal design and computational fluid dynamics code was used with a goal-driven optimisation tool to numerically investigate the thermal performance of a novel design of combining microchannels and micro pin fins for microelectronic cooling applications. Existing designs of microchannels were first optimised and thereafter, three to seven rows of micro pin fins were inserted into the microchannels to investigate whether there was further improvement in thermal performance. The microchannels and micro pin fins were both embedded in a highly conductive solid substrate.
The three-dimensional geometric structure of the combined micro heat sink was optimised to achieve the objective of maximised thermal conductance, which is also minimised thermal resistance under various design conditions. The micro heat sinks investigated in the study were the single microchannel, two-layered microchannels with parallel and counter flow configurations, three-layered microchannels with parallel and counter flow configurations, the single microchannel with circular-, square- and hexagonal-shaped micro pin-fin inserts and the two-layered microchannels with circular-shaped micro pin-fin inserts. A numerical computational fluid dynamics (CFD) package with a goal-driven optimisation tool, which employs the finite-volume method, was used to analyse the fluid flow and heat transfer in the micro heat sinks investigated in this work. The thermal performances of all the micro heat sinks were compared for different application scenarios.
Furthermore, the temperature variation on the heated base of the solid substrate was studied for the different micro heat sinks to investigate which of the heat sink designs minimised the temperature rise on the heated base best. This is very important in microelectronic cooling applications because temperature rise affects the reliability of the device. The heat sink design that best maximised thermal conductance and minimised temperature rise on the heated base was chosen as the best for microelectronic cooling. For all the cases considered, fixed volume constraints and manufacturing constraints were applied to ensure real-life applicability. It was concluded that optimal heat sink design for different application scenarios could be obtained speedily when a CFD package which had an optimisation tool was used.
Keywords: Microchannels, micro pin fins, computational fluid dynamics, goal-driven optimisation, heat transfer, fluid flow, constraints, optimal heat sink, thermal conductance, thermal resistance, constructal theory, temperature variation
Supervisor: Prof. T. Bello-Ochende
Co-Supervisor: Prof. J.Meyer
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