PhD's completed

The following is a list of PhD's dissertations that were recently completed in this Department. 

All these dissertations can be found at UPSpace, the University of Pretoria Institutional Repository.The abstracts of older PhD from the department are summarized in the following archive pages:

[2019] [2018] [2017] [2016] [2015] [2014-2011] [2010-2004

2020

 

D Kafka, 2020."Automated learning rates in machine learning for dynamic mini-batch sub-sampled losses"

Learning rate schedule parameters remain some of the most sensitive hyperparameters in machine

learning, as well as being challenging to resolve, in particular when mini-batch subsampling is considered. Mini-batch sub-sampling (MBSS) can be conducted in a number of ways, each with their own implications on the smoothness and continuity of the underlying loss function. In this study, dynamic MBSS, often applied in approximate optimization, is considered for neural network training. For dynamic MBSS, the mini-batch is updated for every function and gradient evaluation of the loss and gradient functions. The implication is that the sampling error between mini-batches changes abruptly, resulting in non-smooth and discontinuous loss functions. This study proposes an approach to automatically resolve learning rates for dynamic MBSS loss functions using gradient-only line searches (GOLS) over fifteen orders of magnitude. A systematic study is performed, which investigates the characteristics and the in ability of GOLS to resolve learning rates. GOLS are shown to compare favourably against the state-of-the-art probabilistic line search for dynamic MBSS loss functions. Matlab and PyTorch 1.0 implementations of GOLS are available for both practical training of neural networks as well as a research tool to investigate dynamic MBSS loss functions.

Supervisor: Prof DN Wilke


S M M Osman, 2020. "Experimental investigation into convection heat transfer in the transition flow regime by using nanofluids in a rectangular channel"

The growing demand for energy worldwide requires attention to the design and operating of heat exchangers and thermal devices to utilise and save thermal energy. There is a need to find new heat transport fluids with better heat transfer properties to increase convective heat transfer, and nanofluids are good alternatives to conventional heat transport fluids. Although extensive research has been done on the properties of nanofluids in recent decades, there is still a lack of research on convection heat transfer involving nanofluids, particularly in the transitional flow regime. This study focused on the application of nanofluids in heat exchangers as heat transport fluids by investigating forced convective heat transfer of alumina-water and titanium dioxide-water nanofluids prepared by using the one-step method. The particle size used was 46 nm and 42 nm for the aluminium oxide and the titanium dioxide respectively. Uniform heat flux boundary conditions were used by uniformly heating the rectangular channel electrically. Nanofluids with volume concentrations of 0.3, 0.5 and 1% were used for the alumina-water nanofluids, and volume concentrations of 0.3, 0.5, 0.7 and 1% were used for the titanium dioxide-water nanofluids. The viscosity of the nanofluids under investigation was determined experimentally, while the thermal conductivity and other properties were predicted by using suitable correlations from the literature. A Reynolds number range of 200 to 7 000 was covered, and the investigated flow rates included the laminar and turbulent flow regimes, as well as the transition regime from laminar to turbulent flow. Temperatures and pressure drops were measured to evaluate heat transfer coefficients, Nusselt numbers and pressure drop coefficients. Heat transfer and hydrodynamic characteristics in the transition flow regime were carefully studied and compared with those in the transition regime when flowing pure water in the same test section.

The study also investigated another approach of enhancing heat transfer in heat exchangers by increasing the heat transfer area of the heat exchanger itself, and this was done by filling the rectangular test section with porous media to increase the heat transfer surface area and thus enhance heat transfer. Hence in this study, the effect of using porous media was also studied by filling the rectangular test section with high-porosity nickel foam. The permeability of the used nickel foam was determined by conducting pressure drop measurements through the nickel foam in the test section, and heat transfer and pressure drop parameters were measured and compared with those in the empty test section.

The results showed that all the nanofluids used enhanced heat transfer, particularly in the transition flow regime. The 1.0% volume concentration alumina nanofluid showed maximum enhancement of the heat transfer coefficient, with values of 54% and 11% in the turbulent regime. The maximum enhancement of the heat transfer coefficient was 29.3% in the transition regime for the 1.0% volume concentration titanium dioxide-water nanofluid. The thermal performance factor in the transition flow regime was observed to be better than that in the turbulent and laminar flow regimes for all the nanofluids.

The results of the nickel foam test section showed that the values of the friction coefficient were 24.5 times higher than the values of the empty test section, and the Nusselt number was observed to be three times higher when using nickel foam than without foam in the test section. No transition regime was observed for the foam-filled test section on either the heat transfer results or the pressure drop results; however, transition from laminar to turbulent was found for the test section without foam. The results of the thermal factor of the foam-filled test section showed a thermal performance factor higher than unity through the entire Reynolds number range of 2 000 to 6 500, with better thermal performance factor at lower Reynolds number.

Supervisor: Prof Mohsen Sharifpur

Co-supervisor: Prof Josua Meyer


A.S. Shote, 2020, "Aerodynamic losses and endwall heat transfer in a linear vane cascade with endwall film-cooling and endwall-leading edge contouring"

Elevated inlet temperature is desirable in turbine passages. This is not without a whole lot of aerodynamic loss, endwall thermal stresses consequences and pressure loss penalties most especially downstream of the turbine passage. Film cooling and endwall modification are needed to increase the efficiency and prolong the material life of the turbine endwall and components. The main objective of this work is to investigate the heat transfer influence at the near endwall region by employing the leading edge contouring and film-cooling at the endwall.

The hub-side blade profile and passage geometry of the 1st stage nozzle guide vane of a GE-E 3 turbine engine are scaled up and employed in a linear cascade for the experimentation at a Reynolds number of 2.1E+05 based on turbine vane- blade chord and reference velocity. The hot endwall of the turbine engine was replicated experimentally by supplying constant heat flux at the endwall. A numerical model of the cascade is employed in a commercial computational fluid dynamics (CFD) code STAR CCM+ TM to optimize fillet shape. CFD is employed to predict the baseline case (no film-cooling, no endwall modification) and Fillet-1 endwall modification case (no film-cooling). Experimentally, the efficacy of four unique upstream film-cooling schemes and geometries are investigated with two linear fillets employed at the endwall-blade junction. The film-cooling is employed at five different inlet blowing ratios (M = 1.0, 1.4, 1.8, 2.2 and 2.8). The interaction of the film-cooling jets with the main flow inside the cascade are captured and analyzed. The leading-edge film cooling schemes tested include the flush slots, the slot-discrete holes combination, the discrete holes and the linear inlet beveled/curved holes. The distance travelled and spread of the film cooling along the endwall are assessed with respect to the effectiveness and non-dimensional temperature. Two linear contoured endwall geometries known as the fillets were employed at the blade-endwall junction along with the film-cooling configuration at the endwall for the experimental measurement.

The computational results of static pressure on the vane blade surface at the mid- span of the baseline and Fillet-1 cases (M = 0) shows good agreement with the experimental cases. The simulation also captures appropriately the passage vortex as presented by the experimental result of the coefficient of the total pressure loss. The location and the magnitude of the total pressure loss distribution of the computational result are comparable with that of the experiment. The linear variations of Fillet-1 in the axial and spanwise directions have significant effect on the approaching endwall boundary layer of the horse shoe vortex. The
optimized contoured endwall reduces the size and magnitude of the horse shoe vortex at the near endwall of the vane-blade. High Nusselt number (Nu) is recorded at the throat region of the endwall for the baseline without fillet case in the experiment. This is attributed to the flow acceleration effect. However, with the installation of Fillet-1 and Fillet-2, the magnitude of the Nusselt number at the throat region reduced by (20 – 40)% from that of the baseline case. Fillet-2 has the lowest reduction effect on the Nu at the throat region of the endwall. The non-dimensional temperature of the flow field near the endwall shows that Fillet-1 and Fillet-2 improve the endwall film cooling coverage in both pitchwise and axil directions. In general, high film cooling jet flux, provides better cooling and endwall coverage as the momentum of jet reduces the pitchwise cross flow that is responsible for high heat transfer and aerodynamic loss. With the injection of the high momentum film cooling flow (M = 2.8), the result shows excellent improvement in the adiabatic effectiveness for all the leading-edge film cooling configurations. The curved holes produced the best effectiveness distributions at the endwall. However, the slot-discrete hole configuration with the fillet or without the fillet are having the best reduction influence on the passage vortex at the exit plane of the turbine cascade passage. Therefore, the magnitude and size of the passage vortex are reduced significantly. At blowing ratio ≤ 2.2, with the fillet and without the fillet, the main flow has more influence on the film-cooling jets thereby reducing the adiabatic effectiveness coverage of the endwall. Generally, Mass fractions are found to increase as the blowing ratio increases and decrease with endwall modification. Fillet-1 recorded the least mass fraction for all blowing ratios investigated.

Supervisor: Dr G.I. Mahmood

Co-Supervisor: Prof. J.P. Meyer

Published by Barbara Huyssen

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