3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
11 results
Search Results
Item Stagnation heat transfer on a concave surface cooled by unconfined slot jet(2015) Aziz, Maurel NguepnangHeat transfer at the stagnation point on a concave surface and on a flat plate subjected to un-confined slot jet impingement is characterized and compared at a fixed jet Reynolds number of ReB = 20000. The concave surface diameter-to-slot width ratio is fixed at 9.0 whilst the slot exit-to-target surface distance, H/B, varies from 0.5 to 20.0. In particular, the non-monotonic variation of heat transfer at the stagnation point with H/B is fluidically explained. The present results clarify that the deflection zone formed on the target surface as a result of the jet impingement leads to the upstream shift of the peak in turbulence strength that exactly coincides with the peak of heat transfer at the stagnation point. With the concave surface, the impinging jet deflected radially on the surface is re-entrained into the incoming jet due to the curvature, which causes the shortening of the potential core of the slot jet and the longitudinal upstream shift of the peak in turbulence strength compared to the flat plate. Therefore, the peak in heat transfer at the stagnation point occurs at shorter H/B on the concave surface than on the flat plate.Item Delays in the nuclear power plant procurement process in South Africa(2020) Kheswa, NkosikhonaThe constant evolution and development of nuclear-power projects has necessitated changes to contemporary projects. The Republic of South Africa has included the development of nuclear-power projects in its development goals as detailed in the Integrated Resource Plan. However, these projects have been delayed for years. This research is aimed at investigating the root causes of these delays. The relevant literature was analysed and reviewed while several nuclear projects were studied in order to elicit viable information about the root causes of the delays observed in the country’s own nuclear power project. The research method used to address the problem is qualitative. Internet websites, research papers and interviews were used to obtain the information. The status of different management models in power construction were analysed and reviewed in order to determine the delays in the procurement process in South Africa’s nuclear project since 2006. This research found gaps and issues for the current stakeholders and flaws in the current procurement process. Nuclear-project completion in South Africa is crucial for South African industry and citizens because of its contribution to economic growth and job opportunities, necessitating strict adherence to procedures to avoid delays. Recommendations were made for the future approach for the procurement of nuclear power stations in South AfricaItem Mathematical models for the transmission dynamics of bovine schistosomiasis with contaminated environment(2020) Kadaleka, SolomonSchistosomiasis is a chronically debilitating infection of humans and animals, caused by different species of blood flukes. The main purpose of this thesis is to gain some insights into the transmission dynamics of the disease by exploring the role of contaminated environment. We first propose and rigorously analyze a generic deterministic mathematical model with snail and bovine hosts as a system of non-linear ordinary differential equations. Second, we extend our generic model to incorporate humans and control measures, namely treatment of humans and bovines and mollusciciding of the contaminated environment. The basic reproduction number R0 of the two proposed models is computed and used to theoretically investigate the existence and stability of the models’ steady states. By constructing a suitable Lyapunov function, we prove the global stability of the endemic equilibria. Sensitivity analysis is performed to determine the relative importance of model parameters to disease transmission. The basic reproduction number is most sensitive to model parameters biased towards the contaminated: the bovine recruitment rate, the fecal output parameter, the snail-Miracidia effective contact rate and the cercariae to miracidia survival probability. Pontrayagin’s Maximum Principle is used for the optimal control analysis in order to determine the strategies which yield optimal results in controlling the spread of schistosomiasis. Analytical results are supported with numerical simulation using Matlab, Maple and Mathematica software. Numerical simulations indicate that mollusciciding proves to be more effective in containing the spread of the disease in humans and bovines, however a combined application of treatment of infected bovines and humans and mollusciciding will be most effective in controlling the spread of schistosomiasis. Finally, uncertainty analysis on the non-dimensional system parameters is graphically represented using the Latin Hypercube Sampling and Partial Rank Correlation Coefficient techniquesItem Thermal analysis of continuously moving solid and porous fins using approximate analytical methods(2019) Ndlovu, Partner LuyandaIn various industrial and engineering applications, fins (extended surfaces) are frequently adopted to enhance the rate of heat dissipation between a system and its surroundings. The heat transfer mechanism of a fin is to conduct heat from a heat source to the fin surface via conduction, and then dissipate heat to the surrounding fluid via convection, radiation, or simultaneous convection-radiation. In order to improve the rate of heat transfer through finned surfaces, it is necessary to understand a fin’s dynamic response to change in temperature. The study of heat transfer though fins continues to be of scientific interest and recently, the study of moving fins has attracted a lot of research interests. The study of heat transfer through fins is modeled by differential equations. In search for solutions to differential equations arising in physics and engineering, analytical methods are very useful as it is difficult if not impossible to find the exact solutions. In recent years, the availability of faster processing equipment further means that we are able to compute analytical solutions to highly nonlinear equations that are more accurate in representing the physical phenomena. The modeling of heat transfer through fins reduces the experimental costs and gives insight into various parameters influencing the heat transfer process. In this thesis, the Variational Iteration Method (VIM) and the Differential Transform Method (DTM) are used to solve the nonlinear boundary value problems describing heat transfer in solid and porous fins undergoing convective-radiative heat dissipation. Validation of analytical solutions is also obtained by comparison with numerical solutions. The aim is to derive mathematical models describing heat transfer though fins, analyze the impact of the embedding thermo-physical parameters, compare the accuracy and computational efficiency of these two modern day analytical methods. The study of porous fins is performed using Darcy’s model to formulate the governing heat transfer equations. As far as we know, the transient study of heat transfer through moving fins has not been performed anywhere in literature. Related work on finned heat transfer is modeled using steady state models with the assumption that the transient response dies out quickly. Since a broad range of governing parameters are investigated, the results could be useful in a number of industrial and engineering applications.Item Heat integration of multipurpose batch plants through multiple heat storage vessels(2018) Sebelebele, NthabisengIn most industrial processes, energy is an integral part of the production process; therefore, energy consumption has become an intensified area in chemical engineering research. Extensive work has been done on energy optimisation in continuous operations; unlike in batch operations because it was believed that due to the small scale nature of batch plants, small amounts of energy is consumed. Certain industries such as the brewing and dairy industries have shown to be as energy intensive as continuous processes. It is, therefore, necessary for energy minimisation techniques to be developed specifically for batch processes in which the inherent features of batch operations such as time and scheduling are taken into account accordingly. This can be achieved through process integration techniques where energy consumption can be reduced while economic feasibility is still maintained. Most of the work done on energy minimisation either focuses on direct heat integration, where cold and hot units operating simultaneously are integrated, or indirect heat integration, where units are integrated with heat storage. The schedules used in these models are, in most cases, predetermined which leads to suboptimal results. This work is aimed at minimising energy consumption in multipurpose batch plants by using direct heat integration together with multiple heat storage vessels through mathematical programming. The proposed approach does not use a predetermined scheduling framework. The focus lies on the heat storage vessels and the optimal number of heat storage vessels together with their design parameters, namely size and the temperature at which the vessels are initially maintained, are determined. The formulation developed is in the form of a mixed integer non-linear program (MINLP) due to the presence of both continuous and integer variables, as well as non-linear constraints governing the problem. Two illustrative examples are applied to the formulation in which the optimal number of multiple heat storage vessels is not known beforehand. The results rendered from the model show a decrease in the external utilities, in the form of cooling water and steam, compared to the base case where no integration is considered and the case where only one heat storage vessel is used.Item Effects of increasing rejection temperatures on electricity demand for ventilation and cooling in automated metalliferous underground mines(2014) Mochubele, Ekgatlamang MothusiThe South African power crisis and corresponding rising costs experienced since 2008, created a paradigm shift in terms of electricity use. The mining sector is the second highest consumer of electricity with metalliferous mines being accountable for 80% of the total power. Recent studies revealed that underground ventilation and cooling accounts for 30% to 40% of total electricity costs in an underground metalliferous mine. Hence the need to look at ways to reduce electricity consumption in ventilation and cooling. Work has been done on optimising efficiencies of ventilation and refrigeration systems of underground mines. Currently, the high energy consumption is driven by efforts to achieve a thermally acceptable environment for workers (manned) in deep metalliferous mines which is currently between 27ºC (wb) and 29ºC (wb). However, no detailed study has been done looking at increasing thermally acceptable environments for deep level metalliferous mine. In this study the impact of increasing rejection temperature to 40ºC (db) was assessed in the automated (unmanned) scenario at a maximum depth of 2811 metres. Then the power demand was compared with the manned scenario. The results proved that automation in an underground mine has the potential of reducing electricity cost of ventilation and cooling by more than 50%. For example, the production rate of about 200 kilo-tons per month yield an annual cost saving of R71 million on electricity. These cost savings can be used to justify automation systems. Automation removes workers from the hazardous environment and replaces them with equipment which can withstand harsher conditions. The introduction of an automation system in underground mines would not come risk-free. Currently, automation systems have not yet reached a level of removing workers completely from underground.There are instances that would require workers to enter production zones. A Bow-Tie risk analysis was used to show the hazards that workers would be exposed to and prevention controls and responses to mitigate the impact the risks.Item Parameteric thermal process models of friction stir welding(2017) Uslu, Mehmet YildirimThe Friction Stir Welding process is a rotating tool, that consists of a specialy designed shoulder and pin, that is plunged into the joining line of the required material and traverses along this line. The friction is induced by the rotating tool causes the workpiece material to rise to an operating temperature of 70% to 90% of the workpiece material's melting temperature and resulting in, no phase change, nor any defects associated with phase change, occurs in the workpiece. The increased temperature of the material causes the shear yield strength to drastically decrease thus allowing the two pieces to plasticise, easily stir around the tool and subsequently join. As the tool traverses along the workpiece, the softened material cools in the wake of the rotating tool and recrystallises, forming a ne grained microstructure. Attempts to develop an innovative tool to correlate the resulting of thermal models with process parameters are scarce. In this work, 6056-T4 and 6082-T6 Aluminum alloy sheets are friction stir welding at different rotational and translational speeds during the experimental aspect and material 2024-T3 for the analytical calculations. The effects of process parameters on the resulting thermal and mechanical properties are investigated. The results show that the use of coolant during the friction stir weld decrease heat generation substantially, this can also affect the force of the weld. It is also observed that the shear strenght of the processed sheet depends strongly on the rotational and translational speeds as weld as the thermal aspect and varies widely within the processed region, this was shown in this study by evaluating the thermal aspects of different weld types namely the Standard tool, Bobbin tool and the innovative tool. In addition. The proposed approach involves determination of the use of the friction stir welding in different thermal conditions and championing the use of an innovative tool.Item The development of surface based measurements for monitoring self heating of fuel stockfiles(1991) Anderson, PaulAnalysis of temperatures measured in an experimental coal bed (using the classical conductive-convective approach) confirm previously published permeabilities of similar beds, and furthermore validate the use of heat- transfer coefficients at exposed surfaces of coal stockpiles, The range of the estimated heat transfer coefficients is similar to natural convective coefficients at flat horizontal surfaces, which is expected. [Abbreviated Abstract. Open document to view full version]Item Analysis of heat transfer in a hot body with non-constant internal heat generation and thermal conductivity(2016-09-19) Lourenco, Marcio AlexandreHeat transfer in a wall with temperature dependent thermal conductivity and internal heat generation is considered. We rst focus on the steady state models followed by the transient heat transfer models. It turns out that the models considered are non-linear. We deliberately omit the group-classi cation of the arbitrary functions appearing in the models, but rather select forms of physical importance. In one case, thermal conductivity and internal heat generation are both given by the exponential function and in the other case they are given by the power law. We employ the classical Lie point symmetry analysis to determine the exact solutions, while also determining the optimal system for each case. The exact solutions for the transient models are di cult to construct. However, we rst use the obtained exact solution for the steady state case as a benchmark for the 1D Di erential Transform Method (DTM). Since con dence in DTM is established, we construct steady state approximate series solutions. We apply the 2D DTM to the transient problem. Lastly we determine the conservation laws using the direct method and the associated Lie point symmetries for the transient problemItem An investigation into the heat transfer aspects of transpiration cooling(2015-08-06) Hobson, G. VAlthough transpiration cooling has been demonstrated to keep the metal surface temperatures, in a gar turbine, below that at which oxidation occurs even though the hot gas temperatures are in excess of the metals melting temperature, few experimental studies have been conducted on the heat transfer aspects of transpiration cooling. Especially the effect blowing has on a turbulent boundary layer that has developed over a porous surface that is heated by the mainstream. Many studies have involved the blowing or suction oi the boundary layer through heated porous plates. Trans. an cooling was exper cntally investigated by making use of an existing wind tunnel which was modified so as to simulate the heat transfer phenomenon resulting from blowing coolant through a porous wall into a heated mainstream. The existing wind tunnel was designed and commissioned by Krieg (13) who considered the momentum transfer aspects of transpiration cooling. The solution of the momentum equation by Krieg forms the basis from which this investigation was developed, the original solution procedure being put forward by Cebeci and Smith (7). Krieg developed a generalised two-dimensional finite-differunce compute* program to solve the incompressible momentum equations describing a blown boundary layer. This program was further developed as part of this effort to solve the compressible momentum and neigy equations so as to account for the heat transfer in the blown boundary layer. The program is used to predict the experimental results obtained from the literature as well is those ottained during the nine experimental runs on the wind tunnel. Freestream flow velocities varied from 4,^5 m/s to 14,95 m/s with correspoi ;.ng temperatures of 44,95°C and 33,00eC respectively. The blowing fr tion F, was varied from 0,0059 to a maximum of , temperature of 40°C. The numerically calculated profiles and tho: » obi lined expurim ntally, as well as one set presented by another researcher, compared well. Finally, recommendations for future studies have been suggested.