3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Numerical solution of unsteady turbulent free convection over a vertical flat plate(2015) Remar, JaroslavA theoretical treatment of the problem of unsteady turbulent free convection over a vertical flat plate is presented in this dissertation. An exhaustive review of the relevant publications revealed, that at the present time no solution of this problem has been given. The development of a method, by which the abovementioned problem could be tackled, is a substantial part of the dissertation. The equations of conservation of mass, momentum, and energy, written in a general form, were the starting point of the derivation. Various assumptions, simplifying the partial differential equations, were introduced. In the end, boundary layer equations were obtained. Turbulence was simulated by a phenomenological model, consisting of an algebraic law of the wall and a partial differential rate equation. The turbulence model is based on the concept of effective viscosity. Also, a constant turbulent Prandtl number was employed. The problem of an isothermal plate in a stagnant non-stratified fluid was treat; d, and appropriate initial and boundary conditions were formulated„ The system of equations was solved by an explicit finite- difference method. The numerical stability criteria were established. A computer programme, based on the numerical scheme, was developed and employed for calculations. The calculations were carried out for dry air, water, and mercury, representing gases, liquids, and liquid metals, respectively. In this way, a broad range of Prandtl numbers was covered. Temperature velocity, and effective viscosity profiles are presented here together with some other results of the calculations* An important observation is that the overall heat transfer coefficient goes through a temporary minimum before attaining its steady state value. The transient, which is extremely fast, can be divided into throe characteristic stages: the initial conduction regime, an intermediate stage, and the steady state. Our results were verified by comparison with data available from other independent sources. Due to the lack of data covering ■the whole transient, only the first and third stages were considered, The initial conduction regime was compared with an analytical solution and the final steady state results with experimental data of various authors, respectively. The agreement is good and no serious discrepancies were discovered. Although the present method produces reliable results, it cannot be widely employed, because the computing times are almost prohibitive with the present-day computers.Item Synthesis of carbon nano-structured materials(2012-02-24) Shaikjee, AhmedABSTRACT Page | iiAbstract i The deposition of carbon during catalytic reactions has a long history, with major efforts initially focused towards their prevention rather than synthesis. However the discovery of fullerenes and later that of carbon nanotubes by Iijima, shifted scientific focus towards the synthesis, characterization and application of carbon deposits. This renewed interest in carbon based materials, has revealed a universe of extraordinarily shaped carbon materials (SCMs) in the nano and micro range, from tubes and helices to horns and most recently graphene. It has been noted that there exists a relationship between the morphology of the carbon material and its inherent properties, making them highly prized for numerous technological applications. However before these carbon materials can be effectively exploited control over their selective synthesis is necessary, a problem that has been solved with only limited success. As such, there still exists a need to develop synthetic strategies that would yield shaped carbon materials selectively. More importantly, it is essential that a better understanding of the growth factors that lead to differently SCMs is obtained. In this study we have highlighted the parametric conditions for optimum growth of carbon helices, as well as that of carbon fibers with unique structure. We have found that catalyst morphology and the carbon source are key aspects, which control carbon material growth and morphology. The synthesis of carbon materials using bi and tri-metallic supported catalyst systems revealed that Cu was an effective promoter for obtaining helices, particularly at low temperatures (≤ 550 ˚C). On further investigation, Cu was shown to exhibit incredible carbon deposition capabilities at temperatures as low as 200 ˚C. Adjustments of the catalyst preparation conditions (support, metal counter ion, solvent and reduction temperature) and synthesis temperature, revealed that the yield and morphology of the carbon deposit could be altered to selectively produce both straight and helical carbon fibers. A TEM tomography study revealed that the copper particles that gave distorted decahedra formed helical fibers, while trigonal bi-pyramidal particles gave linear fibers. Various plate-like particles revealed that as the number of sides of a catalyst particle varied (3, 4, 5 or 6) there was a corresponding change in the Abstract Page | iv carbon fiber helicity. A relationship between catalyst particle morphology and fiber morphology was thus established. TEM analysis also revealed that catalyst particles underwent rapid reconstruction during carbon fiber synthesis, and that the carbon source (gas environment) was influential in this reconstruction event. A NiOx (unsupported) catalyst was prepared and reactions with various substituted alkyne hydrocarbons were undertaken. Analysis revealed that different alkynes produced carbon fibers with varying morphologies. Using different alkynes in a sequential manner led to the formation of ‘co-block’ carbon fibers with an A-B-A-B... or A-B-C... morphology. Using different alkynes followed by acetylene led to the selective synthesis of straight, Y-junction or irregular carbon fibers. Accompanying these results was the observation that in each case the catalyst particle morphology was unique. Reaction of NiOx with trichloroethylene, in which trichloroethylene acted as a source of carbon for fiber growth, also restructured the Ni catalyst into a tetrahedral shape that gave tripod-like carbon growth. It was found that, substituted alkynes (and alkenes) provided a means for controlling catalyst particle morphology and hence carbon fiber morphology. The study has highlighted the relationship that exists between catalyst and SCM morphology, as well as the effect of hydrocarbons, not only as a source of carbon for SCM growth but also as a means of controlling catalyst morphology and SCM structures.Item Closure of tailings facilities: Current practice review and guidelines for success(2006-11-17T10:12:04Z) Mark, RobinsThis research project has been prepared to satisfy the requirements of the University of the Witwatersrand, Faculty of Engineering and the Built Environment for the degree of Master of Science (Civil) in Geotechnical Engineering and Materials (50/50). Contained within the project is a review of international past and current tailings facility closure techniques. Guidelines and minimum requirements for successful closure have been developed and are presented.Item Affording or constraining epistemological access: An analysis of a case-based approach in a first year process and materials engineering course.(2006-11-14T11:52:07Z) Kotta, Linda ThokozileThe focus of this study was a case-based approach used in the first year course Introduction to Process and Materials Engineering, PRME1002, at the University of the Witwatersrand in 2005. This approach attempted to promote epistemic access to Process and Materials Engineering by moving away from the more traditional decontextualised and contrived engineering problems and introducing context-rich cases entailing more authentic engineering problems. The study investigated the extent to which the context rich problem-solving environment afforded the students epistemic access to Process and Materials Engineering. This was done through an analysis of the form and content of students’ knowledge and problem-solving skills as evidenced in their written responses to case-based problems. A modified form of the Structure of Learning Outcomes (SOLO) taxonomy was used as the instrument of analysis. The research showed that students tended to work in fragmented ways despite the context. They tended not to fully explore the context and as such could not successfully identify the salient aspects. They frequently ignored evidence in the context and invented their own in order to be able to use strategies that they were most familiar with. These findings suggest that that while the case-based approach introduced in the course, theoretically has the hallmarks of an ideal approach with which to create a favourable environment for learning, if students treat knowledge as fragmented and aren’t persuaded by the context to change their ways of working, the case-based approach does not afford students optimal epistemological access.Item Optimisation of regeneration systems for a range of Cassava (Manihot Esculenta Crantz) cultivars suitable for growth in South Africa and transformation with SACMV N-REP gene(2006-11-13T12:08:37Z) Makwarela, MurunwaCassava (Manihot esculenta Crantz) is a vegetatively propagated root crop used as a staple throughout the tropics and subtropics. It is the fourth most important and cheapest staple food crop after rice, wheat and maize in developing countries, providing food for over 600 million people. However, its production is severely limited by a wide variety of viral and bacterial diseases, especially Cassava Mosaic Disease (CMD) which is caused by several geminivirus species including, South African cassava mosaic virus (SACMV), African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), Indian cassava mosaic virus (ICMV) and the Ugandan recombinant virus (UgV). In South Africa (SA), there has recently been an enormous upsurge of interest in cassava for industrial applications such as the manufacture of starch, animal feeds, and in its potential as a food security crop for marginalised farmers. However, due to serious losses in cassava yields by begomoviruses, such as SACMV, there is an urgent need for the development of appropriate systems that allows for transformation and regeneration of virus-resistant transgenic cassava cultivars suitable for diverse needs and growth requirements in different geographical areas in southern Africa. The potential application of cassava tuber disks as an alternative system to leaf tissue for transformation and regeneration was investigated. Furthermore, the antibiotic, carbenicillin, was tested as a possible shoot inducing factor. Disks from freshly-harvested cassava tubers were cultured on 25 different sets of MS supplemented with zeatin (0.01-5 mgl-1) and indole-3-acetic acid (0.01-5 mgl-1). Carbenicillin at 500 μgl-1 was included in each treatment as a potential viii organogenesis inducing factor. The results observed after 21 days in culture indicated that non-embryogenic friable callus formed readily on MS medium supplemented with MS vitamins, 30 gl-1 sucrose, 0.01 mgl-1 indole-3-acetic acid (IAA), 0.01 mgl-1 zeatin (ZEA), 500 μgml-1 carbenicillin and 0.8% agar, pH 5.8. Shoots or somatic embryos were never formed and only adventitious roots developed at a frequency of 60% on shoot induction medium supplemented with 2μM copper sulphate (CuSO4), 1 mgl-1 6-benzylaminopurine (BAP) and 0.5 mg-1 indole-3-butyric acid (IBA). The current study also investigated infection of cassava and tobacco by the SA begomovirus species SACMV, dimer A and B using the particle inflow gun. Full-length head-to-tail dimers of DNA-A and DNA-B of SACMV were constructed by digestion with SalI or EcoRI, respectively. The DNA-coated particles were used to shoot 3-week-old cassava plantlets (cv. TMS60444) at a pressure of 1500 psi using the Bio-Rad biolistic device. Thirty-day-old N. benthamiana seedlings were also inoculated in the same manner. In both cases young tender uppermost leaves were targeted (five plants inoculated and another 5 as control). Disease symptoms were recorded daily on the first emerging leaves. Cassava plantlets and tobacco seedlings showed infection by visibility of symptoms. On the other hand, control plantlets that were not inoculated were symptomless. Symptoms appeared 7 dpi in tobacco whereas mosaic symptoms became visible 14 dpi in cassava. The pre-requisite for any cassava transformation program that proposes to develop improved plants is the availability of a reliable regeneration system. Presently many laboratories that prioritize cassava research are able to reliably