Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 1999 / The simulation of Plasma Arc Cutting is presented in this study. The plasma arc cutting process employs a plasma torch with a very narrow bore to produce a transferred arc to the workpiece. A technique for modelling plasma arc cutting has been developed by applying the thermo-metallurgical model to the process and integrating a model of material removal to this model. The model is solved using the finite element method using the FE package SYSWORLD, more specifically SYSWELD. The objective is to determine the minimum energy required to cut a plate of some thickness using this virtual model. The characteristics of the cut need to exhibit the characteristics of a "high quality cut". The model presented can predict the kerf size given certain process variable settings. The numerical results obtained are assessed by conducting experiments. By maintaining Ill1rumum energy input cost savings can be made through energy savings, limiting additional finishing processes and reducing expense of shortening the electrode and nozzle lifetimes. The modelling of the PAC process using virtual design techniques provides a cost-effective solution to the manufacturing industries with respect to process specification development. This plays an important role in South Africa's transition into a competitive global market. It is envisaged that the model will provide an alternative more efficient, non-destructive means of determining the optimum process variable settings for the plasma arc cutting process.

The characterization, properties and applications of the novel nanostructured microgel (nanoparticle network and microgel crystal) composed of poly-N-isopropylacrylanmide-co-allylamine (PNIPAM-co-allylamine) and PNIPAM-co-acrylic acid(AA) have been investigated. For the novel nanostructured hydrogels with the two levels of structure: the primary network inside each individual particle and the secondary network of the crosslinked nanoparticles, the new shear modulus, drug release law from hydrogel with heterogeneous structure have been studied. The successful method for calculating the volume fraction related the phase transition of colloid have been obtained. The kinetics of crystallization in an aqueous dispersion of PNIPAM particles has been explored using UV-visible transmission spectroscopy. This dissertation also includes the initial research on the melting behavior of colloidal crystals composed of PNIPAM microgels. Many new findings in this study area have never been reported before. The theoretical model for the columnar crystal growth from the top to bottom of PNIPAM microgel has been built, which explains the growth mechanism of the novel columnar hydrogel colloidal crystals. Since the unique structure of the novel nanostructured hydrogels, their properties are different with the conventional hydrogels and the hard-sphere-like system. The studies and results in this dissertation have the important significant for theoretical study and valuable application of these novel nanostructured hydrogels.

The composition of volcanic products can provide critical information about the source and the conditions of melting. This information is used to highlight differences in melting environments from volcanic regions around the globe. Volcanic lavas and other products from the Rungwe Volcanic Province, in southwest Tanzania (9.13S,33.67E), were collected and studied to test a number of lingering questions about the role of magmatism in a continental rift tectonic environment. The Rungwe Volcanic Province is the only region in this portion of the East African Rift (EAR) system with apparent magmatism. Is magmatism here the product of rifting, like melts generated in oceanic rift tectonic environments (Mid-ocean ridge basalts, MORB), or is melting here facilitated by the upwelling asthenospheric mantle, like melts generated at hotspots or plumes (oceanic intraplate basalts, OIB)? To address this, contributions from the continental lithosphere must also be identified and addressed. Each chapter of this dissertation approaches this fundamental question using different aspects of the comprehensive chemical and isotopic dataset from this study. The second chapter outlines a novel thermobarometer that is then applied to Rungwe samples to estimate the temperatures and depths at which the melts equilibrated. Laboratory melt experiments of garnet peridotite, some containing CO2, create melt with major element characteristics applicable for pressure and temperature estimation of Rungwe samples. The parameterization of Al2O3 and MgO from the experimental melt compositions provides a thermobarometer with a temperature range of 1100-1500C (16C, 1), and a pressure range of 2-5 GPa (0.2 GPa, 1). The maximum potential temperature reached for Rungwe samples is 1372C. Potential temperatures at Rungwe overlap with the ambient asthenospheric mantle, as sampled by the global range of MORB. Potential temperature range for Rungwe is too high for melts to have a derivation from the continental lithosphere, and too low for melts to be derived from the thermally-driven plume. The pressures of melt equilibration for Rungwe span a range from GPa, when converted to depths is 55-101 km. Depth estimates can be compared to the estimated depths of the lithosphere-asthenosphere boundary (LAB) from seismic tomography models. Rungwe melts appear to be derived from the depths at or below the LAB, supporting their derivation from an asthenospheric source. Under the same parameters, other volcanic regions from the Western Branch of the EAR give similar results, while maximum potential temperatures from the Eastern Branch exceed estimates from the ambient asthenospheric mantle, providing more support for a thermally-derived mantle plume there. The third chapter provides a timeline of volcanism at Rungwe including ages from Ar-Ar geochronology performed on samples from this study, as well as dates of two precursor carbonatite bodies in the vicinity of the volcanic province. Most of the Rungwe Volcanic Province was emplaced between present-9Ma, with emerging evidence for eruptions between 9Ma and ~25Ma. A proposed broadening of the age range of each volcanic stage definition helps to include eruptions prior to 9Ma, and encompass eruptions shown to have occurred between the original volcanic stage age ranges. Two carbonatite bodies in the northwest edge of the volcanic province date to 169.0 0.6 Ma and 154.4 0.9 Ma, and show no evidence of Cenozoic reactivation. The emplacement ages of the majority of Rungwe samples coincide with accelerated rifting and basin formation present-9Ma. The updated timeline of Rungwe volcanism suggests that eruptions prior to 9Ma are still tied to tectonic extension, based on comparison to thermochronology cooling ages from the major border faults. The fourth chapter characterizes and provides context about the chemical and isotopic composition of the mantle source of Rungwe melting. Isotopic Sr-Nd-Pb-Hf, as well as major and trace elemental compositions provide a fingerprint for Rungwe melts in which to compare to the range of global OIB and to other EAR melts. The majority of Rungwe melts possess isotopic traits that are consistent with an asthenospheric plume-derived source. Many isotopic and trace element ratio characteristics identified are not shared with any identified OIB-source volcanic region, but are present in other EAR volcanoes. These indicators suggest that some Rungwe melts, together with some EAR volcanoes, share a common source characteristic or melt process that the global OIB does not sample or experience. Homogeneity of plume source or continental lithosphere over the large geographic distances between volcanic provinces in the EAR are not expected. No OIB emplaced on oceanic crust must traverse Archaean or Proterozoic subcontinental lithosphere or crust. The influence of melt interaction with these elements are explored in detail as the main cause of differences between OIB and Rungwe compositions. Metasomatic phases accumulated by melt interaction at the LAB interface over eons create compositions that can influence low-volume melts that traverse them. It appears that no Rungwe melt evaded this overprint from the subcontinental lithospheric mantle, despite large-scale preservation of the plume-derived melt origin.

Long time annealing of semicrystalline PEEK above the glass transition results in the observation of several time dependent phenomena - "physical aging", "secondary crystallization", "multiple melting" of lamellae with different thermal stability etc. Their interrelation - common origin and kinetics of development, is characterized extensively for the first time in this study. The evolution of the crystallinity during the secondary crystallization process was monitored by DSC and density measurements. Crystallinity was characterized according to the standard two-phase model of semicrystalline polymers and analyzed with respect to the failure of the model to adequately describe the physical state of the polymer. A discrepancy was observed between DSC and density crystallinity values and their respective rates of development during the secondary crystallization stage. WAXS reveals that the crystal density is not a physical constant, but depends on the crystallization and/or annealing temperature. Furthermore, the crystalline lamellae densify with time during crystallization and/or annealing. This observation leads to the conclusion that there is no one-to-one correspondence between density and crystallinity and necessitates the application of a revised equation for density crystallinity which accounts for the dynamics of crystal densification. The characteristics of the low temperature endothermic peak in the DSC scan of PEEK (peak maximum, transition enthalpy etc.) were found to evolve with annealing time and temperature during the secondary crystallization process in a way similar to the kinetics of development of the enthalpy relaxation process in amorphous polymeric glasses. This study reports for the first time in the literature the observation of "physical aging" above the glass transition in the case of PEEK (according to the definition of this term given by Struik). An extensive investigation of the "double melting"/"multiple melting" phenomenon, which is observed as a result of isothermal treatment of the polymer above Tg, was performed and several new observations reported. After the end of the primary crystallization process, the semicrystalline polymer is a nonequilibrium system due to the fact that crystallinity is less than unity. The system's continuing approach to equilibrium and its response to mechanical perturbations follow kinetics similar to that of segmental relaxation below the glass transition. / Ph. D.

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This thesis makes a novel contribution to the state-of-the-art literature on EHD melting enhancement of PCMs showing the effects of electroconvection flow and solid extraction during the melting process. The details of the contribution made by this work have been disseminated in the form of three journal publications, which have been integrated into this sandwich Ph.D. thesis. / Latent heat thermal energy storage plays an important role in bridging the gap between the energy supply and consumer demands. The latent heat storage systems use phase change materials (PCMs) which are characterized by their high latent heat and therefore lead to higher energy densities. However, one major disadvantage of PCMs is their low thermal conductivities which affects the rates of charging and discharging. Electrohydrodynamics (EHD) offers an opportunity as an active heat transfer enhancement method which can significantly enhance the melting rates while being able to control the heat transfer as per the system’ needs with a very low power consumption. The application of EHD in two-phase solid liquid systems results in generating electroconvection flow in the liquid medium which increases the heat transfer coefficient and decreases the melting time. The main objective of the current work is to study the heat transfer enhancement and the role of EHD forces during the melting of phase change materials (PCMs) under constant temperature boundary conditions. There are two main investigations performed in the current study. First is experimentally studying the EHD melting enhancement of PCMs while applying high voltages through two rows of electrodes embedded inside the PCM. Moreover, in the experiments, solid extraction was investigated using high-speed imaging conducted at various locations with respect to the electrodes. In the second investigation, PCM melting in a rectangular cavity under the effect of EHD and constant temperature boundary conditions is studied numerically. The flow field, temperature field, and phase field are simulated during the melting process until a steady state condition is reached. Additionally, the effect of the applied voltage and temperature boundaries on the electroconvection flow is illustrated. Experimentally, the EHD melting enhancement of paraffin wax is examined under different applied DC voltage magnitudes and polarities, and different temperature gradients. In addition, the role of EHD forces was investigated by applying DC and AC square waves with different frequencies and offset values. The results showed that the melting enhancement increases with a nonlinear relation with voltages, wherein the maximum effective thermal conductivity was found to be 0.95 W/m-K at -10 kV in comparison with the value of 0.2 W/m-K for the pure liquid paraffin wax, with an enhancement ratio of 4.75. The Coulomb force was concluded to be the dominant EHD force in the study while the dielectrophoretic effect was negligible. Characterization of solid extraction was performed by measuring the intensity of extraction, and the size and velocity of dendrites after extraction at different applied voltages and temperature boundaries for different phase change materials having different mushy zone thickness. For paraffin wax, solid extraction was detected for all the applied DC voltages. Small dendrites were observed to be pulled out from the mushy zone melt front and rise upwards in a rotational manner. The extraction intensity was found to be high at locations of high Coulomb force near the electrodes. In addition, solid extraction measurements showed that the size and velocity of the extracted dendrites increase alongside the applied voltage while the velocity decreases at higher temperature boundaries. Finally, it was found that the existence of a large mushy zone results in higher solid extraction intensities. A numerical model was conducted using the finite element method to investigate the EHD melting of PCMs. In the model, the non-autonomous charge injection assumption is used with the Coulomb force being the only electrical body force considered. First, phase-change modeling is conducted to simulate the melting of paraffin wax without EHD under constant temperature boundary conditions until a steady-state condition is achieved. Next, the whole set of coupled EHD equations is introduced to the model to simulate the EHD melting process. The results revealed that two electroconvection cells were generated between each two successive electrodes in the liquid PCM. The EHD flow leads to the redistribution of the temperature field which enhances the heat transfer. EHD melting continues until a steady-state condition is regained after one hour of EHD time, at which point the enhancement ratio was found to be 2.33 at 6 kV. The influence of the applied voltages and temperature boundaries on the electroconvection flow showed that the fluid velocity increases significantly by increasing the voltage while it decreases under higher temperature gradients across the liquid region. This thesis makes a novel contribution to the state-of-the-art literature on EHD melting enhancement of PCMs showing the effects of electroconvection flow and solid extraction during the melting process. The details of the contribution made by this work have been disseminated in the form of three journal publications, which have been integrated into this sandwich Ph.D. thesis. / Thesis / Doctor of Philosophy (PhD)

Identification of fungal isolates is critical in studying Aspergillus flavus ecology and for developing methods to reduce aflatoxin contamination. In our efforts to track biocontrol applications of the atoxigenic A. flavus K49 (NRRL 30797), we have developed a rapid and accurate classification system for A. flavus based on PCR product melting temperatures (Tm). Using 18 primers and a total of 59 Aspergilli strains, including all 49 representatives of the Georgian peanut Vegetative Compatibility Groups (VCGs), a decision tree Tm flowchart was generated. The decision tree can classify all 59 strains using only 9 of the SSR primers and an average of 3.4 primers for each definitive classification. To confirm the effectiveness of the decision tree for strain identification, unknown samples isolated from experimental fields inoculated with various A. flavus strains in Stoneville, MS were analyzed. Ninety-six percent of the samples could be placed into a VCG using Tm(s) coupled with the decision tree. This dynamic system is an excellent tool for researchers studying biodiversity of A. flavus.

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