Nguyen, Vinh Q.
16 April 2001
This thesis examines the numerical solution to Burgers' equation on a finite spatial domain with various boundary conditions. We first conduct experiments to confirm the numerical solutions observed by other researchers for Neumann boundary conditions. Then we consider the case where the non-homogeneous Robin boundary conditions approach non-homogeneous Neumann conditions. Finally we numerically approximate the steady state solutions to Burgers' equation with both the homogeneous and non-homogeneous Robin boundary conditions. / Master of Science
The photoconductive properties of undoped hydrogenated amorphous silicon have been extensively studied. Measurements of optical absorption, dark d.c. conductivity, steady state photoconductivity, step response transient rise photoconductivity and impulse response flash decay photoconductivity have been made. In addition, computer simulation has been used to give an insight into the physical processes involved in the photoconductivity experiments.Two materials were used in the study, to provide a comparison. All the above measurements, except the transient measurements were made on sputtered material prepared by the author. a-Si:H prepared elsewhere by the glow discharge decomposition of silane was measured by all the above experiments, except optical absorption.The results obtained from the flow discharge material were interpreted as due to recombination in distributed states, which are restricted in energy, extending from the dark Fermi level upwards to 0.6eV below Ec. The capture cross section of these states was of a value expected for the neutral dangling bond, so they have been denoted as Do states. States outside this energy were seen to be ineffective as recombination centres. The model thus has features intermediate between a simple 2-trap system and a distributed density of states. Chapter 2 presents a detailed analysis of steady state photoconductivity for the case of a single correlated defect level, and demonstrates its near equivalence to a simple 2-defect one electron system. This partly justifies the subsequent use of one electron states in computer modelling. Computer simulation successfully predicts the form of the experimental step response. The flash decay required the existence of an extra discrete state at 0.4eV, but gave a better quantitative fit.The sputtered material appeared to have the recombination in the steady state controlled by discrete states 0.6eV below Ec, but inconsistencies remain between the interpretations of different measurements on this material.
Potter, Christopher Robert
No description available.
Experimental investigation of free-surface jet-impingement cooling by means of TiO2-water nanofluidsWilken, Nicolas John January 2019 (has links)
The exponential advancements in the field of electronics and power generation have resulted in increased pressure on the thermal management of these systems where the desire for enhanced heat transfer is prevalent. A technique for enhancing heat transfer that has gained sufficient attention over the past two decades is to suspend nano-sized metallic particles in a base fluid in order to enhance its thermophysical properties. Fluids produced in such a manner are commonly termed nanofluids. Due to the promising heat transfer capabilities of nanofluids, many industrial applications are beginning to implement these fluids in their thermal practices. One of the potential applications where nanofluids may be used which has received a great deal of research attention is jet-impingement heat transfer. Concerning the existing publications on nanofluid jet impingement, most works within the steady state regime are limited to the cooling of Al2O3-water nanofluids, while transient studies do not account for cooling without the effects of boiling phenomena and for surfaces other than steel. In this study, six particle volume fractions of TiO2-water ranging between 0.025 and 1% were prepared and characterised for appropriate cooling tests. The study was conducted within both the steady and transient state with the main objective of evaluating the thermal performance of the selected nanofluid and to determine the optimum particle concentration for jet-impingement cooling applications. Therefore, an experimental rig was designed and manufactured where a copper target surface of 42 mm was impinged upon by a 1.65 mm orifice nozzle at a non-dimensional nozzle-to-target height of 4. The results indicated that the use of nanofluids in impingement applications produced adverse effects, depending on the particle fraction considered. With respect to the steady-state cooling tests, the copper surface was subjected to a constant heat flux of 145 watt and cooled by the different fluids at Reynolds numbers ranging between approximately 10 000 and 30 000. A maximum enhancement of 14.75% was observed in the measured Nusselt numbers, which occurred at a particle volume concentration of 0.05%. When increasing the volume fraction above 0.1%, unfavourable effects were observed for the heat transfer of the system in comparison with the base case tests of DI-water. Such trends were characterised by the trade-off between the enhancement in thermal conductivity and viscosity, both of which were increased with an increase in particle concentration. As for the effect of Reynolds number on the resulting thermal performance, a directly proportional relation was shown and could be described by the forced convection effect. The transient impingement tests showed that particle concentrations less than 0.1% produced an enhancement in cooling efficiency, while those of higher volume fractions showed negative effects. According to these tests the maximum enhancement was also obtained at a volume fraction of 0.05% and produced an average cooling efficiency enhancement of 16%. The results of the investigation clearly showed that the use of TiO2-water nanofluids in jet-impingement cooling applications produced thermal enhancement depending on the selected particle concentration. / Dissertation (MEng)--University of Pretoria, 2019. / NRF / Mechanical and Aeronautical Engineering / MEng / Unrestricted
17 July 2008
Analytical three-dimensional temperature field in the liquid and heat-affected zones and prediction of the three-dimensional fusion zone shape around the keyhole produced by a moving high-intensity beam are provided. Determination of the fusion zone shapes is of fundamental and practical importance to understand properties and microstructures of joints. In this work, the keyhole is idealized by a paraboloid of revolution in a finite workpiece subject to an incident flux of a Gaussian distribution.Introducing analytical solutions of three-dimensional analytical temperature field, the dimensionless width, leading and rear edges, and depth of the fusion zone are analytically found to be a function of the dimensionless parameters governing beam power per unit penetration, location of the workpiece surface and shape of the keyhole. The dimensionless parameters governing the keyhole shape can be evaluated from a force balance at the keyhole base. The results show the effects of welding parameters, such as the dimensionless beam power, Peclet number, cavity opening radius, Biot number, thickness of workpiece, and the parameter approximating convection, on the shape of the fusion zone and the temperature of keyhole surface. A significant difference in the fusion zone shapes predicted between the line-source solution and this work indicates the strong effects of three-dimensional heat transfer. Agreement between the prediction from this work and available experimental data is achieved.
Plaisted, Michael David
09 February 2015
Three testing procedures were developed in this research to allow expeditious characterization of soil unsaturated flow properties using steady state centrifuge methods. The first testing procedure, referred to as the “instrumented” procedure, focuses on using in-flight measurement of the suction and volumetric water content of soil samples under centrifugation. The measurements are used to calculate the soil water retention curve and hydraulic conductivity function (K-function) of soil samples. A good agreement was found between results determined using the “instrumented” procedure and standard testing methods. Several possible sources of inaccuracy were determined with the “instrumented” procedure. The void ratio, the changes of which were not measured, was found to decrease during centrifugation and the lower boundary condition, which was not accounted for in the evaluation, was found to affect a large portion of the sample. In order to improve the accuracy of results, two additional testing procedures were developed that accounted for these issues and incorporated the void ratio of the soil as an additional variable. The first additional procedure was used to measure the soil water retention surface (SWRS) of soil samples while the second was used to measure the unsaturated hydraulic conductivity surface (K-surface) of soil samples. Both new procedures, referred to as the “hydrostatic” and “imposed flow” procedures, were used to characterize the unsaturated flow properties of a low plasticity clay (“RMA” soil). The unsaturated flow characteristics of the RMA soil were evaluated for a wide range of void ratio and three compaction moisture conditions. As a result, the effects of void ratio and compaction moisture content on the unsaturated flow characteristics could be determined for the RMA soil. The compaction water content was shown to have significant effects on both the retention behavior and the unsaturated hydraulic conductivity of the RMA soil. In general, increases in compaction water content resulted in a decrease of large pore sizes in the soil, resulting in higher water retention and lower unsaturated hydraulic conductivity. The void ratio was found to have comparatively lesser, but still significant, effects on both retention and conductivity characteristics. Specifically, decreases in void ratio were shown to reduce the unsaturated hydraulic conductivity. In addition, decreases in void ratio were shown to result in either increases or decreases on the soil water retention, depending on the level of suction in the soil. A good agreement was found between results obtained using standard methods and those from the hydrostatic and imposed flow procedures. Accordingly, steady state centrifuge methods were ultimately found to provide a both expeditious and accurate method for characterizing the unsaturated flow properties of soil. / text
The Method of Distributed Volumetric Sources for Forecasting the Transient and Pseudo-steady State Productivity of Multiple Transverse Fractures Intersected by a Horizontal WellFan, Diangeng 2010 December 1900 (has links)
This work of well performance modeling is focused on solving problems of transient and pseudo-steady state fluid flow in a rectilinear closed boundaries reservoir. This model has been applied to predict and to optimize gas production from a horizontal well intercepted by multiple transverse fractures in a bounded reservoir, and it also provides well-testing solutions. The well performance model is designed to provide enhanced efficiency with the same reliability for pressure transient analysis, and well performance prediction, especially in complex well fracture configuration. The principle is to simplify the calculation of the pressure response to an instantaneous withdraw, which happens in other fractures, within a shorter computational time. This pressure response is substituted with the interaction between the two whole fractures. This method is validated through comparison to results of rigorous Distributed Volumetric Sources (DVS) method in simple symmetric fracture configuration, and to results of field production data for complex well/fracture configuration of a tight gas reservoir. The results show a good agreement in both ways. This model indicates the capability to handle the situations, such as: various well drainages, asymmetry of the fracture wings, and curved horizontal well. The advantage of this well performance model is to provide faster processing - reducing the computational time as the number of fractures increase. Also, this approach is able to be applied as an optimization and screening tool to obtain the best fracture configurations for reservoir development of economically marginal fields, in terms of the number and dimensions of fractures per well, also with external economic and operational constraints.
07 July 2003
Abstract Spray cooling is frequently encountered in a number of engineering applications. An experimental study was made to investigate the effect of liquid sprays used to cool a hot surface. Both pure water and R-134a were served as a working medium sprayed from a single circular nozzle onto a Pt plated surface of an electrically heated surface. Spray cooling tests were performed for steady state and transient experiment. Cooling characteristics curves were obtained over a range of Weber number(Water¡G80¡B148¡B231¡AR-134a¡G50¡B96¡B152),pressure drop of liquid(0.1Mpa¡B0.15Mpa¡B0.2Mpa),degree of subcooling (Water¡G55¢J¡B60¢J¡AR-134a¡G2¢J¡B4¢J) and initial temperature(Water¡G240¢J¡AR-134a¡G60¢J). Thermal design data of high performance as well as more and further physical insight of the above-stated spray cooling heat transfer can be acquired. The results will hopefully be helpful not only for the academia but for the industry.
The role of residue Y955 of mitochondrial DNA polymerase [gamma] in nucleotide binding and discriminationEstep, Patricia Ann 14 February 2012 (has links)
The human mitochondrial polymerase (pol γ) is a nuclearly-encoded polymerase that is solely responsible for the faithful replication and repair of the mitochondrial genome. The Y955C mutation in pol γ results in early onset progressive external ophthalmoplegia, premature ovarian failure, and Parkinson’s disease. It is believed that the position of this Y955 residue on the catalytic helix in the polymerase makes it responsible for stabilizing the incoming nucleotide. I have investigated the kinetic effect of the Y955C mutation. Mutation of the tyrosine to a cysteine resulted in a decreased maximum rate of polymerization and increased the dissociation constant for incoming nucleotide. In turn, this decreased catalytic efficiency by 30 to 100-fold. In addition, the polymerase did not incorporate all bases with the same efficiency, it was most efficient when incorporating dGTP opposite a dC, but showed less efficient catalysis when faced with an A:T or T:A base-pair. The polymerase also showed reduced discrimination against misincorporation events. However, when presented with an oxidatively-damaged base, 8-oxo-deoxyguanosine, the polymerase chose to incorporate the base in the correct conformation opposite a dC, discriminating against the mutagenic incorporation of 8-oxo-dGTP opposite a dA. The results presented in this thesis suggest that the severe clinical symptoms of patients with this mutation are at least due in part to the reduced efficiency and discrimination of this polymerase γ mutation. / text
A GIS-based estimation of steady-state non-point source bacteria pollution in the Lower Rio Grande below Falcón ReservoirLynch, Robin Shaw 06 August 2012 (has links)
This report estimates the steady-state, non-point source bacteria pollution along the international river system of the Lower Rio Grande / Río Bravo between Falcón Reservoir and the Gulf of Mexico. The results from this report may be used by environmental agencies in the United States and México in order to develop a steady-state water quality model of the bacterial load in this river system. This report creates a GIS-based estimation of the steady-state, non-point source pollution from sources such as failing septic tanks, untreated sewage, grazing animals, and wildlife in the watershed. This report also provides recommendations for environmental agencies when developing the water quality model. The results and methodology developed for this report may be used as part of the Lower Rio Grande / Río Bravo Watershed Initiative, a binational pilot project to develop a plan to restore and protect the quality of the Rio Grande/Río Bravo. / text
Page generated in 0.1293 seconds