Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

An important component in understanding the laser sintering process is knowledge of the thermal properties of the processed material. Thermal conductivity measurements of pure polyamide 12 and polyamide11 with multi-wall carbon nanotubes were conducted based on transient plane source technology using a Hot Disk® TPS500 conductivity measurement device. Polyamide powder was packed to three different densities in nitrogen at room temperature. Thermal diffusivity and conductivity were measured from 40°C to 170°C for both fresh powder and previously heated (“recycled”) powder. The fresh powder tests revealed that thermal conductivity increased linearly with temperature whereas for previously heated powder, more constant and higher thermal conductivity was observed as it formed a powder cake. Tests were also performed on fully dense polyamide 12 to establish a baseline. Polyamide 12 powder had a room-temperature thermal conductivity of approximately 0.1 W/mK which increased with temperature, whereas the bulk laser sintered polyamide 12 room-temperature value was 0.26 W/mK and generally decreased with increasing temperature. / text

Electron heat transport at the scale of electron gyroradius are investigated via numerical simulation of a fluid model and a role of E x B shear flow with intermediate E x B shearing rate is explored in Euler's equation. The anomalous transport, enhanced transport due to turbulent electro-magnetic fields caused by plasma instabilities, has been a focus of the inter-national fusion research communities. Among the instabilities, the drift-type instabilities from the pressure-gradient universal in magnetic fusion devicesare considered responsible for the anomalous transport. In the current status of wide use of wave heating on electrons and subsequent high core electron temperature, the turbulent heat loss through electrons has one of the most important science element in preventing the large fusion tokamaks from reaching breakeven in the past decade. The Electron Temperature Gradient fluid model consists of electrostatic potential, toroidal magnetic flux function and electron temperature (or pressure) describing electron drift waves. The fluid model proves to be highly useful to electron heat flux analysis in fusion machines. We analyze the discharges in National Spherical Tokamak eXperiment(NSTX) and Tokamak Configuration Variable (TCV) and found that the electron thermal diffusivities can be explained in terms of the mixing length argument based on electron gyroradius, linear theory and our nonlinear fluid simulation. The nonlinear fluid model predicts reasonable heat flux observed in the experiments. Based on the analysis, we investigate the dependences of the dynamics on the ratio of electron and ion temperature T[subscript e]/T[subscript i] and plasma beta [beta subscript e-]. The nonlinear dynamics such as saturation mechanism of the ETG turbulence and the electromagnetic dynamics in terms of micro-tearing at the scale of electron gyroradius are discussed briegly. In most of plasma confinement devices, the equilibrium radial electric field exists and the turbulence-generated electric field is observed. The coherent structure, called as zonal flow, has been know to be effective to suppress the micro-turbulence. But at intermediate E x B shear, where the vortex eddy turn-over time is comparable to E x B shearing rate, the suppression is weak and the flow shear can leads to vortex amplification through interaction of nonlinear dynamics and shear flow. / text

High oil-viscosity is a major concern for the recovery of oil from heavy-oil reservoirs. Introducing energy to the formation has proven to be an effective way of lowering the oil viscosity by raising the temperature in the formation. The application of low-frequency heating, also known as electrical resistance heating, is limited by water vaporization near the wellbore which breaks the conductive path to the reservoir, and limits the heating rate as well as the resulting production rates. Electromagnetic (EM) heating, also called high-frequency heating, can be used instead. Although its potential was recognized during the late 70’s, no simulation results or detailed modeling studies have yet been published that completely model the complex interactions of EM energy and multiphase flow. One of the main drawbacks of proposed models is the use of the EM adsorption coefficient as a constant regardless of the properties of the medium, which can obscure the important effect of this parameter on the extension of the reservoir area heated. This dissertation presents a multiphase, two-dimensional radial model that describes the three-phase flow of water, oil, and steam and heat flow in a reservoir within confining conductive formations. The model accounts for the appearance and/or disappearance of a phase, and uses the variation in temperature and water saturation to update the EM absorption coefficient. This model allows determining the temperature distribution and the productivity improvement from EM heating when multiple phases are present. For the numerical simulations of EM heating, I used COMSOL Multiphysics, a Lagrange-quadratic finite element simulator, and its partial differential equations (PDE) application. Several simulations were made for hypothetical reservoirs with different fluid and rock properties. Also, analytical solutions for a single-phase EM heating model were developed and used to validate the numerical solutions. Special attention is focused on reservoirs with characteristics for which steam injection is not attractive or feasible such as low permeability, thin-zone, and extra-heavy oil reservoirs. Results showed that EM heating is feasible based on the power source and frequency used to maintain an optimum absorption coefficient and to obtain higher production rates. Comparisons showed that cumulative oil production and recovery factor obtained by EM heating are better than what is achieved by cyclic steam stimulation (CSS) for reservoirs with the above mentioned characteristics. / text

The stable isotope compositions (chlorine, oxygen, and hydrogen), major anion concentrations, and major/minor cation concentrations of 37 thermal (any spring water with temperature at least 6.5° C above mean ambient air temperature) and mineral springs from the Cascade volcanic arc system were measured in order to better determine chlorine sources within the Cascades hydrothermal systems, and thus place better constraints on halogen flux through the subduction zone. Typically, most subduction zone flux calculations have been limited to the study of the erupted magmas and gases from fumarole vents, yet magmatic discharge through thermal springs may be considerable, particularly those in the often ignored forearc. Additionally, 9 geochemically well characterized lavas from across the Mt. St. Helens/Mt. Adams region of the Cascade arc (Leeman et al. 2001, 2005) were analyzed for their halogen concentrations, as well as their Cl stable isotope composition. Cl concentrations in the thermal springs range from 6 to 13,850 ppm and have δ37Cl values that range from -0.1‰ to + 1.9‰ (average = +0.8 ± 0.4‰; error = ± 0.2‰), with no systematic variation along or across the arc. The slightly positive values (~0.0 to +0.9‰) may be explained by fluid-rock interaction with underlying lithologic units, such as 37Cl-enriched volcanic sequences, and/or serpentinites or oceanic crust of accreted oceanic terranes. Another process possibly contributing to these positive δ37Cl values, particularly those with δ37Cl > 1‰, is magmatic HCl fractionation during degassing generating an enriched 37Cl vapor which mixes with thermal waters. We cannot completely rule out slab-derived altered oceanic crustal chlorine that has degassed into the springs, although most slab Cl is believed to have already been devolatilized from the slab before reaching sub-arc depths corresponding to longitudes where these springs are located at the surface. Lavas from the Columbia transect across the arc exhibit highest Cl concentrations at the volcanic front compared to the forearc and backarc. Br, like Cl, exhibits highest concentrations along the volcanic front. F and I show a progressive decrease in concentration from forearc to backarc which may demonstrate the putative early surge of fluids/fluid mobile element loss early in subduction at relatively shallow depth. δ37Cl values range from -0.1 to +0.8‰ (error = ± 0.2‰) and may reflect a component of assimilation of crustal material, or is derived from an enriched mantle, although we cannot completely rule out some isotopic fractionation and/or slab-derived chlorine. / text

The driving force behind research in alternative clean and renewable energy has been the desire to reduce emissions and dependence on fossil fuels. In the United States, ground vehicles account for 30% of total carbon emission, and significantly contribute to other harmful emissions. This issue causes environmental concerns and threat to human health. On the other hand, the demand on fossil fuel grows with the increasing energy consumption worldwide. Particularly in the United States of America, transportation absorbs 75% of this energy source. There is an urgent need to reduce the transportation dependence on fossil fuel for the purpose of national security. Polymer electrolyte membrane (PEM) fuel cells are strong potential candidates to replace the traditional combustion engines. Even though research effort has transferred the fuel cell technology into real‒world vehicle applications, there are still several challenges hindering the fuel cell technology commercialization, such as hydrogen supply infrastructure, cost of the fuel cell vehicles, on‒board hydrogen storage, public acceptance, and more importantly the performance, durability, and reliability of the PEM fuel cell vehicles themselves. One of the key factors that affect the fuel cell performance and life is the run‒time thermal and water management. The temperature directly affects the humidification of the fuel cell stack and plays a critical role in avoiding liquid water flooding as well as membrane dehydration which affect the performance and long term reliability. There are many models exists in the literature. However, there are still lacks of control‒oriented modeling techniques that describe the coupled heat and mass transfer dynamics, and experimental validation is rarely performed for these models. In order to establish an in‒depth understanding and enable control design to achieve optimal performance in real‒time, this research has explored modeling techniques to describe the coupled heat and mass transfer dynamics inside a PEM fuel cell. This dissertation is to report our findings on modeling the temperature dynamics of the gas and liquid flow in the porous media for the purpose of control development. The developed thermal model captures the temperature dynamics without using much computation power commonly found in CFD models. The model results agree very well with the experimental validation of a 1.5 kW fuel cell stack after calibrations. Relative gain array (RGA) was performed to investigate the coupling between inputs and outputs and to explore the possibility of using a single‒input single‒output (SISO) control scheme for this multi‒input multi‒output (MIMO) system. The RGA analyses showed that SISO control design would be effective for controlling the fuel cell stack alone. Adding auxiliary components to the fuel cell stack, such as compressor to supply the pressurized air, requires a MIMO control framework. The developed model of describing water transport in porous media improves the modeling accuracy by adding catalyst layers and utilizing an empirically derived capillary pressure model. Comparing with other control‒oriented models in the literature, the developed model improves accuracy and provides more insights of the liquid water transport during transient response. / text

There are 1,305 Superfund Sites on the United States Environmental Protection Agencies National Priorities List that may require remediation due to the environmental or human health risks associated with subsurface contamination. The contaminants present at these sites and others vary with respect to their physical and chemical properties which dictate the selection of appropriate remediation technologies. In-Situ Thermal Desorption (ISTD) has been studied as a remediation technique for removing many recalcitrant contaminants from soil. ISTD involves passing electrical current through heating elements in wells and removing contaminants through heater/vacuum wells. Heating occurs by heat conduction through the soil. At high temperatures, even relatively low volatility contaminants can be vaporized, removed by vacuum and treated with an on-site recovery system. The main objective of this research was to design and test a laboratory apparatus scaled to a typical ISTD field site and to use it to conduct experiments that could be used to aid in the validation of the STARS numerical simulator. A dimensional analysis was done on the governing energy balance equation to determine the most important scaling groups for the ISTD process so the laboratory experiments could be scaled up to the field. The laboratory apparatus was modeled after a symmetry element of the hexagonal field pattern and a triangular glass prism was constructed for heated sandpack experiments. Temperature data was measured in dry sand, sand partially saturated with water, and sand with both water and PCE added to it. The apparatus was made of glass so that the behavior of the PCE contaminant could be observed when the sand was heated. / text

Thermally rearranged polymers are formed from the solid-state thermal reaction of polyimides and polyamides that contain reactive groups ortho position to their diamine. These polymers have shown outstanding transport properties for gas separation applications. The thrust of this work is to critically examine the chemical and morphological structure of these polymers and to identify the fundamental contributions of gas sorption to permeability. To accomplish this goal, a series of TR polymers and TR polymer precursors have been synthesized and investigated for transport properties. As a function of conversion, diffusivity increases more dramatically than sorption, which explains the outstanding permeabilities observed for these samples. Modifications to the polymer backbone structure, which can be achieved by adding rigid functional groups such as hexafluoroisopropylidene-functional linking groups, can further be used to improve permeabilities. The precursor used to form TR polymers has dramatic effects on the final polymer transport properties. Despite having nearly identical polymer structure, TR polymers formed from polyamide precursors have lower combinations of permeability and selectivity than TR polymers formed from polyimide precursors. In addition to structure-property studies with TR polymers, this thesis also present comparisons of permeability, diffusivity, and sorption of sparingly soluble gases (i.e., hydrogen and helium) for hydrocarbon-based polymer, highly fluorinated polymers, perfluoropolymers, and a silicon-based polymer. An explanation for the unique transport properties of perfluoropolymers is presented from the standpoint of the solution-diffusion model, whereby perfluoropolymers have uniquely different solubility selectivities than hydrocarbon-based polymers. Additionally, a large database of sorption, diffusion, and permeability coefficients is used to determine the contributions of free volume on solubility selectivity in polymers. / text

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