Thesis (Ph.D.)--McMaster University, 2003. / Advisor: G. P. Johari. Includes bibliographical references (leaves 191-199). Also available via World Wide Web.

Thesis (Ph. D.)--Drexel University, 2006. / Includes abstract and vita. Includes bibliographical references (leaves 157-163).

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 155-161).

Thesis (Ph. D.)--Drexel University, 2007. / Includes abstract and vita. Includes bibliographical references (leaves 140-145).

by K. E. Bett, J. S. Rowlinson and G. Saville. / Includes bibliographical references and index.

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<p> Magnetic refrigeration is one of the alternative cooling technologies that is environmental friendly and has high theoretical coefficient of performance values. This thesis study focuses on magnetic field and heat transfer enhancement of a reciprocating-type magnetic refrigeration system. A set of NdFeB 52 MGOe permanent magnets were employed to form a Halbach magnet array. Gadolinium (Gd) was used as the magnetocaloric material. It was placed concentrically within the Halbach array aperture with the working fluid running through the gap in between the magnet assembly and Gd yielding annular flow. Three different annular flow geometries namely; circular, octagonal and hexagonal cross-sections were studied. Magnetization process was analyzed theoretically, numerically and experimentally for k = 4 configuration. Numerical analysis was done by Finite Element Method Magnetics (FEMM), theoretical analysis was conducted by a mathematical model, and experimental analysis was performed on a Halbach magnet array. Obtained magnetic field results were used to calculate corresponding entropy changes and heat flux values. These values were compared to numerical heat transfer results from ANSYS and a close agreement between results were observed.</p><p>

<p> The work presented in this thesis is a topological approach for understanding the formation of structures from the liquid state. The strong difference in the thermal transport properties of non- crystalline solid states as compared to crystalline counterparts is considered within this topological framework. Herein, orientational order in undercooled atomic liquids, and derivative solid states, is identified with a quaternion order parameter. </p><p> In light of the four-dimensional nature of quaternion numbers, spontaneous symmetry breaking from a symmetric high-temperature phase to a low-temperature phase that is globally orientationally ordered by a quaternion order parameter is forbidden in three- and four-dimensions. This is a higher-dimensional realization of the Mermin-Wagner theorem, which states that continuous symmetries cannot be spontaneously broken at finite temperatures in two- and one-dimensions. </p><p> Understanding the possible low-temperature ordered states that may exist in these scenarios (of restricted dimensions) has remained an important problem in condensed matter physics. In approaching a topological description of solidification in three-dimensions, as characterized by a quaternion orientational order parameter, it is instructive to first consider the process of quaternion orientational ordering in four-dimensions. This 4D system is a direct higher-dimensional analogue to planar models of complex <i>n</i>&ndash;vector (<i> n</i> = 2) ordered systems, known as Josephson junction arrays. </p><p> Just as Josephson junction arrays may be described mathematically using a lattice quantum rotor model with <i>O</i>(2) symmetry, so too can 4D quaternion <i>n</i>&ndash;vector (<i>n</i> = 4) ordered systems be modeled using a lattice quantum rotor model with <i> O</i>(4) symmetry. <i>O</i>(<i>n</i>) quantum rotor models (that apply to <i>n</i>&ndash;vector ordered systems that exist in restricted dimensions) include kinetic and potential energy terms. It is the inclusion of the kinetic energy term that leads to the possible realization of two distinct ground states, because the potential and kinetic energy terms cannot be minimized simultaneously. </p><p> The potential energy term is minimized by the total alignment of <i> O</i>(n) rotors in the ground state, such that it is perfectly orientationally ordered and free of topological defects. On the other hand, minimization of the kinetic energy term favors a low-temperature state in which rotors throughout the system are maximally orientationally disordered. </p><p> In four-dimensions, the <i>O</i>(4) quantum rotor model may be used to describe a 4D plastic crystal that forms below the melting temperature. A plastic crystal is a mesomorphic state of matter between the liquid and solid states. The realization of distinct low-temperature states in four-dimensions, that are orientationally-ordered and orientationally-disordered, is compared with the realization of phase-coherent and phase-incoherent low-temperature states of <i>O</i>(2) Josephson junction arrays. Such planar arrays have been studied extensively as systems that demonstrate a topological ordering transition, of the Berezinskii-Kosterlitz-Thouless (BKT) type, that allows for the development of a low-temperature phase-coherent state. </p><p> In <i>O</i>(2) Josephson junction arrays, this topological ordering transition occurs within a gas of misorientational fluctuations in the form of topological point defects that belong to the fundamental homotopy group of the complex order parameter manifold (<i>S¹</i>). In this thesis, the role that an analogous topological ordering transition of third homotopy group point defects in a four-dimensional <i>O</i>(4) quantum rotor model plays in solidification is investigated. Numerical Monte-Carlo simulations, of the four-dimensional <i>O</i>(4) quantum rotor model, provide evidence for the existence of this novel topological ordering transition of third homotopy group point defects. </p><p> A non-thermal transition between crystalline and non-crystalline solid ground states is considered to exist as the ratio of importance of kinetic and potential energy terms of the <i>O</i>(4) Hamiltonian is varied. In the range of dominant potential energy, with finite kinetic energy effects, topologically close-packed crystalline phases develop for which geometrical frustration forces a periodic arrangement of topological defects into the ground state (major skeleton network). In contrast, in the range of dominant kinetic energy, orientational disorder is frozen in at the glass transition temperature such that frustration induced topological defects are not well-ordered in the solid state. </p><p> Ultimately, the inverse temperature dependence of the thermal conductivity of crystalline and non-crystalline solid states that form from the undercooled atomic liquid is considered to be a con- sequence of the existence of a singularity at the point at which the potential and kinetic energy terms become comparable. This material transport property is viewed in analogue to the electrical transport properties of charged <i>O</i>(2) Josephson junction arrays, which likewise exhibit a singularity at a non-thermal phase transition between phase-coherent and phase-incoherent ground states. </p><p>

Single phase geothermal fluids sampled in 2007 from 1500m depth in Well RN-12 of the Reykjanes geothermal system in Iceland show large differences in dissolved copper, zinc and iron concentrations when compared with fluid sampled from the wellhead. Equilibrium modeling of the samples taken at depth indicate that the fluid was supersaturated in sulfide minerals even at moderately acidic pH values, suggesting that the deep samples, as collected, are out of equilibrium. Wellhead sample reconstructions indicate a well-bottom pH of about 5.5 at 295°C, but a pH of 3.6 at saturation with chalcopyrite, bornite, pyrite and sphalerite would be required to account for the large concentrations of Cu, Zn and Fe in the down-well samples. This acidic value needed for the high metal concentrations is not realistic in this naturally buffered system, likely indicating contamination in the downhole analysis.

Thesis submitted in fulfilment of the requirements for the degree Doctor of Technology: Mechanical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology 2014 / Thermoacoustic heat engines provide a practical solution to the problem of heat management where heat can be pumped or spot cooling can be induced. This is new among emerging technology with a strong potential towards the development of sustainable and renewable energy systems by utilising solar energy or wasted heat. The most inhibiting characteristic of current thermoacoustic cooling devices is the lack of efficiency. Although simple to fabricate, the designing of thermoacoustic coolers involves significant technical challenges. The stack has been identified as the heart of the device where the heat transfer takes place. Improving its performance will make thermoacoustic technology more attractive. Existing efforts have not taken thermal losses to the surroundings into account in the derivation of the models. Although thermal losses can be neglected for large-scale applications, these losses need to be adequately covered for small-scale applications. This work explores the use of a multi-objective optimisation approach to model and to optimise the performance of a simple thermoacoustic engine. This study aims to optimise its geometrical parameters—namely the stack length, the stack height, the stack position, the number of channels and the plate spacing—involved in designing thermoacoustic engines. System parameters and constraints that capture the underlying thermoacoustic dynamics have been used to define the models. Acoustic work, viscous loss, conductive heat loss, convective heat loss and radiative heat loss have been used to measure the performance of the thermoacoustic engine. The optimisation task is formulated as a five-criterion mixed-integer nonlinear programming problem. Since we optimise multiple objectives simultaneously, each objective component has been given a weighting factor to provide appropriate user-defined emphasis. A practical example is provided to illustrate the approach. We have determined a design statement of a stack describing how the design would change if emphasis is placed on one objective in particular. We also considered optimisation of multiple objective components simultaneously and identified global optimal solutions describing the stack geometry using the augmented ε-constraint method. This approach has been implemented in GAMS (General Algebraic Modelling System). In addition, this work develops a novel mathematical programming model to optimise the performance of a simple thermoacoustic refrigerator. This study aims to optimise its geometrical parameters—namely the stack position, the stack length, the blockage ratio and the plate spacing—involved in designing thermoacoustic refrigerators. System parameters and constraints that capture the underlying thermoacoustic dynamics have been used to define the models. The cooling load, the coefficient of performance and the acoustic power loss have been used to measure the performance of the device. The optimisation task is formulated as a three-criterion nonlinear programming problem with discontinuous derivatives (DNLPs). Since we optimise multiple objectives simultaneously, each objective component has been given a weighting factor to provide appropriate user-defined emphasis. A practical example is provided to illustrate the approach. We have determined a design statement of a stack describing how the geometrical parameters described would change if emphasis is placed on one objective in particular. We also considered optimisation of multiple objective components simultaneously and identified global optimal solutions describing the stack geometry using a lexicographic multi-objective optimisation scheme. The unique feature of the present mathematical programming approach is to compute the stack geometrical parameters describing thermoacoustic refrigerators for maximum cooling or maximum coefficient of performance. The present study highlights the importance of thermal losses in the modelling of small-scale thermoacoustic engines using a multi-objective approach. The proposed modelling approach for thermoacoustic engines provides a fast estimate of the geometry and position of the stack for maximum performance of the device. The use of a lexicographic method introduced in this study improves the modelling and the computation of optimal solutions and avoids subjectivity in aggregation of weight to objective functions in the formulation of mathematical models. The unique characteristic of this research is the computing of all efficient non dominated Pareto optimal solutions allowing the decision maker to select the most efficient solution. The present research experimentally examines the influence of the stack geometry and position on the performance of thermoacoustic engines and thermoacoustic refrigerators. Thirty-six different cordierite honeycomb ceramic stacks are studied in this research. The influence of the geometry and the stack position has been investigated. The temperature difference across the stack and radiated sound pressure level at steady state are considered indicators of the performance of the devices. The general trends of the proposed mathematical programming approach results show satisfactory agreement with the experiment. One important aspect revealed by this study is that geometrical parameters are interdependent and can be treated as such when optimising the device to achieve its highest performance. The outcome of this research has direct application in the search for efficient stack configurations of small-scale thermoacoustic devices for electronics cooling.