Thesis--Wisconsin. / Includes bibliographical references (leaf 167).

The effects of viscous dissipation on the flow phenomena and heat transfer rate for fully developed laminar flow through vertical ducts and passages has been analysed under the condition of combined free and forced convection. The fluid properties are considered to be constant except for the variation of density in the buoyancy term of the momentum equation. The thermal boundary condition of uniform heat flux per unit length in the flow direction has been considered. The investigation is carried out for two geometries; (a) Circular ducts and (b) Concentric annuli. The governing momentum and non-linear energy equations are solved for the circular duct by three methods; (i) Power Series Method (ii) Galerkin's Method and (iii) Numerical Integration Method. The solutions for the concentric annuli are obtained by Numerical Integration Method. Results for the velocity and temperature distribution in the flow field are obtained, and information of engineering interest like Nusselt numbers have been evaluated. For combined free and forced convection, the momentum and energy equations are coupled, and hence viscous dissipation affects both the velocity and temperature fields. The effect of viscous dissipation on the velocity field is to reduce the flow velocity near the heated wall(s) and thus it counteracts the effect of free convection on the velocity field for the present study of heating in upflow. The effect of viscous dissipation on the temperature field is to act as a heat source in the fluid and reduce the temperature differences in the system. Viscous dissipation opposes the externally impressed heating and reduces the heat transfer rate when the surface transfers heat to the fluid. Consequently, lower Nusselt number values are obtained when viscous dissipation is taken into consideration. The quantitative effect of viscous dissipation on Nusselt number is found to be small for the case of circular ducts. However, for flow through annular passages and for the corresponding values of the same parameters, the effect of viscous dissipation on the heat transfer rate may not be ignored. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

This thesis examines the underlying physics that gives rise to the Hawking and Unruh effects. The Rindler coordinate system is constructed from a physical argument that shows how an observer would actually go about building such a coordinate system out of scaffolding and clocks. Quantum theory is discussed in detail with particular relevance to quantum entanglement as this is an important issue relating to information loss in black holes. The thesis demonstrates the general impossibility of utilising quantum entanglement to transmit information faster than light. Bell's theorem is also reviewed from the perspective of anti-correlated spin-half particles. This theorem shows the impossibility of describing nature by a local hidden variable theory, and hence emphasises the importance of the topic of information loss in black holes as a bridge between general relativity and quantum theory. The Unruh effect is a purely quantum field theoretic effect that displays considerable mathematical similarities to the Hawking effect. The effects are nevertheless quite dissimilar in some respects and this thesis examines some of these differences. The other aim of the thesis is to discuss the possible loss of information in a black hole. The Hawking effect raises the possibility that a black hole may evaporate and potentially disappear completely. This raises a significant problem related to how the information that entered the black hole may escape, if at all. If information cannot escape the black hole then this implies a violation of one of the principles of quantum mechanics: a pure quantum state cannot undergo unitary evolution to become a thermal distribution of radiation but this is what the Hawking effect essentially predicts. These two conclusions are new and are important contributions to the understanding of the coupling between gravity and quantum theory. The thesis also looks at a number of subsidiary topics to do with the underlying physics of these effects along the way, always with an emphasis on the physical. In particular, the method for quantizing a field is developed in a physical manner by examining the continuum limit of a quantized discretely modelled string. Two other topics within the thesis that are of interest are a demonstration of the coordinate independence of the Euler-Lagrange equations and a heuristic method of 'deriving' the Lorentz transformation equations that is presented in an appendix. These two presentations are new and have not appeared elsewhere to my knowledge. / Doctor of Philosophy (PhD) (Science)

No description available.

Thesis (Ph.D.)--The University of Utah, 2008. / Includes bibliographical references.

Thesis (M.S. in Electrical Engineering)--Naval Postgraduate School, March 2006. / Thesis Advisor(s): Alexander Julian. "March 2006." Includes bibliographical references (p. 103-104). Also available online.

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.

No description available.

Thesis (M.Eng. (Materials Science and Metallurgical Engineering)) -- University of Pretoria, 2000.

Thesis (M.S.)--Ohio University, August, 1996. / Title from PDF t.p.