Doctor of Philosophy

dissertationThe primary focus of this work is an assessment of heat transfer to and from a reversible thermosiphon imbedded in porous media. The interest in this study is the improvement of underground thermal energy storage (UTES) system performance with an innovative ground coupling using an array of reversible (pump-assisted) thermosiphons for air conditioning or space cooling applications. The dominant mechanisms, including the potential for heat transfer enhancement due to natural convection, of seasonal storage of "cold" in water-saturated porous media is evaluated experimentally and numerically. Winter and summer modes of operation are studied. A set of 6 experiments are reported that describe the heat transfer in both fine and coarse sand in a 0.32 cubic meter circular tank, saturated with water, under freezing (due to heat extraction) and thawing (due to heat injection) conditions, driven by the heat transfer to or from the vertical thermosiphon in the center of the tank. It was found that moderate to strong natural convection was induced at Rayleigh numbers of 30 or higher. Also, near water freezing temperatures (0°C-10°C), due to higher viscosity of water at lower temperatures, almost no natural convection was observed. A commercial heat transfer code, ANSYS FLUENT, was used to simulate both the heating and cooling conditions, including liquid/solid phase change. The numerical simulations of heat extraction from different permeability and temperature water-saturated porous media showed that enhancement to heat transfer by convection becomes significant only under conditions where the Rayleigh number is in the range of 100 or above. Those conditions would be found only for heat storage applications with higher temperatures of water (thus, its lower viscosity) and large temperature gradients at the beginning of heat injection (or removal) into (from) soil. For "cold" storage applications, the contribution of natural convection to heat transfer in water-saturated soils would be negligible. Thus, the dominant heat transfer mechanism for air conditioning applications of UTES can be assumed to be conduction. An evaluation of the potential for heat transfer enhancement in air-saturated media is also reported. It was found that natural convection in soils with high permeability and air saturations near 1 becomes more important as temperatures drop significantly below freezing

Electromechanical Evaluation of a Double-Core Motor With Ceramic Elements

This study investigated the prospect of embedding ceramic elements in the body of a double-core transverse-flux machine to mitigate heat in the machine’s coil. In addition, the ceramic elements were used to hold the coil above the permanent magnets in order to reduce the negative flux leakage of the magnets into the coil. Three types of ceramics were investigated for this purpose: glass ceramic, aluminum oxide, and silicon nitride. Steady-state thermal analysis demonstrated a significant temperature drop in the motor coil when the ceramic element was embedded in the design. The back electromotive force of the winding improved by 30% as a result of reduced leakage flux. Structural analysis of the motor demonstrated high endurance of the ceramic elements to thermal stress and motor vibration. The results of computer thermal analysis were verified in the laboratory by performing a test on a section of a motor

Techno-economic feasibility analysis of an extreme heat flux micro-cooler

Summary: An estimated 70% of the electricity in the United States currently passes through power conversion electronics, and this percentage is projected to increase eventually to up to 100%. At a global scale, wide adoption of highly efficient power electronics technologies is thus anticipated to have a major impact on worldwide energy consumption. As described in this perspective, for power conversion, outstanding thermal management for semiconductor devices is one key to unlocking this potentially massive energy savings. Integrated microscale cooling has been positively identified for such thermal management of future high-heat-flux, i.e., 1 kW/cm2, wide-bandgap (WBG) semiconductor devices. In this work, we connect this advanced cooling approach to the energy impact of using WBG devices and further present a techno-economic analysis to clarify the projected status of performance, manufacturing approaches, fabrication costs, and remaining barriers to the adoption of such cooling technology