Browsing by Author "Sewall, Evan Andrew"

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    Development of a Thermal Management Methodology for a Front-End DPS Power Supply
    Sewall, Evan Andrew (Virginia Tech, 2002-10-11)
    Thermal management is a rapidly growing field in power electronics today. As power supply systems are designed with higher power density levels, keeping component temperatures within suitable ranges of their maximum operating limits becomes an increasingly challenging task. This project focuses on thermal management at the system level, using a 1.2 kW front-end power converter as a subject for case study. The establishment of a methodology for using the computer code I-deas to computationally simulate the thermal performance of component temperatures within the system was the primary goal. A series of four benchmarking studies was used to verify the computational predictions. The first test compares predictions of a real system with thermocouple measurements, and the second compares computational predictions with infrared camera and thermocouple measurements on a component mounted to a heat sink. The third experiment involves using flow visualization to verify the presence of vortices in the flow field, and the fourth is a comparison of computational temperature predictions of a DC heater in a controlled flow environment. A radiation study using the Monte Carlo ray-trace method for radiation heat transfer resulted in the reduction of some component temperature predictions of significant components. This radiation study focused on an aspect of heat transfer that is often ignored in power electronics. A component rearrangement study was performed to establish a set of guidelines for component placement in future electronic systems. This was done through the use of a test matrix in which the converter layout was varied a number of different ways in order to help determine thermal effects. Based on the options explored and the electrical constraints on the circuit, an optimum circuit layout was suggested for maximum thermal performance. This project provides a foundation for the thermal management of power electronics at the system level. The use of I-deas as a computational modeling tool was explored, and comparison of the code with experimental measurements helped to explore the accuracy of I-deas as a system level thermal modeling tool.
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    Large Eddy Simulations of Flow and Heat Transfer in the Developing and 180° Bend Regions of Ribbed Gas Turbine Blade Internal Cooling Ducts with Rotation - Effect of Coriolis and Centrifugal Buoyancy Forces
    Sewall, Evan Andrew (Virginia Tech, 2005-11-28)
    Increasing the turbine inlet temperature of gas turbine engines significantly increases their power output and efficiency, but it also increases the likelihood of thermal failure. Internal passages with tiny ribs are typically cast into turbine blades to cool them, and the ability to accurately predict the flow and heat transfer within these channels leads to higher design reliability and prevention of blade failure resulting from local thermal loading. Prediction of the flow through these channels is challenging, however, because the flow is highly turbulent and anisotropic, and the presence of rotational body forces further complicates the flow. Large Eddy Simulations are used to study these flows because of their ability to predict the unsteady flow effects and anisotropic turbulence more reliably than traditional RANS closure models. Calculations in a stationary duct are validated with experiments in the developing flow, fully developed, and 180° bend regions to establish the accuracy and prediction capability of the LES calculations and to aid in understanding the major flow structures encountered in a ribbed duct. It is found that most flow and heat transfer calculations come to within 10-15% of the measurements, typically showing excellent agreement in all comparisons. In the developing flow region, Coriolis effects are found to destabilize turbulence and increase heat transfer along the trailing wall (pressure side), while decreasing leading wall heat transfer by stabilizing turbulence. Coriolis forces improve flow turning in the 180° bend by shifting the shape of the separated recirculation zone at the tip of the dividing wall and increasing the mainstream flow area. In addition, turbulence is attenuated near the leading wall throughout the bend, while Coriolis forces have little effect on trailing wall turbulence in the bend. Introducing and increasing centrifugal buoyancy in the developing flow region increases trailing wall heat transfer monotonically. Along the leading wall, buoyancy increases the size of the recirculation zones, shifting the peak heat transfer to a region upstream of the rib, which decreases heat transfer at low buoyancy parameters but increases it as the buoyancy parameter is increased beyond a value of 0.3. Centrifugal buoyancy in the 180° bend initially decreases the size of the recirculation zone at the tip of the dividing wall, increasing flow area and decreasing flow impingement. At high buoyancy, however, the recirculation zone shifts to the middle of the bend, increasing flow resistance and causing strong flow impingement on the back wall. The Boussinesq approximation is used in the buoyancy calculations, but the accuracy of the approximation comes into question in the presence of large temperature differences. A variable property algorithm is developed to calculate unsteady low speed flows with large density variations resulting from large temperature differences. The algorithm is validated against two test cases: Rayleigh-Bénard convection and Poiseuille-Bénard flow. Finally, design issues in rotating ribbed ducts are considered. The fully developed assumption is discussed with regard to the developing flow region, and controlling the recirculation zone in the 180° bend is considered as a way to determine the blade tip heat transfer and pressure drop across the bend.