Title of Dissertation: HEAT TRANSFER AND MECHANICAL DESIGN ANALYSIS OF SUPERCRITICAL GAS COOLING PROCESS OF CO2 IN MICROCHANNELS

Title of Dissertation: HEAT TRANSFER AND MECHANICAL DESIGN ANALYSIS OF SUPERCRITICAL GAS COOLING PROCESS OF CO2 IN MICROCHANNELS Guohua Kuang, Doctor of Philosophy, 2006 Directed By: Professor Michael Ohadi, Department of Mechanical Engineering Professor Balakumar Balachandran (Co-Advisor), Department of Mechanical Engineering An extensive review of the literature indicates a lack of systematic study of supercritical CO2 gas cooling and no prior work on CO2/oil mixture in supercritical region, suggesting a lack of fundamental understanding of supercritical gas cooling process and a lack of comprehensive data that would help quantify the performance potential of CO2 microchannel heat exchangers for engineering applications. This dissertation presents a systematic and comprehensive study on gas cooling heat transfer characteristics of supercritical CO2 in microchannels. Semi-empirical correlation is developed for predicting heat transfer performance of supercritical CO2 in microchannels. The effect of oil addition on heat transfer performance has been experimentally investigated as well. It is shown that presence of lubricant oil mixed with supercritical CO2 in the heat exchangers can substantially affect heat transfer and pressure drop coefficients. Because of the outstanding performance of supercritical CO2 and its promising potential as a substitute for current refrigerants, attention has been paid to the design of CO2 microchannel heat exchangers. The extensive review of the literature also indicates no previous study in systematically developing a simulation model for structural design of microchannel heat exchangers. The dissertation extends the research to the mechanical design analysis of microchannel heat exchangers. A finite-element method (FEM) based mechanical design analysis of tube-fin heat exchangers is carried out to develop a simulation model of the heat exchangers. The solid modeling and simulation scheme can be served as a guide for mechanical design of CO2 heat exchangers. Experiments are conducted to validate the developed models as well. HEAT TRANSFER AND MECHANICAL DESIGN ANALYSIS OF SUPERCRITICAL GAS COOLING PROCESS OF CO2 IN MICROCHANNELS