An extensive review of the literature indicates a lack of systematic study of supercritical CO₂ gas cooling and no prior work on CO ₂/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 CO ₂ 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.