Study On Heat Transfer And Flow Characteristics Of CO₂ Micro-channel Gas Cooler

As a natural substitution working fluid for CFCs and HCFCs, carbon dioxide is always the research hot spot. The carbon dioxide transcritical cycle has been successfully applied in automobile air conditioning and heat pump system. Using an integrated fin and micro-channel gas cooler as the research object, its heat transfer and flow characteristics is studied by both experimental and numerical method in present work, aiming to provide theory support to the application of micro-channel heat exchanger in CO₂ automobile air conditioning system.Compared with the traditional micro-channel heat exchanger, the fins are integrated with micro-channel for the integrated fin and micro-channel gas cooler. The local heat transfer and flow characteristics should be understood on one hand. On the other hand, research of CO₂ micro-channel gas cooler performance in refrigerant system and the model is also the focus of this paper.The main difference between integrated fin and micro-channel heat exchanger and traditional micro-channel heat exchanger is air-side, and the air-side heat resistance occupied a great part of the total heat resistance. By CFD method, the air-side heat transfer and flow characteristic is simulated to investigate the effect of fin length, fin height, fin pitch and fin thickness. It is found that fin length and fin pitch have great effect on heat transfer performance. The enhanced heat transfer enhancement mechanism is analyzed and optimal fin parameters are obtained. The heat transfer and flow empirical correlations are summed up to give guidance to heat exchanger design.The heat transfer and flow characteristics of supercritical carbon dioxide in the micro-channel are complex and few research have been done. In this work, CFD method is used to study in depth the heat transfer and flow mechanism of supercritical carbon dioxide in the micro-channel. The mainly parameters that affect the supercritical carbon dioxide heat transfer such as operation pressure, mass flow, flow direction and wall temperature are investigated in detail. .It is suggested the factors that affect the heat transfer characteristics of supercritical carbon dioxide in micro-channel can be attributed to the dramatic changes of physical properties.Based on the CFD research of local heat transfer and flow characteristics, experiment is carried out to study the performance of integrated fin and micro-channel heat exchanger in CO₂ refrigerant system. The effects of working pressure, refrigerant mass flow rate, air inlet temperature and velocity on heat capacity and pressure drop are investigated. COP of the system is also analyzed to research the heat exchanger performance.Finally, the numerical model of CO₂ micro-channel gas cooler is developed. Compared with the experiment results, the model predicted the gas cooler heating capacity within 5% and pressure drop on the refrigerant-side within 8% at the range of experimental error. It shows the model is accurate enough to predict performance of the heat exchanger. The validated model is used to investigate the impact of the flow setting and airside mal-distribution on the performance of the heat exchanger give guidance to heat exchanger design. Based on the model, the heat transfer performance of the integrated fin and micro-channel gas cooler is compared with traditional micro-channel gas cooler. It indicates that the integrated fin and micro-channel gas cooler has good performance.The present work researches on the integrated fin and micro-channel gas cooler and analyzes the heat transfer and flow characteristics of CO₂ micro-channel gas cooler. The theory of CO₂ micro-channel gas cooler is carried out to establish the basis of integrated fin and micro-channel gas cooler using in CO₂ automotive air conditioning.