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This paper demonstrates that the implementation of Flash Gas Bypass method can improve the performance of conventional direct expansion R134a mobile air-conditioning system with a microchannel evaporator. This method uses flash gas tank after expansion valve to separate and bypass flash refrigerant vapor around the evaporator, and feed the evaporator with only liquid refrigerant. Pressure drop is reduced and refrigerant distribution is significantly improved, resulting in higher evaporator effectiveness and evaporation pressure. Both lower pressure drop and lifted evaporation pressure allows the compressor to work with lower pressure ratio, saving required compressor work. An experimental comparison of the direct expansion system shows that Flash Gas Bypass method increases the cooling capacity and COP at the same time by up to 16% and 11%, respectively.

This paper develops a model for a parallel microchannel evaporator that incorporates quality variation at the tube inlets and variable mass flow rates among tubes. The flow distribution is based on the equal pressure drop along each flow path containing headers and tubes. The prediction of pressure drop, cooling capacity, and exit superheat strongly agree with 48 different experimental results obtained in four configurations using R134a. Predicted temperature profiles are very close to infrared images of actual evaporator surface. When compared to the uniform distribution model (that assumes uniform distribution of refrigerant mass flow rate and quality) results from the new model indicate superior prediction of cooling capacity, and exit superheat. Model results indicate maldistribution of refrigerant mass flow rate among the parallel tubes, caused primarily by pressure drop in the outlet header.