Numerical Simulation Analysis Of CO₂ Microchannel Parallel Flow Evaporator

Replacing CFCs and HCFCs with carbon dioxide has always been a research hotspot.In recent years,in the field of automotive air conditioning and heat pump technology,the application of carbon dioxide transcritical cycle system has made great progress.As an important part of carbon dioxide transcritical cycle system,microchannel parallel flow evaporator is an important part to determine its system performance.Therefore,the study of microchannel parallel flow evaporator has a great significance for the automotive air conditioning industry.In this paper,the carbon dioxide microchannel parallel flow evaporator is taken as the research object,and the flow heat transfer characteristics of the microchannel flat tube side and the louver fin side are studied using Fluent software.The main research work and results are as follows:?1?The UDF program of carbon dioxide evaporation was introduced into Fluent,and its evaporative heat transfer characteristics in the parallel channel evaporator microchannel flat tube were simulated and studied.The results show that:the evaporation of carbon dioxide in the flat tube experiences intermittent flow,annular flow and foggy flow in turn;When the mass flow rate is increased,the evaporative heat transfer coefficient of carbon dioxide increases,and the dryness of the evaporation drying point decreases.When the mass flow rate is 500 kg·m^-2·s^-1,the corresponding heat transfer coefficient is the largest,and the maximum value is14436 W·m^-2·K^-1;With the increase of heat flow density and evaporation temperature,the evaporation heat transfer coefficient of carbon dioxide increased and the dryness of evaporation point decreased.?2?Fluent software was used to study the louver fin side,and a three-dimensional model of air and fin coupling was established to explore the distribution law of velocity,temperature and pressure when air flows through louver fins.The results show that:With the frontal wind speed increasesthe,the flow resistance increases,and the air flow efficiency increases,and the air deflected through the louver fins increases;With the air flowing along the fins,the air temperature decreases,the air temperature decreases and the heat transfer performance of the first half of the fin greater than the second half of the fins;With the frontal wind speed increases,the temperature difference between the air inlet and outlet decreases.When the speed is 1.0 m·s^-1,the temperature difference between the inlet and the outlet is 9.7 K.When the speed is 3.0 m·s^-1,the temperature difference between the inlet and the outlet is 5.2 K.?3?A new type of variable cross-section louver fin is proposed,and its flow and heat transfer characteristics are studied by Fluent software.The influence of structural parameters on its internal flow and heat transfer characteristics is discussed,and the optimization of structural parameters were obtained when the comprehensive performance factor JF is largest.It is concluded that compared with the traditional rectangular fins,the heat transfer factor j,resistance factor f and comprehensive performance factor JF of the novel variable section louver are improved by 7.3%,2.6%and 7.65%,respectively,which indicates that the overall performance of the novel variable section louver is better.It is acquired that the comprehensive performance factor JF is the optimal when the curvature number R*is 0.51,the louver angle?is27°,the louver spacing L_p is 1.1 mm and the fin spacing F_p is 1.2 mm.