Research On Micro-channel Evaporator For Automotive Air Conditioning

Micro-channel evaporators are widely used in automotive air conditioners because of their light weight and high efficiency.Because the multi-channel small diameter flat tube and louver fins are used in the evaporator,the structure is complex,and the flow of air and refrigerant is complex.Meanwhile the phase change occurs during the heat transfer process,resulting in a more complicated flow and heat transfer.At present,there is no unified understanding of the mechanism of micro-channel phase transition,and the flow and heat transfer in the micro-channel evaporator is studied in this paper.First,the heat transfer formula of the louver fin and the influence of the structural parameters of the louver fin on the flow and heat transfer are summarized.The research results on the uniformity of flow distribution on the refrigerant side are summarized.The basic flow patterns and heat transfer characteristics of the two-phase flow in the process of evaporation are analyzed,and discuss the boiling heat transfer mechanism.The test of micro-channel evaporator was carried out by using the comprehensive performance test bench of automobile air conditioning.Tests under different conditions were carried out to obtain parameters such as evaporator inlet/outlet enthalpy,temperature,pressure drop,and air resistance,and the heat transfer capacity of the evaporator was analyzed.The temperature of the evaporator outlet air temperature measured by the thermocouple network arranged on the surface of the evaporator was used to study the uniformity of the outlet temperature of the evaporator,and the optimization direction of the evaporator structure was established to improve the uniformity of the temperature distribution and the overall heat transfer capacity.In view of the problem of non-uniform evaporator surface temperature,the structural characteristics of the manifold in the evaporator have been optimized,and the flow distribution of the refrigerant has been adjusted by changing the position and throttle hole size of the throttle plate in the manifold.Based on the three-dimensional simulation software STAR-CCM+,the three-dimension simulation was performed.The results show that the two throttle plates in the flow direction of the header are reasonable,can improve the flow mal-distribution phenomenon obviously,so that the pressure loss can be controlled within a reasonable range,the installation position of the throttle plate also had best value.The throttle hole size of the throttle plate greatly influences the flow distribution uniformity.If the throttle hole diameter is too large,the effect of the throttle plate is not obvious.If the throttle hole diameter is too small,the pressure loss is large,and the flow distribution before the throttle plate is too much,resulting in a worsening of the distribution.In the case of the same opening area,the circular throttle hole has the best diversion effect and the minimum pressure loss.The uniformity of flow distribution deteriorates as the inlet dryness and the inlet mass flow increase,but the influence of the mass flow on the flow distribution is not significant.The three dimensional simulation model is modified to get more accurate simulation results by using the test data of multiple verification as the boundary conditions of the 3D simulation.In this paper,the physical parameters are introduced into the calculation model in the form of polynomial,in order to improve the calculation accuracy.In the flat tube part,STAR-CCM+ specific directional grids are used for meshing to improve the calculation speed while ensuring the calculation accuracy.According to the comparison between the test results and the simulation results,the air side error is within 7%,and the refrigerant side heat exchange capacity error is within 6%,which fully validates the correctness of the calculation model.By comparing the heat transfer capacity before and after the structural optimization,and the heat exchange capacity increased by 304 W.Comparing the heat transfer capacity under different working conditions,as the mass flow increases,the enthalpy difference at the evaporator inlet and outlet becomes smaller,but the heat exchange capacity increases and the heat exchange capacity increases.