Numerical Simulation And Experimental Test Of Convective Heat Transfer Characteristics In Microchannel Evaporator

The microchannel heat exchanger is a new type of high efficiency and energy saving device with the advantages of high heat transfer efficiency,compact structure,and light weight,etc.The further research for microchannel heat exchanger applying to domestic air conditioners system is conducted,and the results can provide a data support for the optimization,development design and operational parameter optimization of air conditioning heat exchangers.It has an important practical significance that reducing energy consumption of air conditioning and improving heat transfer efficiency.Based on the literature research,the microchannel evaporator model is built and the numerical simulation methods is employed to analyze and discuss the effects of the header model on flow distribution,the refrigerant distribution uniformity and structure optimization in the evaporator,and gas-liquid two-phase flow of the refrigerant distribution characteristics.According to numerical result,the microchannel evaporator is designed and processed,and an experimental system is setup.The performance of microchannel evaporator is tested and compare with conventional tube-fin evaporator.The result show that:（1）In the four header models（Type A,Type B,Type C and Type D）,the inlet velocity（flow rate）has a large influence on the flow distribution of the flat tubes in the heat exchanger.For low inlet velocity,the distribution characteristic of flow rate in each flat tube present small in middle and large in two side.With increasing the inlet flow rate,the flow distribution of the flat tubes present low in inlet and gradually increasing along the flow direction of the header.The slope of static pressure change at the inlet of each flat tube is basically consistent with the trend of increase or decrease in internal flow.（2）The microchannel evaporator with parallel header inlet type has bad flow distribution,and theη₀ that is the ratio of the maximum flow and the minimum flow of the refrigerant is large and the refrigerant distribution in some regions is too much or too little.For improved structure（vertical header inlet type）,the uniformity of the refrigerant flow distribution of the flat tube is improved,and the resistance loss at the total inlet end of the evaporator is reduced about 21.39%.In addition,the flow distribution at different depths of combination is analyzed and the result shows that the best combined depth T is 0.42（the optimal depth of the flat tube inserted into the header is 8.64 mm）,which can meet the processing requirements.（3）For refrigerant two-phase distribution of the microchannel evaporator,the optimized evaporator has small outlet liquid volume fraction（V_L=0.27）,only about49.16%compare with the original structure（V_L=0.55）.Therefore,the optimized evaporator has high evaporation efficiency and better heat transfer performance.As for original structure,the temperature difference between the inlet and outlet is low（ΔT=3.2°C）,and the temperature drops slowly.The inlet temperature of each flat tube gradually decreases along the flow direction of the refrigerant in the lower header,and the farther from the refrigerant inlet,the more of flow rate,and the overall heat transfer performance of the evaporator is poor.For the optimization structure,it has high temperature difference between the inlet and outlet（ΔT=9.8°C）,and larger refrigerant distribution area（3#¹7#flat tube）,and present better heat transfer performance.The final calculation results before and after optimization show that the microchannel evaporator of the improved structure heat transfer capacity increase735W and improve about 5.44%.（4）The experimental test results show that the temperature of the flat tube near the inlet of the evaporator is low,that is to say the refrigerant flow near the inlet is more.After the 4#flat tube,the temperature change of the system gradually decreases and the flow distribution is more uniform.For low surrounding temperature of the evaporator surface,the dryness of the refrigerant in the channel is very small,the fluid state is mainly in the liquid phase and the resistance is small.With increasing of the temperature in the test chamber,the surrounding temperature at the evaporator surface is gradually increasing,the dryness of the refrigerant in the channel increases and a complete gas-liquid two-phase flow is formed.Therefore,the resistance increases and the pressure drop increases.（5）The pressure drop between inlet and outlet of the microchannel evaporator is smaller than the tube-fin evaporator.At the same time,the temperature difference between the inlet and outlet is maintained at about 4.7°C and the cooling capacity is better.Compared with tube-fin evaporators,the cooling capacity of the microchannel evaporator system and the COP increase about 2.95%and 4.73%,respectively.Meanwhile,the input power and the windward area of the core decrease about 1.82%and 27.5%,respectively.The refrigerant charge decrease about 21.9%and the weight is only about 18.3%of tube fin evaporators.（6）The economic analysis of the evaporator shows that the annual total reimbursement for the microchannel evaporator is about 1547.47 RMB/year and the tube fin evaporator is about 1361.78 RMB/year,which is higher 13.63%than the tube fin evaporator.In terms of economic benefits,the maximum revenue of microchannel evaporators is 3,488 RMB/year,and the tube-fin evaporators is 2,624 RMB/year,which is higher 24.77%than the tube fin evaporator.In general,the ultimate benefit of microchannel evaporators is better than the tube-fin evaporators.