Study On Flow Distribution Characteristics Of Microchannel Evaporator And Heat Transfer Enhanced By Gas Diversion

Due to the large specific surface area,the heat transfer efficiency of micro-channel evaporator has been greatly improved,and it has a broad application and development prospect in the fields of air conditioning refrigeration,electronic components and aeroengine thermal management.But in the process of actual application,because many branch and prone to gas-liquid two-phase flow stratification results in uneven flow distribution,dry steaming and fluid for too much,cannot make full use of the heat exchange area,makes the heat transfer performance deteriorates.And in the process of studying the flow distribution characteristics,it is difficult to directly measure the flow of each flat tube,while,the refrigerant in the evaporator is gas-liquid two-phase flow,so it is difficult to make theoretical analysis and simulation calculation..Therefore,this paper builds a micro-channel evaporator performance test rig,uses infrared thermal imaging camera to capture the surface temperature distribution cloud map of the evaporator,explores the method of characterizing the flow distribution law,changes the refrigerant mass flow and wind speed,and analyzes the refrigerants flow distribution law of the first and second pass under different working conditions,and design a gas diversion pipe in the middle header of the evaporator to export part of the gas phase refrigerant to improve the uneven flow distribution,and their heat transfer and flow characteristics are studied.The specific research results are as follows:(1)In the first-rate process of dual-flow microchannel evaporator,the flow distribution law is calculated by combining the wall temperature of the evaporator with the thermal balance relationship between the air side and the refrigerant side,which is consistent with the distribution law of the actual wall temperature,the undercooled section and the two-phase section.The difference between the calculated total flow and the actual measured total flow is no more than 30%.However,in the second process,because most of the refrigerant in the flat tube did not evaporate completely,the flow distribution of the flat tube could not be accurately obtained,so the two-phase length could be used to represent the flow distribution law.(2)Under different inlet refrigerant flow and wind speed,the flow of the first pass of the double pass microchannel evaporator is mainly distributed at the bottom,and the flow distribution among the flat tubes is uneven,and the flow unbalance degree of the flat tubes is up to 1.3.In the second process,the long two-phase flat pipe is mainly distributed at the bottom.In the flat pipe near the top of the second header,With the increase of the number of flat tube,the closer to the top of collecting tube,the length of two-phase section becomes smaller.(3)under the different wind speed,the gas diversion pipe flow rate on avera ge 1.4% of the microchannel evaporator inlet refrigerant flow,adding gas guiding device of microchannel evaporator superheat are lower,compared with not addin g gas flow guide device,the biggest drop of 45.4%.However,there is little differ ence in heat transfer between the two,and the maximum increase is not more tha n 0.5%.At the same time,the heat transfer coefficient between the two is not mu ch different,no more than 6.9%.After gas diversion is installed,the pressure drop of the evaporator with microchannel with gas diversion device is smaller.Compa red with the evaporator without gas diversion device,the pressure drop is up to 7.4%.(4)under different inlet refrigerant flow rate,gas diversion pipe flow rate,on average,1.2% of the microchannel evaporator inlet refrigerant flow,adding gas guiding device of microchannel evaporator heat transfer coefficient is higher,but did not add gas guiding device in microchannel evaporator heat exchange slightly higher on the whole,the difference is not big,most no more than 0.3%,and the degree of superheat is also higher.At the same time,the pressure drop of the micro-channel evaporator with gas diversion device is smaller,which is up to 9.2% compared with that without gas diversion device.(5)After the gas-phase refrigerant is led out of the middle header by adding a gas guide device,when the flow of the gas diversion pipe is 1.4% of the refrigerant flow at the inlet of the micro-channel evaporator,the length of the two-phase section of the flat pipe at the top of the second flow process is significantly larger than that of the flat pipe without gas diversion device,with the maximum increase of 87.3%..When the average flow rate of the gas diversion pipe is 1.2% of the inlet refrigerant flow rate of the microchannel evaporator,the two-phase area of the flat pipe at the top of the second process also increased,with the maximum increase of 67.3%,and the dry steaming of the flat pipe at the top of the second process was relieved.