Study On Heat Transfer Characteristics And Influencing Factors Of Microchannel Evaporator

As a condenser,microchannel heat exchanger has been widely used.It has many advantages,such as light weight,small volume,low cost,anti-corrosion,compact structure,high heat exchange efficiency,less refrigerant charge,high recovery and utilization rate.When it is used as an evaporator,because of its special structure,the refrigerant flows into multiple parallel flat tubes at the same time,and the refrigerant flows in the microchannel evaporator There is a problem of uneven flow distribution in the flat tube.However,the gas-liquid two-phase flow in the flat tube of microchannel evaporator is a very complex problem,and its flow mechanism,distribution mechanism,and heat transfer characteristics have not been fully mastered.In view of the above problems,this paper studies the heat transfer characteristics and influencing factors of the double flow microchannel evaporator by building a test-bed for the heat transfer characteristics of the microchannel evaporator.It can provide theoretical reference for the research and application of microchannel evaporator.Through the experimental test,the influence of three parameters,air side facing wind speed,refrigerant flow rate and subcooling degree,on the surface temperature,heat transfer characteristics and refrigerant flow distribution of the two-way microchannel evaporator are studied.The following conclusions are drawn:(1)In the first process of heat exchange,the refrigerant in the two-way microchannel evaporator undergoes the supercooled section and the two-phase section.The second process undergoes the two-phase section and the superheated section.The length of the supercooled section reflects the flow distribution characteristics of the first process,and the length of the two-phase section reflects the flow distribution characteristics of the second process.The temperature nephogram and the temperature distribution on the surface of the microchannel evaporator show that the flow distribution of each process of the microchannel evaporator is obviously uneven.The RDP of the first process and the second process varies from 0.64 to 0.93.(2)The surface temperature of microchannel evaporator was obtained by infrared thermal imager,and the flow rate of liquid refrigerant in each flat tube of microchannel evaporator under different working conditions was calculated.In the first process,the flow rate difference between the bottom flat tube and the upper flat tube can reach 181.6%.In the second process,the flow rate of the bottom part is still higher than the top part.There are almost no liquid refrigerants in the top flat tubes of 3~ 5,and there is obvious dry evaporation.The flow rate difference can reach 210.8%.(3)The flow distribution of the double flow microchannel evaporator is affected by the air velocity,the subcooling degree of the evaporator inlet and the refrigerant flow.With the increase of air speed at the air side,the uniformity of flow distribution becomes worse.The RDP of the first process changes from 0.91 to 0.77,and the RDP of the second process changes from 0.78 to 0.64.With the increase of inlet flow,the uniformity of flow distribution improves.The RDP of the first process changes from 0.76 to 0.90,and the RDP of the second process changes from 0.64 to 0.77.With the increase of inlet subcooling,the uniformity of flow distribution improves.The RDP of the first process changes from 0.84 to0.93,and the RDP of the second process changes from 0.84 to 0.93 From 0.67 to 0.77.It can be seen that the influence of evaporator inlet flow on flat tube flow distribution is greater than air side velocity and refrigerant inlet subcooling.(4)The refrigerant flow rate has the greatest influence on the pressure drop at the inlet and outlet of the microchannel evaporator,followed by the air crosswind speed and the refrigerant subcooling degree.When the refrigerant flow increased from 30 kg / h to 50 kg /h,the pressure drop at the inlet and outlet of the microchannel evaporator increased by112%;the air speed at the air side increased from 1.5m/s to 3.5m/s,and the pressure drop at the inlet and outlet increased by 63%;the refrigerant subcooling increased from 2.5? to6.5?,and the pressure drop at the inlet and outlet decreased by 14%.(5)The heat transfer mainly comes from the latent heat process of refrigerant phase change.The refrigerant inlet flow has the greatest influence on the heat transfer and the overall heat transfer coefficient of the evaporator.The air side face-to-face wind speed and refrigerant subcooling have little influence on the heat transfer of the microchannel evaporator.When the wind speed is between 1.5m/s and 3.5m/s and the refrigerant subcooling is between 2.5? and 6.5?,the change of heat exchange rate is between 1%and 4%,the total heat transfer coefficient increases by 12.5% and decreases by 28.3%respectively;when other conditions remain unchanged,the refrigerant flow rate increases from 30 kg /h to 50kg/h,the heat exchange rate increases by 63%,and the total heat transfer coefficient increases by 30%.