| Microchannel heat exchanger has the advantages of small volume size,small floor space,low cost,electrochemical corrosion resistance and so on.Under the background of " carbon neutralization and carbon peak " proposed by the state,reducing the emission of pollutants from microchannel heat exchanger and improving the heat transfer efficiency have become the inevitable requirements of its development and innovation.As an evaporator,the problem of easy frosting and difficult defrosting of microchannel heat exchanger is particularly prominent.When the surface frosting will cause the performance of heat pump air conditioning system to decrease,the stability will decrease,the utilization rate will decrease,and the indoor environment temperature fluctuation will increase.In order to improve the system performance of the microchannel heat exchanger and reduce its energy consumption to make it fully heat exchange,this paper explores the distribution of refrigerant inside the microchannel heat exchanger and the frosting characteristics of the outer wall of its flat tube by numerical simulation.The main research contents are as follows:Firstly,the two-phase distribution of refrigerant in microchannel heat exchanger is simulated.According to the relevant experimental literature,a two-dimensional calculation model of microchannel heat exchanger is established.The accuracy of the calculation model is verified by the experimental parameters such as outlet superheat,heat transfer,inlet and outlet pressure drop and gravity heat pipe temperature.On this basis,the distribution characteristics of refrigerant in the heat exchanger are obtained,and the influence of inlet flow rate,inlet dryness and subcooling parameters on the distribution of refrigerant is explored.The results show that with the flow and heat transfer of the refrigerant,the heat transfer between the refrigerant and the tube wall undergoes a phase change,and the phase change continues in the flat tube until almost all of it is converted into gas at the outlet.With the increase of inlet flow rate and subcooling,the heat transfer in the tube increases,and the gas volume fraction and temperature in the heat exchanger decrease gradually.With the increase of inlet dryness,the gas volume fraction and temperature gradually increase.The above parameters have a certain influence on the non-uniformity of refrigerant distribution in the heat exchanger.Secondly,on the basis of the above simulation,the frost growth prediction model proposed in this paper is verified and the air side frost calculation model of microchannel heat exchanger is established.The results show that the frosting model established in this paper is suitable for the simulation of air side frosting in microchannel heat exchanger within the allowable error range of simulation calculation.Finally,the numerical simulation of the frosting outside the flat tube of the microchannel heat exchanger is carried out.The results show that the frost thickness decreases gradually along the refrigerant flow direction.At the same time,the frost thickness decreases gradually with the increase of inlet dryness and flow rate,and the frost thickness increases with the increase of subcooling.On the air side,the closer to the outer wall of the flat tube,the higher the density of the frost layer,and the lower the density of the frost layer extending to the air domain side.In addition,the velocity of wet air increases gradually with its flow direction.The volume fraction of water vapor in moist air is lower near the frost surface.In summary,this paper proves the uneven distribution of refrigerant in microchannel heat exchanger and obtains the frosting outside the tube by numerical simulation method from the mechanism,which provides technical reference for solving the energy saving problems such as heat transfer frosting and defrosting. |
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