Simulation And Experimental Study On Frost Growth Characteristics Of Microchannel Heat Exchanger

With the popularity of small heat pump systems such as automotive air conditioning system,the compact microchannel heat exchangers are more and more widely used.Due to the narrow fin spacing of the microchannel heat exchanger,when the heat pump system operates under the frosting condition in winter,the frost layer hinders the air circulation,reduces the heat exchange efficiency of the microchannel heat exchanger and increases the defrosting energy consumption.In order to explore the growth characteristics of the frost layer on the surface of the microchannel heat exchanger,and reduce the defrosting energy consumption and system energy consumption,in this paper simulation and experimental research of the frost layer growth on the surface of the microchannel heat exchanger was carried out,The growth characteristics of the frost layer on the surface and the influence of frosting on the heat exchange performance were analyzed.Firstly,the heat transfer model of microchannel heat exchanger was established by using the ?-NTU efficiency heat transfer unit number method.Based on the quasi steady state assumption,the mathematical models of the frost layer growth and air side pressure drop were established.The calculation program was written with MATLAB software to simulate the frosting characteristics on the finned tube heat exchanger.The simulation results were compared with the experimental data obtained earlier to verify the rationality of the models.Secondly,the frosting characteristics on the microchannel heat exchangers with different geometric dimension were simulated,and the influence of the fin spacing and fin opening angle on the frosting characteristics and heat exchange performance of the microchannel heat exchangers were analyzed.Finally,the experimental platform was built,and the flat-fin microchannel heat exchanger was selected as the prototype.The effects of different environmental relative humidity and ambient dry bulb temperature on the thickness of frost layer on the surface of microchannel heat exchanger,frost mass,heat exchange,air side pressure drop,frost layer surface temperature and boundary layer water vapor partial pressure were analyzed experimentally.The following conclusions are obtained from simulation and experimental results:（1）The comparison between the simulation value and the experimental results of the surface frosting of the fin tube heat exchanger and flat-fin microchannel heat exchanger shows that the curve change trend of the simulation value and the experimental results is consistent.The errors between the experimental and simulation values of the frost thickness,frost mass and average heat transfer of the fin and tube heat exchanger is less than 23.4%,13% and 14.5% respectively,while for the flat-fin microchannel heat exchanger the errors of the frost thickness,frost mass and average heat exchange are less than 22%,15.7% and0.41% respectively,which verifies the rationality of the frosting model.（2）The simulation of frosting of microchannel heat exchanger with different geometric parameters show that with the decrease of the fin spacing,the average heat transfer of heat exchanger increases,but the frosting period get short,and the air side pressure drop increases rapidly.The average heat transfer of the microchannel heat exchanger with the fin spacing of 1.0mm is about 3.4% higher than that of the heat exchanger with fin spacing of 1.5mm,and the pressure drop on the air side is about 64%higher than that of the heat exchanger with fin spacing of 1.5mm.The thickness of the frost layer,the frost mass,the air side pressure drop and the average heat transfer of the heat exchanger increase with the increase of the fin opening angle of the microchannel heat exchanger.The average heat transfer of the heat exchanger with 27 ° opening fins is 3.4%higher than that of the flat-fin heat exchanger,while the air side pressure drop is 38.5%higher than that of the flat-fin heat exchanger.（3）The experimental results show that the effect of the relative humidity on the growth of frost layer is obvious.The higher the relative humidity,the faster the frost layer grows and the shorter the frosting period.At the same time,the rapid growth of the frost layer thickness leads to the rapid increase of the air side pressure drop.After frosting for 35 minutes,the air side pressure drop under the condition of relative humidity of 80% is 2.3times that under the condition of relative humidity of 60%.（4）The experimental results show that when the outdoor temperature is 0 ℃ ～ 3 ℃,the growth rate of the frost layer is the fastest,the corresponding frosting period is the shortest,and the growth rate of the air side pressure drop is the fastest,which has influence on the average heat exchange during the frosting period.The frosting period under condition of outdoor temperature of 3 ℃ is 50% shorter than that under-5 ℃.