Experimental Study On Frosting Characteristics Of Finned Evaporator

When the evaporator of a refrigeration unit operates under high humidity and low temperature conditions,the cold surface is prone to frost growth.In the early stage of frost growth,the operation of the refrigeration unit will not be affected.As the working time prolongs,the surface of the evaporator and air for heat exchange will be gradually covered by the frost layer.In addition,the thermal conductivity of the frost is only 0.1160.139W/(m2·℃),which is determined by the heat transfer process heat transfer formula φ=KF Δt shows that the continuous frost formation process of the evaporator will simultaneously reduce the heat transfer coefficient K and the heat transfer area F.Therefore,frost formation phenomenon will weaken the heat exchange between the evaporator and the evaporator coil,affect the normal operation of the refrigeration unit,and pose a great threat to the entire refrigeration unit.It is necessary to conduct a careful analysis and study of evaporator frost formation.This experiment focuses on the common finned evaporator and places it in the air duct.The air duct and evaporator are placed in the enthalpy difference simulation room,and the condensing compressor unit is placed outdoors.The evaporator and condensing compressor unit are a complete steam compression refrigeration system,and the refrigerant is R404A.The enthalpy difference chamber simulated the frosting conditions of low temperature and high humidity,and the following experiments were mainly conducted:1)By observing local frosting photos taken by CCD cameras and analyzing the theories of relevant scholars,the growth behavior of frost layer on the end of fins in finned tube evaporators was studied;2)Study the operation of a steam compression refrigeration system during frosting and defrosting processes,with relevant research parameters including evaporation temperature and pressure of the refrigeration system,condensation temperature and pressure,compressor suction and exhaust temperature,refrigeration capacity and performance coefficient;3)Control other environmental conditions unchanged,and observe and record the effect of wind speed on the frost formation process of the evaporator.In order to quantitatively analyze and represent the impact of different wind speeds on the frost formation process,the captured images are binarized;4)Control other environmental conditions unchanged,set different air temperatures,and take frosting photos of the evaporator as the research object to explore the critical temperature of the frosting ambient air.The main conclusions are as follows:1)Through experimental observation,due to the small fin spacing of the evaporator used in this experiment,there is less frost inside the fin space,and frost mainly occurs at the tip of the fins;2)As the thickness of frost at the end of the fins increases,the frost layer will block the space of the evaporator fins,reducing the performance of heat exchange;Frosting in the evaporator leads to a decrease in heat exchange area and an increase in heat exchange resistance,resulting in the refrigerant inside the evaporator not overheating and a decrease in evaporation pressure and temperature.Compared with the benchmark operating condition,the refrigeration capacity and COP of the refrigeration unit under frosting conditions decreased by 23.9%and 25%,respectively.The total refrigeration capacity reduced by frosting within one hour was 11.27%;3)During the process of increasing the head-on wind speed from 0m/s to 1m/s,and then from 1m/s to 2m/s,a slight increase in the surface temperature of the evaporator fins dominates,which is not conducive to frost formation.Therefore,the increase in wind speed leads to a decrease in frost formation.After one hour of frost formation,the frost blockage rates for frontal wind speeds of 0m/s,1m/s,and 2m/s were 0.8899,0.7493,and 0.3988,respectively.The blockage rate decreased with the increase of wind speed;4)Analyzing the frost situation on the windward surface of the entire evaporator at air temperatures of 0℃,5℃,and 10℃(corresponding operating conditions are F,G,and H respectively),it can be observed from the pictures that there is frost layer growth on the surface of the evaporator fins at F and G,and the frost under F operating condition is much greater than that under G operating condition.Under H operating condition,the COP of the cooling system remains between 3.9 and 4.1.Under F and G operating conditions,the COP of the cooling system decreases with frost formation time.When the air humidity is 85%and the wind speed is 0.5m/s,The ambient temperature for frost formation is between 5 and 10℃.