Study On Comprehensive Heat Transfer Performance Of Finned Tube Heat Exchanger Under Frost

The air source heat pump uses low-level energy stored in the air to provide heat or cold to the target system.Compared with other heating methods,it reduces environmental pollution and reduces energy consumption by using an air source heat pump.Therefore,air-source heat pumps have been widely used in China.However,when the air source heat pump is running in hot summer and cold winter,there is a problem of frosting on the outdoor evaporator,which will affect the heat transfer performance of the unit.At the same time,it will need to increase the energy consumption of defrost and bring poor indoor thermal comfort.The purpose of this paper is to explore that the heat transfer performance of fin heat exchangers has been improved under low humidity conditions,and that the frost rate can be delayed under frost conditions.In this paper,it is studied by using experimental methods and numerical calculation methods.The conclusions are as follows:Experiments were conducted with flat fin heat exchangers and corrugated fin heat exchangers as outdoor evaporators under the conditions of inlet air temperature 0??6?,relative humidity 60%?90%and inlet air speed 1.5 m/s?4.5m/s.The results show that:?1?Due to the rapid growth of frost layer and the rapid increase in resistance,eventually the corrugated fins enter a serious frost block deterioration stage 50 minutes earlier than the flat fins.This shows that the flat fin heat exchanger has a significant effect of delaying frost formation,and can also effectively reduce the number of defrosts and reduce the impact on indoor environmental parameters.?2?Under low-temperature and high-humidity conditions,compared with corrugated fin heat exchangers,the average heat transfer capacity,average COP,and average equivalent convection heat transfer coefficient of flat fins have increased by 8.2%,8.1%,and 30.4%,respectively.The cumulative frosting of the fins was reduced by10.1%.The comprehensive performance evaluation factor of the flat fin heat exchanger is24.2%higher than that of the corrugated fin,indicating that the comprehensive heat transfer performance of the flat fin heat exchanger under frost conditions is better than that of the corrugated fin heat exchanger.The characteristics have a significant influence on the frost formation of the heat pump outdoor unit.?3?The most basic condition for frosting is that the surface temperature of the fin is lower than the saturated dew point temperature of the inlet air,and the frost speed is related to the absolute moisture content of the air,indicating that the influence of the inlet air temperature and humidity on the frost speed is coupled relationship.In this experiment,the heat transfer performance of the sample with an inlet velocity of3.0m/s is better than 1m/s and 4.5m/s.It shows that when the air intake speed is increased to a certain value,continuing to increase the air intake speed has little effect on improving the heat transfer performance.Based on the experimental study,in order to solve the frosting problem of the current air source heat pump fin heat exchanger in winter,based on the principle of reducing the area of the openings,retaining the high heat exchange area at the front end,and increasing the area of influence of the openings,the elliptical tube perforated fin is designed.The fin model was simulated with FLUNET software.The results are as follows:?1?On the premise of retaining the high heat exchange area at the front of the fin to the maximum,the sickle-shaped opening increases the disturbance of the fluid and reduces the thickness of the thermal boundary layer on the surface of the fin,thereby significantly enhancing the heat transfer.The angle between the elliptical tube and the direction of air flow is small,and the air cylinder is located at the rear end of the elliptical tube,so the fin flow loss of the elliptical tube is smaller than that of the circular tube.?2?When the air flow velocities range between 1m/s and 4m/s,the heat transfer factor j of the elliptical tube perforated fin is higher than that of the circular tube plain fins by 7.1%to 15.3%.The pressure drop of the elliptical tube exchanger reduced by 30.1%to 36.6%,the comprehensive performance index5)?1)^1/3 is higher than that of the circular tube plain fin by mean 18%.The maximum rate is near to 31.9%,which indicates that the performance of the elliptical tube perforated fin is better than that of the circular tube plain fin.?3?The numerical simulation results provide new ideas for the transformation of the fins of the air-source heat pump outdoor unit,and provide a basis for experimental research on the operation of the new type fins at low temperature and high humidity.