Research On The Frost Suppression Performance Of Evaporator In Air Source Heat Pump System Based On The Superhydrophobic Surface Modification Technology

Air source heat pump is an energy saving device that can transfer heat from low level heat source air to high level heat source with the help of high level energy,but when operating in low temperature and humid conditions,frost problem will occur on its outdoor coil surface.The growth of frost layer makes the heat exchange thermal resistance between the coil and air increase,thus triggering problems such as rapid decrease of energy efficiency ratio of heat pump unit,insufficient heat production and decrease of indoor thermal comfort.In order to improve the practicality of heat pumps in cold regions of China,scholars have conducted a lot of research on the frost problem of air source heat pumps.A review of the research status at home and abroad shows that the use of superhydrophobic surface modification technology to suppress frost on cold surfaces is feasible.However,due to the fact that the preparation method of superhydrophobic surface is not popular and the mechanism of superhydrophobic surface frost inhibition has not been clarified,the research on superhydrophobic surface modification frost inhibition technology is still insufficient.Based on the two key preparation steps of rough structure and low surface energy modification,the superhydrophobic modification method-chemical etching method was optimized.With the help of scanning electron microscopy characterization,static contact angle characterization and surface compound element composition characterization,the surface property changes during the modification process were clearly defined.The results show that the etchant concentration affects the surface rough structure construction effect,and the etchant hydrochloric acid concentration of 3 mol/L has the best etching effect;on the other hand,different modifiers have different modification effects on the rough surface,and fluorosilane has the best modification effect.An experimental bench for the study of frost formation/defrosting in superhydrophobic fins and three-dimensional finned tubes was established.The bench consists of a refrigerant circulation system,a cold air supply system and a visual observation and data acquisition system.The refrigerant circulation system can provide a variable temperature environment of-25～25 °C for the surface of the fin,and the cold air system can provide humid air with a dry bulb temperature of-5～25 °C and a relative humidity of 60%～90%.Through the visual observation and data acquisition system,the frost layer of one-dimensional fin and threedimensional fin tube was observed and photographed,and the parameters of various working conditions of the experiment were measured.Based on the experimental platform above,the similarities and differences between one-dimensional superhydrophobic fins and ordinary fins in the process of condensation and droplet freezing in humid air are compared.The results showed that when the cold surface temperature was 0 °C,the number of condensate droplets on the superhydrophobic surface was 77.2% of the ordinary surface,the droplet weight was 28%of the ordinary surface,and the droplet coverage was 35.5% of the ordinary surface,and the higher the cold surface temperature,the better the anti-condensation effect of the superhydrophobic surface.In this paper,a model of moist air condensation and droplet freezing on superhydrophobic surface was established,and it was found that the condensation of wet air on superhydrophobic surface into nuclear energy barrier and the density of condensate nucleus were small,which inhibited the condensation process of moist air on superhydrophobic surface.The heat transfer resistance of the droplets on the superhydrophobic surface is large,resulting in a slow freezing rate of the droplets.Taking a single tube of finned tube heat exchanger with round copper tube and annular aluminum fin as the research object,the effects of fin wetting properties,fin spacing,cold surface temperature and wind speed on the frosting characteristics of the finned tube were explored.Experiments showed that the frosting amount of superhydrophobic finned tubes was81.1% of that of ordinary finned tube.When the fin spacing was 5 mm,the frost amount of superhydrophobic finned tube was 82.1% of that of 3 mm fin spacing.When the cold surface temperature was-5 °C,the frost amount of superhydrophobic finned tube was 48.9% of the cold surface temperature of-15 °C.When the wind speed was 0.5 m/s,the frost amount of the superhydrophobic finned tube was 71.8% of that at the wind speed of 2 m/s.The results showed that the wetting properties,fin spacing,cold surface temperature and wind speed of the finned tube had significant effects on the frosting characteristics of the finned tube.The defrosting efficiency of superhydrophobic finned tube can reach more than 90%,which is significantly better than ordinary finned tube,which is of great significance for preventing secondary freezing of heat pump chamber coils.The research results of this paper are helpful to further explore the mechanism and characteristics of defrosting,defrosting and defrosting of superhydrophobic finned tube heat exchangers,and provide important technical data for surface superhydrophobic modification and frost layer growth of finned tube heat exchangers.It has important theoretical guiding significance and practical application value for improving the performance of air source heat pump air-conditioning system in cold area and promoting its application in frosting area.