Experimental and numerical investigation on frosting of finned-tube heat exchanger considering droplet impingement

Air source heat pumps are prone to frosting under low-temperature, and high-humidity conditions. Frosting in finned-tube heat exchangers increases their heat transfer and airflow resistances. Moreover, it leads to reductions in the evaporation and condensation temperatures of the unit, increased energy consumption, working performance deterioration, and even shutdown. Therefore, it is necessary to simulate the frost formation process, in order to provide a reference for frost-prevention design of finned-tube heat exchangers in cold, high-humidity areas. However, fin microchannel frosting is a complex process that involves heat and mass transfer. It is difficult to describe the frost layer formation process completely. This is, especially true for droplet formation during nucleation. This paper describes the mass transfer process that occurs during nucleation via a droplet impingement model and uses the enthalpy porosity method to solve the solidification problem. Air-source heat pump frosting tests performed in the enthalpy difference laboratory verify the numerical methods. It is determined that the model in this paper can simulate the frost growth process more comprehensively and accurately than a mathematical model that does not consider droplet formation. Then, the effects of the mass transfer rate, air temperature, humidity, inlet velocity, and fin surface temperature on the finned-tube heat exchanger frosting process are described. Frost growth is related positively to the rate at which water vapor impacts the fin surface. The frost is thickest when the moist air temperature range is between 1 ? and 3 ?. Moreover, the frost density varies little when the wet air temperature is above 0 ?. The variation of frost density and frost thickness shows an opposite trend under the wind speed factor.