The effect of PM2.5 air pollution on the frosting process of outdoor finned-tube evaporator

Air source heat pumps (ASHPs) have worldwide applications due to their superior performance in energy saving and environmental friendliness. Two challenges of frosting and fouling for outdoor coils of ASHPs were of attention individually. This study investigated the effect of PM2.5 air pollution on the frosting process of the outdoor evaporator of ASHPs. The system performance and frosting characteristics of ASHPs with different environmental parameters were measured experimentally. The results showed that PM2.5 could significantly promote the frosting process when the concentration was greater than 146 μg/m³. Under the condition of severe pollution, both the experimental data and evaporator surface images reflected the shortening of the frost column generation period and frost layer growth period. On the theoretical basis of the three periods of the frosting process, the frost layer growth period was further divided into transition period and frost thickening period. At the same time, in order to quantitatively analyze the difference of frosting rate between severe pollution condition and no pollution condition, frosting rate ratio (FRR) was defined to reflect the influence of PM2.5 on frosting in different periods. Based on the analysis of the experimental results under different temperature and humidity conditions, it was considered that the frosting zone could be divided into two regions. In the Region I where the moisture content of the air was very low, the effect of PM2.5 on the frosting was weakened, and the FRR was positively correlated with the moisture content of the air. While in Region II with a relatively high moisture content of air, the FRR was positively correlated with the mass ratio of PM2.5 to moisture.