Experimental study of dust effect on frosting and frost melting characteristics of air source heat pump fin surface

The outdoor heat exchanger of the air source heat pump (ASHP) will be affected by frost and dust accumulation, which restricts its performance. Currently, researchers have conducted in-depth independent research on both. However, there has been little research on their coupling effects. Thus, in this study, an experimental platform is set up to study the frosting characteristics of dusty fin surfaces visually. At the initial stage, the water vapor will permeate into the dust particles, and the distribution of condensation water droplets will be affected. The initial frost crystals on the surface of dust particles primarily result from the freezing of condensed water, whereas those on the clean fin surfaces mainly originate from the sublimation of water vapor. The frost growth process on dust particles can be divided into three periods: crystal growth period, frost layer growth period, and frost layer cyclic growth period. On the particle surface, frost tends to grow upwards rather than sideways. While the frost on dust particles melts, the adjacent particles will be drawn together by the melted water. The presence of a dust layer on a fin surface will slow down the rate of frost formation: For every 100 μm increase in dust thickness, the frost formation rate decreases by about 1.57 μm/min. Under the operating condition in this study, the frosting rate initially remains stable, and then decreases when the average frost thickness reaches about 1 mm. Additionally, the heat exchanger with thicker dust layers tends to be quickly blocked by frost.