Effects of frost thickness on dynamic defrosting on vertical hydrophobic and superhydrophobic fin surfaces

•Effect of initial frost thickness on dynamic defrosting was studied.•Influences were different on vertical fin surfaces with varying wettability.•Frost-water motion and droplet distribution density were affected on HF surface.•Peeling off appeared on SHF surface, no matter how thick the frost layer was.•Defrosting temperature was reduced by optimizing nanostructures of SFH surface. The effect of initial frost thickness on dynamic defrosting was studied on vertical hydrophobic (HF) and superhydrophobic (SHF) fin surfaces, respectively, which may further deepen the understanding of defrosting and promote application of surface treatment to deal with the frosting of ASHP. In the experiments, frost layer broke down into blocks with melting after a long time preheating, and formed water droplets on the HF surface with strong shrinkage. The preheating time and total defrosting time all increased with the increase of frost thickness. And the size and distribution density of melting water droplets were different, because of the influence of frost thickness on frost-water movement. Peeling off of frost layer appeared on the SHF surface, no matter how thick the frost layer was. The key to achieve peeling off was the action of air thermal expansion in the enclosed space formed by nanostructures of SHF surface and frost layer. Theoretical calculation showed that defrosting temperature decreased with the increase of nanostructure height or decrease of nanostructure diameter. Thus, defrosting temperature could be reduced by optimizing the nanostructure size of SFH surface to save defrosting energy consumption or find low temperature heat source for ASHP defrosting.