Influence of flow inertia on the frost formation for vertical surface with and without superhydrophobic coating

The aim of this study is to investigate the effect of flow velocity on the heat and mass performance of vertical surfaces. The superhydrophobic surface were prepared by surface chemical spraying, and the copper and superhydrophobic surfaces were placed in an environmental chamber for the experiments. In the experiments, different velocities and surface temperatures for copper and superhydrophobic surface on the frost formation are examined. Test results include heat transfer coefficient (HTC), frost pattern via visualization, and dynamic behaviour. The correlation between the stage of frost crystals, the thickness of the frost, and the HTCs were analysed in detail. It was found that the HTC of the superhydrophobic surface was 8.6% higher than that of copper surfaces at a velocity of 2.5 m s–1 when the surface temperature was higher (surface temperature -5 °C), and the difference of HTCs of these surfaces was within 1.5% when the surface temperature reached -15 °C, which indicated that the superhydrophobic surface effect was offset by a lower surface temperature, and could not delay the frosting effectively. Via visual analysis of the frost crystal shape, the frost pattern can be classified as ice droplets, needle-like, plume-like, plume-like stacking, and irregular shape, it is found that the superhydrophobic surface shows frost formation delay, and the HTCs are higher than that of the copper samples by 88.1% at a surface temperature of -10 °C with a velocity of 2.0 m s–1. In addition, a higher velocity with a higher surface temperature may lead to re-melting of the frost layer, thereby forming a higher density frost layer in a cycle and resulting in a lower frost layer thickness and lower HTC.