Effect of ultrasound on frost formation on a cold flat surface in atmospheric air flow

An experimental study concerning frost formation on a cold flat surface in atmospheric air flow subjected to the effect of 20 kHz ultrasound was conducted. A close observation of the frost nucleation and frost growth processes with and without the effect of 20 kHz ultrasound was made with a microscopic image system. The size and distribution of freezing water droplets during the initial nucleation period were described and the frost layer structure and thickness variation with time were presented. It was found that the freezing water droplets formed on the surface with the effect of ultrasound are smaller and sparser compared with those without the effect of ultrasound. The coverage of freezing droplets is all less than 52% with the effect of ultrasound compared with that all more than 65% without ultrasound under some tested conditions. Furthermore, the frost layer structure with a special pattern like “frost line” is observed on the cold surface and very small growth of the frost layer can be seen with the effect of ultrasound. The rate of frost layer thickness reduction with the effect of ultrasound compared with that without ultrasound is about 75%. The experimental results also showed that the frost formation process on the flat surface is remarkably restrained due to the effect of ultrasound.