An experimental study of frost crystal growth characteristics on different single micro-sized frozen droplets

The frosting phenomenon poses a potential threat to both industrial production and daily life. Typically, a frost layer consists of frost crystals that grow on various micro-sized frozen droplets, and its development is significantly affected by the frost crystal behaviors. To better understand the frosting mechanism, a series of experiments are conducted to investigate the growth characteristics of frost crystals on single frozen droplets with sizes from 1.83 to 0.02 μL. The results show that the whole growth process of frost crystals on a frozen droplet can be categorized into three stages: crystal rapid growth, crystal growth and collapse, and crystal growth fully stages. As the droplet size decreases, the durations of the crystal rapid growth stage and the crystal growth and collapse stage exhibit an upward fluctuation. The maximum height of frost crystals increases with droplet size, while the net crystal height does not follow this trend. Specifically, the maximum net crystal heights on frozen droplets of 1.83, 0.97, 0.43, 0.05, and 0.02 μL at 1,800 s are 1.681, 1.386, 1.476, 1.134 and 1.355 mm, respectively. The growth rate of the highest crystal in the case of 1.83 μL shows a downward trend over time due to the consistently increased crystal surface temperature. Conversely, the growth rates in the cases of 0.97, 0.43, 0.05, and 0.02 μL initially increase and then decrease over time due to the combined effect of crystal surface temperature and water vapor competition. Both the total number of crystal collapses and the number per unit arc length increase with an increase in droplet size. The study contributes to developing accurate frost prediction models and offers insights for optimizing the defrosting control strategy of air source heat pumps or refrigerators.