Experimental study on frost formation on a cold surface in low atmospheric pressure

The role of atmospheric pressure on the frost formation process including the droplet growth, freezing process and frost growth process on the cold surface under natural convection condition is investigated experimentally via a visualization system. The visualization system, configured with the enclosed tank, observation window, microscopy setup and vacuum unit, is used to observe the frost formation process on the cold surface in low atmospheric pressure. The results indicate that the atmospheric pressure plays a significant role in the whole frost formation process. Decreases in atmospheric pressure lead to a smaller phase transition driving force and hence results in a longer time for the process of droplet growth and freezing. The frost crystal shape transforms from irregular to a columnar type and then to a dendritic type when the atmospheric pressure increases from 0.1 bar to 0.5 bar and then to 1 bar. In particular, the frost layer turns to be thinner and sparser in a lower atmospheric pressure.