Experimental study of the frosting characteristic of water on a cold surface in the magnetic field

摘要

In this paper, the effect of the magnetic field on the frost formation over a cold surface was investigated on the basis of an established visualization system equipped with a magnetic field generator. Various factors such as atmosphere temperature, relative humidity, cold surface temperature, and surface wettability were considered in this study. The thickness of the frost layer, equivalent radius and the morphology of the ice crystal were observed and measured during the courses of droplet growth, freezing, and the frost growth. In order to analyze the effect of the magnetic field on frosting, the coverage fraction was proposed to evaluate the final frosting characteristics. It has been found that the coverage fraction decreases slowly with the increase of the magnetic flux density, which means that the frost formation on the cold surface in a magnetic field is less than that in a non-magnetic field. In addition, a mathematical model was proposed to describe the freezing process with a difference of less than 25% compared to the experimental data. It indicates that the magnetic field plays a significant impact on the freezing process, and the increase of the magnetic flux density leads to a small decrease in the phase change force. It takes a long freezing time for the droplet to enter a solid state, and therefore the crystal radius is small. The crystal boundary is much more regular and the thickness of the frost layer is thinner than that under a nonmagnetic condition since the hydrogen bonds of water are stronger and the configuration is more ordered and stable. Under such circumstances, a large phase change driving force is needed to overcome the magnetic force. The correlations are presented to describe the variation of the radius and thickness of ice crystal, which is beneficial to the defrosting control technology.