Study Of Frost Formation On A Vertical Cold Plates With And Without External Electric Field Under Natural Convection Conditions

Frost accumulation has a severe negative effect on refrigeration, cryogenic and many other engineering systems due to the increased flow and heat transfer resistance and the reduced energy transfer rate. In order to seek the effective defrosting and frost-controlling methods, frost growth on hydrophilic surface under natural conditions with and without the existence of external electric field were investigated in this thesis.The physical characteristics of water molecule and ice crystal were described, and properties relating with the ice crystal growth under external electric field were discussed.In order to understand the frost crystal growth, the nucleation process on the cold plate with constant temperature was investigated. The experimental results revealed that the lower the cold plate temperature is, the shorter the frozen time of water droplets, and the smaller the frozen water droplets. If the cold plate temperature is higher than a certain critical value, for instance, -12°C under our observed experimental conditions, the higher the air relative humidity is, the bigger the water droplets concreting on the cold plate, and the later the water droplets froze. However, if the cold plate temperature is lower than a certain critical value, the influence of air relative humidity on the frozen time of water droplets was not observed. If the cold plate temperature is higher than a certain critical value, for example, -20°C under our observed experimental conditions, frost deposition is through condensation, or condensation frosting. However, if the cold plate temperature is lower than a certain critical value, frost deposition is by ablimation, or ablimation frosting. We identified eight different frost crystal patterns under various experiment conditions.Frost formation on cold surface is a typical interface phenomenon, so the cold surface characteristics should have an important influence on the process of frost formation. Using of the strong hydrophilicity property of nano-SiO2/ nano-TiO2, a hydrophilic coating was made. We found that the coating could restrain the frost layer growth to certain extent. The coating shows a good fastness and repetitive-use performance. In addition, the coating has a very small heat transfer resistance, and the heat transfer resistance may be neglected compared with that of the frost layer. The frost growth on copper plate surface under electric field was observed in this thesis. We found that the applied electric field may change the shape of frost crystals, control the frost layer...