Effect Of Surface Characteristics On Frost Formation On Cold Plates Under Natural Convection Conditions

Frost formation is a common phenomenon in nature that takes place when a cold surface with temperature below the triple point of water is exposed to humidity air. This phenomenon is undesirable because of the insulation effect of the frost layer along with the reduction in heat transfer and the increase in pressure loss resulted from the decrease in airflow area. Furthermore, the defrosting system will increase operation cost and energy consumption. So in this paper we tried to find some effective methods and through these methods we could restrain frost formation from the initial period.We designed two new hydrophilic paint formulations and tested their effectiveness in restraining frost deposition. The new paints have a series of advantages over our previous one: the thinner film thickness, the stronger surface hardness and the better ability to cling to metal surface. For example, the coating thickness of the new paint can be as thin as 0.04mm. Furthermore, the second paint presented in this paper has much better effects on retarding frost formation and it could retard frost appearance for two hours under high air humidity and low plate temperature conditions. The effect of air relative humidity and plate temperature on frost formation on the hydrophilic surface was also studied. And the new paint was applied to a refrigerator surface and the anti-frosting performance was still very obvious even after six months. A theoretical analysis was also presented to explain the anti-frosting performance of these paints according to the polarity of water molecule and high polymer colophony.The frost deposition phenomenon on a bionic super-hydrophobic surface was studied experimentally .With the rapid development of advanced material science and technology, a bionic super-hydrophobic surface technology was developed. This surface has a multiple micro-nano binary structure (MNBS) similar to the microstructure shown on a lotus leaf. This kind of the surfaces has a contact angle of water greater than 150o and a roll angle smaller than 5o. As we know, frost deposition on cold solid surface is a typical interface phenomenon and thus the surface characteristics should have strong influences on the frost deposition process. Thus we designed and manufactured a bionic super-hydrophobic surface whose contact angle with water is 162°. The frost deposition phenomenon is studied on the surface. The experimental results show that the super-hydrophobic surface has a strong ability to restrain frost growth. The frost deposition on this bionic surface was delayed for 55 min compared with the plain copper surface under the conditions of a cold plate temperature -10.1℃, air temperature 18.4℃and relative humidity 40%. Especially the frost layer structure on the super-hydrophobic surface shows remarkable differences to that on a plain copper surface: the structure is weaker, looser, thin and easy to be removed and most importantly, it is of a very special pattern, a pattern similar to chrysanthemum, a frost layer structure that has not been reported before as far as the present authors could know. Finally the observed phenomenon was analyzed from the microstructure of the bionic surface.The new direction of frost formation under magnetic field was studied at last in this paper. Through closely observation the bulk of water droplet, the droplet freezing process and the frost shape were all very different from that without magnetic field. The bulk of water droplet is more homogeneous and the frost structure is weaker, looser, thin and easy to be removed under the magnetic field. The experimental results demonstrate that magnetic field has ability to restrain frost deposition process. This is a complete new path to study frost formation and so for that has not been reported as far as the present author knows.