Astudy On The Effects Of Cold Surface Characteristics On Frost Formation

Frost formation is a very common phenomenon in refrigeration, cryogenic equipment, air conditioning and aerospace industries. Frost layer deposited on heat transfer surfaces usually results in a series of negative effects on the related system or equipment; it will increase thermal resistance, block flow passage and reduce heat transfer performance. Therefore, scheduled defrosting is necessary for normal operation of these systems. However, conventional defrosting methods not only increase energy consumption but also decrease heat transfer efficiency. Thus investigations of frost deposition mechanism and effective anti-frosting methods have been receiving more and more attention from scientists and engineers. The present work is to study the effects of surface characteristics on frost formation by use theories of phase change dynamics, heat transfer, thermodynamics and surface chemistry. The main works are summarized as follows.The nucleation process of water vapour condensation on the cold surface is analysed in a great detail and the nucleation site density, droplet growth rate and dropsize distribution are obtained analytically with the consideration of contact angles. The calculation results show the contact angle has direct effects on the nucleation site density while Rose equation only considers the nucleation site density with contact angle of 90°. The contact angle and wall supercooling also have obvious effects on water droplet growth and dropsize distribution. However, the dropsize distribution has no relation on the contact angle after the water droplets coalesces.Detailed observation was made of the freezing process of a water droplet on the cold surface; the effects of contact angle, ambient conditions and droplet size on the droplet freezing time are investigated both experimentally and theoretically. The experimental results show that the larger the contact angle of the surface is, the longer the time for the droplet to freeze. It is observed that a small sharp-pointed protrusion appears on the top of the droplet at the end of the freezing process, and the frost crystals first appear on this protrusion. The dendritic growth is more obvious on the surface with larger contact angle. The experimental results also show the water droplet freezing time increases with the increasing of cold plate temperature and air relative humidity. A mathematic model for the water droplet freezing time was proposed based on energy conversation, and the prediction of the model is in general agreement the experimental results.The effects of surface characteristics on the initial frost formation are investigated systematically, the water droplets formation, freezing process and initial frost crystals growth on various surfaces with contact angles from 76.0°to 156.2°are observed. The experimental results show for the contact angle less than 90°, the water droplet radius decreases and the freezing time increases with contact angle. The contact angle has no obvious effects on the frost crystals shape and distribution of the full frost layer growth period. However, for the contact angle larger than 90°, the water droplet radius increases with contact angle, the water droplet distribution is sparser and freezing time is longer on the surface of larger contact angle. The frost formation on both plain copper surface and super-hydrophobic surface are carried out under the same conditions, the water droplets freezing time on the super-hydrophobic surface is longer than that on the plain copper foil surface, which demonstrates the super-hydrophobic surface can restrain the frost crystals growth at the initial period.An experimental system for frost formation under low air temperature and forced convection conditions is set up, the water droplet freezing process and effects of ambient parameters on crystals shape are investigated. The air temperature can be maintained from -15°C to room temperature and the cold temperature can be as low as -30°C. The experimental results of water droplet freezing time and frost thickness versus ambient parameters enrich the frost investigations under low air temperature.The anti-frosting performance of a strongly water-absorptive paint is tested both on the copper plate under natural convection conditions and the refrigeration systems. The indoor experimental results demonstrate the anti-frosting performance and good cycling of the strongly water-absorptive paint. The paint is also applied to practical refrigeration systems in order to test its anti-frosting performance. When used on the heat exchanger, the strongly water-absorptive paint can prolong the defrosting time and there are no frost crystals on the coated fins during the whole test. The coated home refrigerator surface has little frost crystals after five months operation. The strongly water-absorptive paint can retard the initial frost formation on the evaporator tubes when used on the cold store, However, the paint didn't perform as good as the other equipment tests due to its low-quality mother material.