An Experimental Study Of Frost Deposition Phenomena On Cryogenic Surfaces

Frost deposition is commonly observed in cryogenic, refrigeration, airconditioning, and aerospace industries. In the past few decades, most of theoretical and experimental researches on frost deposition have been focused on ordinary low temperature conditions(?-30 ?), while people seldom study frost deposition on cryogenic-temperature surfaces(?-100?). In this paper, an experimental system of frost deposition on cryogenic-temperature surfaces under natural convection conditions was set up and a series of frost formation experiments were conducted under various conditions. This study focused on the research of frosting phenomena on cryogenictemperature surfaces under natural convection conditions when the cold plate is placed horizontally and vertically, especially the early frosting phenomena. The influences of the surface temperature, the air temperature and the air relative humidity on the frost formation on the vertically-placed plate were also carefully studied. The frost accumulation in cryogenic equipment will bring serious harm to the system and equipment operation. Hence people developed many anti-frosting technologies, such as hydrophilic and super-hydrophobic surfaces which both have good suppression effect on ordinary low temperature conditions. However, these good suppression effects under the condition of cryogenic-temperature were not confirmed through experiments. So the another work of this paper is that, make some frost experiments on super hydrophobic surface by chemical etching method under cryogenic-temperature condition, and explore the effective frost suppression technology on the cryogenictemperature surface.Through a large number of experiments, we found that in the condition of very low temperature, when cold surface temperature was below-165?, it will appear a lot of liquid substances in the cold surface firstly, then the formation of the frosting crystal, this phenomenon is the first discovery in frost field. We use notes with mars to contact the liquid substances, note quickly burn immediately, which shows that the liquid material include oxygen composition. Further experiments, it was found that frost crystal clusters deposited on liquid air droplets are not static, they moves both as and relatively to the droplets. In addition, we make large experiments about influences of surface temperature, air temperature and air relative humidity on frost formation on cryogenic-temperature surface. We found that under very low surface temperature and natural convection conditions, as the surface temperature drops, the frost layer thickness decreases. This is completely different to the phenomenon observed on ordinary cold surfaces. The same experiments were conducted for the cases that the cold surface was placed vertically. As observed for the horizontal surface, the liquid droplets were also observed on the cold surface if the cold surface temperature is below-165?. However, due to gravity effect, these liquid air droplets rolled downwards and off the cold surface. When liquid air disappears, frost began to grow from the cold surface. While the frost layer shows very strong non-uniform growth. The frost layer starts to form at the upper edge of the cold surface, and then ‘push' downwards gradually. The peculiar thing is that the frost layer seems to march downwards only its thickness reaches to a certain value, and the un-occupied ‘naked' area keeps almost clear of frost. This is something totally different to that of the frost layer growth under ordinary low temperature conditions. And it is also different to that of the frost layer growth when the cold plate is placed horizontally.Finally, we conducted the frosting experiments on super hydrophobic surface under natural convection and cryogenic temperature conditions, found that the frost layer thickness between super hydrophobic side and ordinary side is the same basically, that is to say, super hydrophobic surface almost no have suppression effect under cryogenic temperature condition. So we need to explore new frost suppression technology under cryogenic temperature condition from other methods in the future.