Experimental study of frosting process on a cryogenic surface under trace water vapor conditions
To study the frosting process of trace water vapor on the cryogenic surface,a visualized test rig is designed.Under conventional humidity and trace water vapor conditions,the frosting characteristics of water vapor in room-temperature nitrogen gas flow on a cold surface at-180℃are compared.The effects of trace water vapor content and pressure on frost formation are investigated as well.The results indicate that,compared to the frost formation dominated by the condensation-freezing mechanism under conventional humidity conditions,trace amounts of water(volume fraction is 71×10-6)desublimates into crystalline grains over the cryogenic surface to form unevenly distributed frost crystals without obvious dendritic structure,followed by the formation of a flat frost layer.The deposition of crystalline grains mainly promotes the growth of frost thickness,but has little contribution to the densification of frost layer.As the water vapor content increases from 1.53×10-6 to 500×10-6,the frost layer becomes thicker,and the frost morphology transforms from a flat and uniform thin layer to clustered frost masses.The growth rate accelerates due to the enlarged contact area between nitrogen flow and frost layer.With the increase of ambient pressure from 109 kPa to 190 kPa,the absolute content of water vapor increases with the partial pressure of water vapor.The greater mass transfer driving force results in a rise in the unevenness of frost layer and a more diverse deposition pattern of crystalline grains.A dimensionless correlation of frost layer thickness is fitted for trace water vapor frosting under cryogenic conditions,with an overall error within±20%.
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