FROST FORMATION UNDER DIFFERENT GASEOUS ATMOSPHERES

Rates of water frost growth in a vessel with a cooled horizontal plate were experimentally determined under reduced pressure atmospheres of hydrogen, helium, methane and nitrogen, The mass deposited on the cool ed surface under each of the atmospheres was almost in proportion to t ime. The Sherwood number under the condition of no mist formation, Sh( 0), in the atmospheres of methane and nitrogen was in good agreement w ith Catton's equation for natural convection between horizontal parall el plates, Sh(0) in a hydrogen atmosphere was unity, which corresponds to control by molecular diffusion in the stagnant gas. The tendency o f the decrease in Sit due to mist formation could be evaluated well by multiplying Sh(0) by a factor zeta(CSM). The zeta(CSM) value was calc ulated based on the critical supersaturation model as a function of th e two interface temperatures and the total pressure, Frost growth rate s under each atmosphere were in proportion to [(T-S1-T-W1) t/(1+1/A(S1 ))](0.5). The proportional constant for hydrogen was greater than that for any other tested gas, Agreement and disagreement of the frost eff ective thermal conductivity with previous models were discussed.