Frost deposition on cooled surfaces under reduced pressure

Deposition rates and growth rates of frost on cooled surfaces under reduced pressure were obtained by using a freezing apparatus to reproduce natural convection where an upper surface was cooled at constant temperature and water was evaporated from a lower surface. It was designed for a basic study of a cryogenic freezer proposed for a fuel clean-up system of a fusion reactor. The experiment was carried out in the ranges of 6 × 102 Pa to 1 × 105 Pa of total pressure and 77 K to 261 K of the cooling temperature. Sh numbers at the initial stage of frosting could be fitted to values evaluated using a conventional correlation of natural convection multiplied by a factor based on the critical supersaturation model. Frost thickness and average frost density under reduced pressure grew with the square root of time. In addition, frost thickness increased with [(T_s1-T_w1)t]^0.5 over all the ranges of the experimental conditions except where frost melted on the interface. The difference in frost thickness with the total pressure was qualitatively correlated by a parameter which was defined by the ratio of latent heat flux to sensitive heat flux.