Pressure Drop

The pressure drop caused by vapor flow across the ice-free layers in a slab is proportional to dZ/dt. Consequently, it can be shown that

where Ps is the pressure at the sublimation interface, Pa the pressure at the outer surface of the slab, and K is the permeability of the porous ice-free layers, /f is proportional to the volume fraction of ice initially present, roughly inversely proportional to ice-crystal diameter, increases somewhat as the mean partial pressure of water in the pores increases, and decreases if excessive amounts of non-condensibles are present. Typical K range between 0.7 x 10"9 and 10 x MT9 kg • m"1 • s"1 • (/an Hg)"1 (5 x 10~9 to 70 x 10~9 s-1 in SI units).

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