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Fig. 1.9. Dependence of the ice sublimation intensity Is\ a - on duration of the process for samples of coarse-crystalline (1) and fine-crystalline (2) ice; b - on the air flow velocity v at t = — 5°C and various thickness of dry zones £dry in samples of poly mineral clay; c - on the temperatures of air t from 0°C moving with velocity 4.5ms"1 (1) and stationary (2) for samples ofpolymineral clay at £ = 5 mm; d - on pressure of stationary air above samples of ice (1), polymineral clay (2), kaolin (3), loam (4) (the values Is given here are for the fixed instant of time t = 220 h in the process); e - on the molecular composition of the stationary gas medium for samples of ice (1), polymineral clay (2) and kaolin (3); M = molecular weight.

Fig. 1.9. Dependence of the ice sublimation intensity Is\ a - on duration of the process for samples of coarse-crystalline (1) and fine-crystalline (2) ice; b - on the air flow velocity v at t = — 5°C and various thickness of dry zones £dry in samples of poly mineral clay; c - on the temperatures of air t from 0°C moving with velocity 4.5ms"1 (1) and stationary (2) for samples ofpolymineral clay at £ = 5 mm; d - on pressure of stationary air above samples of ice (1), polymineral clay (2), kaolin (3), loam (4) (the values Is given here are for the fixed instant of time t = 220 h in the process); e - on the molecular composition of the stationary gas medium for samples of ice (1), polymineral clay (2) and kaolin (3); M = molecular weight.

unfrozen water with its progressive restoration by the melting of ice. It is known that the process of ice sublimation in soils is associated with the process of phase transitions and desorption proper as well as with the processes of external and internal heat and moisture transfer. Therefore it amounts to a process of ground drying at negative temperatures, or frost drying. At the same time the processes of the internal heat and mass transfer exert the predominant effect on the intensity of the frost drying because the intensity of ice sublimation (phase transitions) under natural conditions is limited by the rate of water vapour removal from the system, that is, by the resistance to moisture transfer rather than the provision of the process with energy.

The intensity of ice sublimation in the soil is defined experimentally as the density of moisture flux through the open surface of a sample, i.e. as a moisture mass Am being lost by the soil from a unit cross-sectional area F during a time interval At: /s = Am/FAr.

The intensity of ice sublimation from the soil is damped out and decreases with time as the thickness of the sublimated (dried) zone increases. Because the process of soil drying proceeds as an interaction with the vapour-gaseous medium, the thermodynamic conditions of this medium exert considerable effect on the intensity of the heat- and mass-exchange process with the exterior. The experimental data show that an increase of Is with increasing air flux velocity is possible up to a particular limit (t) = 4-7ms_1). The effect decreases with the increasing thickness of ground dried zone (see Fig. 1.9b). Lowering of the temperature causes decreasing sublimation intensity on account of the decrease in the moisture transfer coefficient and gradients of transfer potential (see Fig. 1.9c). Experimental investigation of the effect of the overall pressure of the air (in the range from 1 x 105 to 1.2 x 1010 Pa) on Is at a temperature of — 8°C has shown that the sublimation intensity decreases as the pressure is raised (see Fig. 1.9d). This depends mainly on the decrease in the diffusion coefficient. The process of ice sublimation in soils in relation to the vapour-gaseous medium depends fundamentally on the molecular composition of the gas as well. The value Is decreases with the increase of molecular mass from helium (M = 4) to air (M = 29) and to argon (M = 38). This is associated first of all with the decrease of the vapour diffusion coefficient with increasing molecular mass M of the vapour-gaseous medium (see Fig. 1.9e).

The process which is the reverse of ice sublimation is termed ablimation (desublimation), i.e. crystallization of water vapour on soil surfaces accompanied by heat release and ice crystal formation in the form of frost. Ablimation takes place in the event that the soil surface temperature is lower than that of the surrounding vapour-gaseous medium and its intensity grows with increasing value of the temperature difference. In the early stage the process of vapour crystallization has a well-defined selective pattern. Formation of the first crystals of ablimation ice proceeds on edges, vertices and chips of crystals and on other surface faults with excess of surface energy. Then the crystals grow, coalesce and form a continuous cover of frost on the soil surface creating an additional resistance to heat transfer and causing in this connection a decrease in the temperature differences and, consequently, the ablimation intensity 7a decreases with time. The density of the growing ablimation layer varies with its thickness. It decreases with distance from the surface and changes with time because of the metamor-phic process (recrystallization). With time the density of the ablimation layer increases, thus increasing its thermal conductivity. Concurrently with the increase in thickness of the layer under consideration from the surface of the water-unsaturated soil, the inward-directed moisture diffusion can pro

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