INTERFACE THERMODYNAMIC MODEL FOR LOW-PRESSURE EVAPORATION

The evaporation rate of liquids at low pressure is in many cases no longer a decreasing, monotonous function. This phenomenon known as the Smith-Topley effect is explained in terms of the theory of interface irreversible processes. In order to do this, the interface equations are reckoned in a stationary process. Then it can be shown that the expression for the entropy production is mainly affected by two conservative flows. These flows are then expressed in terms of associated generalized forces using phenomenological relations. An explanation of the Smith-Topley effect requires two phenomena to be taken into account: the occurrence of a thermodynamic disequilibrium at the evaporation interface and a dependence of the coefficient of thermal conduction of gases at low pressure, on pressure. The formula obtained gives the evaporation rate versus the pressure and includes, in addition to the phenomenological coefficients, the temperature gradient in the gas. This gradient is assumed to be constant in the pressure range where the investigated effect occurs. The curves obtained account rather well for the evaporation rate variation and agree with experimental results. Comparison between theory and experiment allows several parameters specific to the phenomenon to be determined. © Copyright by Walter de Gruyter & Co.