Nonequilibrium evaporation from a heated liquid layer

The nonequilibrium evaporation or condensation under weightless conditions across an interface between a viscous gas and a viscous liquid, which is heated from below at a finite depth, is analyzed. An exact solution of the problem involving a moving boundary between a liquid and a gas-vapor mixture is obtained. Numerical results are used to demonstrate the dependence of the evaporation rate on the ratios of density, pressure, temperature, and the molecular weights of the two phases separated by the interface. It is shown that in a nonequilibrium evaporation process, heat transfer across the interface is not merely to supply the heat of evaporation as assumed in equilibrium evaporation, but also to effect the change in the internal energy of the gas-vapor mixture and to provide the flow work and kinetic energy required for the removal of vapor by a nondiffusive process. A numerical example is given to show the magnitude of the error arising from the equilibrium assumption. Based on the obtained exact solution, a conceptual experiment for the determination of the evaporation coefficient devoid of natural convection effect at microgravity is suggested.