Molecular dynamics simulation of kinetic boundary conditions and evaporation/condensation coefficients of direct-contact condensation in two-phase jet

Abstract

Using non-equilibrium molecular dynamics simulations, the kinetic boundary condition (KBC) for direct-contact condensation in two-phase jet and its evaporation and condensation coefficients were studied to solve the Boltzmann equation. The evaporating and reflecting molecular behaviors were studied in detail. The normalized velocity distributions of reflecting molecules in the z-direction before and after reflection fit the Maxwell velocity distribution shifted by two large macroscopic velocities toward and away from the liquid surface owing to the strong net condensation rate. A singular definition of the evaporation coefficient has not been obtained in previous studies. We used a two-boundary method to count the evaporation coefficient and define a parameter to connect our result and the spontaneous evaporation coefficient. The condensation coefficient was studied using the condensation probability and showed larger than the evaporation coefficient in direct-contact condensation states. Both the evaporation and condensation coefficients were not independent of the incoming mass flux. When the incoming mass flux was small, the values of these two coefficients were close to the values under equilibrium states. Based on these results, we constructed the KBC of direct-contact condensation.