Nonequilibrium molecular dynamics study on energy accommodation coefficient on condensing liquid surface—Molecular boundary conditions for heat and mass transfer

Abstract

Nonequilibrium molecular dynamics (NEMD) studies have been conducted to determine molecular boundary conditions at vapor–liquid interfaces for the kinetic theory of condensation and evaporation. In previous studies, a microscopic formulation of the condensation coefficient was defined as the condensation probability of vapor molecules based on equilibrium molecular dynamics simulations and transition state theory. The condensation coefficient was presented as a function of the translation energy of incoming molecules and surface temperature. Based on this, the velocity distributions of evaporating and reflecting molecules were theoretically expressed under equilibrium conditions. In a practical nonequilibrium situation, the energy transfer by the reflecting molecules is important along with the condensation/evaporation probability. However, it is unclear whether the results obtained under equilibrium conditions can be applied under nonequilibrium conditions. This study, therefore, defines the energy accommodation coefficient of reflecting molecules by comparing the energy transfer due to reflection with that under equilibrium conditions. NEMD simulations are conducted using two surfaces facing each other, an evaporating surface and a condensing surface, for argon molecules under different nonequilibrium conditions. The results show that the velocity distribution of reflecting molecules deviates from those under equilibrium conditions, and the energy accommodation coefficient decreases as nonequilibrium conditions increase. Additionally, an inverted temperature profile is observed. Reflecting molecules play an important role in the sensible heat transfer on the condensing surface, and they are not accommodated on the condensing surface. Thus, they raise the temperature in the vicinity of the condensing surface under nonequilibrium conditions.