Abstract
Evaporation of a binary liquid into near vacuum conditions has been studied using numerical solutions of a system of two coupled Enskog-Vlasov equations. Liquidvapor coexistence curves have been mapped out for different liquid compositions. The evaporation process has been investigated at a range of liquid temperatures sufficiently lower than the critical one for the vapor not to significantly deviate from the ideal behaviour. It is found that the shape of the distribution functions of evaporating atoms is well approximated by an anisotropic Maxwellian distribution with different characteristic temperatures for velocity components normal and parallel to the liquid-vapor interface. The anisotropy reduces as the evaporation temperature decreases. Evaporation coefficients are computed based on the separation temperature and the maximum concentration of the less volatile component close to the liquid-vapor interface. This choice leads to values which are almost constant in the simulation conditions.
Highlights
The liquid-vapor phase change is a complex, multiscale process which involves length scales spanning many orders of magnitude
The anisotropy of the distribution function of atoms evaporating into near vacuum has already been found by previous molecular dynamics (MD) simulations and it was attributed to the hard-sphere collisions in the liquid-vapor interface, which become more important as the temperature of the system increases [20,55]
In spite of the simplified description of the microscopic dynamics, the proposed modeling approach leads to a unified description of both the liquid and vapor phases and it permits us to critically assess the boundary conditions that are commonly assumed at the liquid-vapor interface by standard kinetic theory studies, while being far computationally cheaper than MD
Summary
The liquid-vapor phase change is a complex, multiscale process which involves length scales spanning many orders of magnitude. The standard kinetic theory literature on evaporation and condensation processes focuses on the vapor dynamics next to the liquid-vapor interface and takes into account the molecular exchanges with the liquid phase through phenomenological boundary conditions. The present paper studies the evaporation of a binary liquid into near vacuum conditions by means of a system of two coupled Enskog-Vlasov equations for a mixture of spherical atoms interacting through Sutherland potentials. The rest of the paper is organized as follows: Section II outlines the standard kinetic theory approach to evaporation and condensation processes and briefly discusses the problem related to the assessment of the boundary conditions commonly adopted at the liquid-vapor interface.
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