Molecular Dynamics Study of Condensation/Evaporation and Velocity Distribution of N-Dodecane at Liquid-Vapour Phase Equilibria*

Some recent results of molecular dynamics simulations of the condensation/evaporation and velocity distribution of n-dodecane (C12H26), the closest approximation to Diesel fuel, at a liquid-vapour interface in equilibrium state are briefly described. It is shown that molecules at the liquid surface need to gain relatively large translational energy to evaporate. Vapour molecules with large translational energy can easily penetrate deep into the transition layer and condense in the liquid phase. The evaporation/condensation coefficient is shown to be controlled mainly by the translational energy. The properties of the velocity distribution functions of molecules at the liquid, interface and vapour regions are summarised. It has been shown that the distribution functions of evaporated and reflected molecules for the velocity component normal to the surface deviate considerably from the Maxwellian, while the distribution function for all molecules leaving this surface (evaporated and reflected) is close to Maxwellian. The evaporation coefficient has been shown to increase with increasing molecular energy in the direction perpendicular to the surface. These properties have been recommended to be taken into account when formulating boundary conditions for kinetic modelling.