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

Abstract

Some recent results of molecular dynamics simulations of the condensation/evaporation and velocity distribution of n-dodecane (C 12 H 26 ), 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.

Molecular dynamics study on evaporation and reflection of monatomic molecules to construct kinetic boundary condition in vapor–liquid equilibria

“…Forf z+ ref , the results for velocity distribution functions obtained by other measurement methods showed a similar tendency, as shown in Refs. [11,18]. Furthermore, before and after the reflection of the molecules, we found that the velocity distribution functions in the z direction almost coincide with each other.…”

“…Moreover, the velocity distribution functions of the evaporating and condensing molecules were almost the same as the Maxwell distribution at a liquid temperature of 85 K. However, the temperatures of the velocity distribution functions were slightly higher than that of the Maxwell distribution: a temperature of approximately 89.5 K was obtained from the nonlinear least square method, which leads to the increase of average velocity of evaporating or condensing molecules. According to the recent studies [16][17][18], when molecules evaporate or condense at the interface, the translational energy of the molecules becomes higher than that of molecules in the bulk phase. Also, the increasing of the temperature obtained from the velocity distribution function of evaporating molecules was reported [11,18], which agrees with the present results.…”

“…According to the recent studies [16][17][18], when molecules evaporate or condense at the interface, the translational energy of the molecules becomes higher than that of molecules in the bulk phase. Also, the increasing of the temperature obtained from the velocity distribution function of evaporating molecules was reported [11,18], which agrees with the present results.…”

“…For KBC, the velocity distribution function must be evaluated in the bulk vapor phase in the vicinity of the bulk liquid phase [6,7]. However, in previous MD simulations [11][12][13][14]18], there were ambiguities in the positions of evaluating the velocity distribution function or related molecular mass fluxes. On the other hand, the position of KBC (vapor boundary position) was defined at z * = 3.0 in the present study, based on our previous studies [6,7].…”

“…[9][10][11][12][13][14][15][16][17][18][19][20]). As one of the methods to avoid the ambiguities introduced in the process of assigning evaporating or reflecting molecules based on MD simulations, a concept of spontaneous evaporation was assumed [12], and the molecular mass flux due to spontaneous evaporation was confirmed with MD simulation based on virtual-vacuum evaporation [10,12].…”