Subcritical and Supercritical Nanodroplet Evaporation: a Molecular Dynamics Investigation

Molecular dynamics simulations are used to investigate the subcritical and supercritical evaporation of a Lennard-Jones (LJ) argon nanodroplet in its own vapor. Using a new technique to control both the ambient temperature and pressure, a range of conditions are considered to define a transition line between subcritical and supercritical evaporation. The evaporation is considered to be supercritical if the surface temperature of the droplet reaches the LJ argon critical temperature during its lifetime. Between ambient temperatures of 300 K and 800 K, the transition from subcritical to supercritical evaporation is observed to occur at an ambient pressure 1.4 times greater than the LJ argon critical pressure. For subcritical conditions, the droplet lifetimes obtained from the simulations are compared to independently predicted lifetimes from the D2 law. NOMENCLATURE cp constant pressure specific heat D droplet diameter E energy hLV latent heat of vaporization kB Boltzmann constant K evaporation coefficient L system cell length ∗Address all correspondence to this author. m mass N number of atoms Nd number of atoms in droplet Ns number of shells ri j particle separation r, r particle position, distance from center of droplet Rd droplet radius Ru universal gas constant t time T temperature v, vi,r, vi,t velocity vector, radial velocity, thermal velocity V volume Greek α shell label δ mass diffusion coefficient εLJ Lennard-Jones energy scale φ potential energy ρ density σLJ Lennard-Jones length scale τd , τe, τh droplet lifetime, evaporation period, heating period Subscripts CM center of mass cr critical point property D drift 1 Copyright c © 2007 by ASME i particle label, inner o outer ∞ ambient (“far-field”) property Superscripts L liquid property V vapor property INTRODUCTION The ability to predict droplet evaporation rates (and thus lifetimes) is important in non-premixed spray combustion applications such as liquid rocket engines, gas turbines, and Diesel engines [1]. Subcritical evaporation in an environment with negligible forced or natural convection effects is well described by the D2 law [1]. This model predicts the time rate of change of the square of the droplet diameter, D, to be constant, i.e.,