A unimolecular evaporation model for simulating argon condensation flows in direct simulation Monte Carlo

In previous work, a cluster evaporation model was derived from the classical nucleation theory (CNT) to simulate condensation in free expanding plumes using the direct simulation Monte Carlo (DSMC) method. However, the use of a CNT evaporation model, especially in a low temperature environment, is problematic because macroparameters such as cluster surface tension and vapor saturation pressure are not physical for small cluster sizes. In this work, we propose a kinetic based evaporation model obtained from unimolecular dissociation theory (UDT) to model argon cluster evaporation processes in a free expanding plume. The UDT argon cluster evaporation model has been directly verified by molecular dynamics and quasiclassical trajectory simulations. It is found that although there is about one order of magnitude difference in the CNT and UDT evaporation rates, these two theories predict similar cluster evaporation rate trends as a function of cluster size and temperature. The verified new UDT evaporation model, as well as the previous CNT model, are applied to a free expanding argon condensation plume simulated with DSMC. The simulation results show that although there are some differences in cluster number density and average cluster size using the CNT and UDT evaporation models, the condensation onset conditions and Rayleigh scattering intensity for both models agree reasonably well with experimental data.