Development of a kinetic nucleation model for a free-expanding argon condensation flow

Using classical nucleation theory (CNT), a homogeneous condensation model has been developed in previous work to simulate condensation in a free-expanding plume using the direct simulation Monte Carlo (DSMC) method. However, the accuracy of the CNT nucleation theory is questionable due to unphysical assumptions such as the use of macroscopic cluster surface tension for small clusters and the lack of an incubation time for nucleation. Molecular dynamics (MD) simulations performed in previous work confirm that the fundamental mechanism for the initiation of condensation is through dimer formation in two-stage ternary collisions of monomers. In this work we propose a kinetic nucleation model based on the mechanism whereby stable dimers are created from triple collisions. The new hybrid MD-DSMC kinetic nucleation model is implemented in a DSMC simulation of cluster growth processes, starting from dimers, in a condensation plume. Comparison of the distributions of steady-state plume cluster number density and size using the kinetic and CNT nucleation models shows that the condensation physics is significantly different for the two nucleation models investigated. The use of a physically more realistic nucleation model is shown to generate terminal cluster properties consistent with experiments in free-expanding jets.