Applicability of the Homogeneous Nucleation Theory to the Condensation in Free Gas Expansions

The traditional application of the classical homogeneous nucleation theory (CNT) to the condensation in rapidly expanding flows involves the use of the steady-state nucleation rate. Since this rate is derived under the assumption of both steady-state and isobaric/isothermal conditions, the applicability of CNT to highly nonequilibrium environments may be questionable. In addition, the usual strategy of CNT – gas dynamics coupling violates the original nucleation theory even in the isothermal/isobaric environment. In this study, we consider condensation in jets freely expanding into a vacuum. Using the isentropic solution, we approximate the time-dependent pressure and temperature in a given small volume of a gas that is moving along the jet axis with the flow velocity. Several possible strategies of CNT implementation are considered within this volume. It is shown that the terminal cluster distributions are strongly affected by the steady-state assumption and that the original CNT rate equations should be integrated into a computational scheme to model the coupled condensation flow.