Influence of velocity slip and temperature difference on internal relaxation in diatomic–monatomic mixture free-jet expansion

Abstract

The supersonic expansion of a mixture of a heavy gas and a light gas may result in a velocity slip and a temperature difference between light and heavy species. The purpose of this paper is to study the influence of these phenomena on the relaxation of internal degrees of freedom which takes place when one of the gases is a diatomic one. The model presented is based on the Wang–Chang–Uhlenbeck theory and on the assumption of a two-velocity and two-temperature model to represent the velocity distributions. The model incorporates the internal degrees of freedom of the diatomic molecule and can be used to study the effect of velocity slip and temperature difference on the mechanism of internal energy transfer involved in a free-jet expansion. The hydrodynamical equations and the master kinetic equations are derived. The formalism is applied to the system I2–He, assuming that a rotational temperature and a relaxation equation to describe the rotational relaxation can be defined. The unexpected high rotational temperatures observed experimentally in I2–He expansions presenting a velocity slip are reproduced.