Rarefied gas mixtures with large species mass ratio: Outflow into vacuum

Abstract

The theoretical study of the processes of the outflow of a binary gas mixture from a source into a vacuum through an orifice in an infinitely thin wall is presented. Two mixtures with a large species mass ratio K are considered: Au–Ne ( K = 9.76) and Au–He ( K = 49.21). The work continues the study of the flow of Ag–He mixture ( K = 26.95) started in Bykov and Zakharov [“Binary gas mixture outflow into vacuum through an orifice,” Phys. Fluids 32, 067109 (2020)]. The results of the direct simulation Monte Carlo made it possible to propose approximations of the mass flow rates of the species and the mixture depending on the species mass ratio, the flow rarefaction degree, and the mole fraction of light species in the source. It is shown that with an increase in the parameter K, an increase in the dimensionless mass flow rate of the mixture referred to the corresponding free molecular value is observed. The maximum dimensionless flow rate corresponds to the near-continuum regime and exceeds the value obtained using the hydrodynamic approximation and the equivalent single gas approach. A variation of K also leads to changes in the spatial distributions of the dimensionless density and velocity of the mixture and some axial focusing of the flow. An increase in the species mass ratio for the case of a small initial mole fraction of the heavy species in the source for a flow regime close to the hydrodynamic one leads to an increase in acceleration and axial focusing of the heavy species.