Time-dependent rarefied gas flow of single gases and binary gas mixtures into vacuum

Time-dependent flows of single gases and binary gas mixtures with various molar fractions from a chamber through a short tube into vacuum are examined by simulations and experiments. The main goal is the comparison of the flow behaviour between pure gases and binary mixtures including the investigation of the gas separation effect. The simulations are based on an explicit hybrid scheme, coupling at each time step the tube flow rate estimated from a database accordingly constructed via a DSMC solver to the gas pressure in the chamber obtained by a mass conservation equation. Computational results describing the expansion process in terms of the temporal evolution of pressure in the chamber as well as of the total and species flow rates through the tube are reported. Comparison between computational and the corresponding deduced experimental pressures deduces a difference of about 10% which may be explained by accumulation of uncertainties in the time sequence. The binary gas mixture pressure evolution curves are always bounded by the corresponding ones of the components of the mixture flowing as single gases and the tube conductance varies in the opposite direction to that of the weighted mean of the component molar masses. Gas separation is monotonically increased as we move from the viscous towards the free molecular regime, while the rate with which gas separation is increased has a maximum in the transition regime. The equivalent single gas approach provides a reasonable estimate provided that the molar fraction of the light gas and the ratio of heavy to light molar mass are not too high.