  The simulation of rarefied gas flow based on the Boltzmann equation is
challenging, especially when the gas mixtures have disparate molecular masses.
In this paper, a computationally tractable kinetic model is proposed for
monatomic gas mixtures, to mimic the Boltzmann collision operator as closely as
possible. The intra- and inter-collisions are modelled separately using
relaxation approximations, to correctly recover the relaxation timescales that
could span several orders of magnitude. The proposed kinetic model preserves
the accuracy of the Boltzmann equation in the continuum regime by recovering
the four critical transport properties of a gas mixture: the shear viscosity,
the thermal conductivity, the coefficients of diffusion and the thermal
diffusion. While in the rarefied flow regimes, the kinetic model is found to be
accurate when comparing its solutions with those from the direct simulation
Monte Carlo method in several representative cases (e.g. one-dimensional normal
shock wave, Fourier flow and Couette flow, two-dimensional supersonic flow
passing a cylinder and nozzle flow into a vacuum), for binary mixtures with a
wide range of mass ratios (up to 1000), species concentrations, and different
intermolecular potentials. Pronounced separations in species properties have
been observed, and the flow characteristics of gas mixtures in shock waves are
found to change as the mass difference increases from moderate to substantial.
