THE treatment of rarefied gas flows by means of equations based on the mechanics of continuum media is desirable because solving such equations requires less computational resources than methods based on a molecular description, eg, direct simulation Monte Carlo (DSMC). 1

A popular way of handling rarefied flow problems consists in solving Navier-Stokes (NS) equations with modified boundary conditions (BCs) to account for velocity slip and temperature jump. 2 Using Burnett equations is expected to lead to better results when the degree of nonequilibrium increases. Burnett equations, however, suffer from a number of disadvantages: they are considerably more complicated and there is no appropriate theory for writing their BCs. Furthermore, there is little evidence that they bring any significant improvement compared with NS equations for problems of practical interest. Both Burnett and NS equations can be considered as approximate models derived from Boltzmann equation using different orders of approximations.