DIRECT SIMULATION OF CONCENTRATION CREEP IN A BINARY GAS-FILLED ENCLOSURE

A Cartesian, two-dimensional enclosure containing an isothermal rarefi ed binary gas mixture is studied as a limiting-case model for actual c rystal growth experiments conducted in reduced gravity environments. B y employing a microscopic approach related to the Boltzmann equation, it is demonstrated that in the presence of appreciable partial concent ration gradients a steady-state flow pattern develops, driven by kinet ic boundary layers adjacent ro solid boundaries. In contrast, a macros copic analysis based on the continuum transport equations and the clas sical no-slip boundary condition would predict no flow whatsoever. For the case of equal mass species, the velocity scales involved are show n to increase with the disparity in accommodation coefficients, in agr eement with expectations based on one-dimensional, linearized Knudsen sublayer theory, while quantitative comparison between simulations and the latter theory reveals significant confinement effects. Simulation of concentration creep in binary mixtures composed of disparate mass species requires an alternative computational procedure, motivated by surface recombination/dissociation reactions. For this case, flow fiel ds and creep coefficient values for a range of mass ratios are also re ported. It is concluded that future continuum-level modelling efforts should more fully exploit the detailed information now available from relevant microscopic simulations. (C) 1996 American Institute of Physi cs.