Dh. Papadopoulos et De. Rosner, DIRECT SIMULATION OF CONCENTRATION CREEP IN A BINARY GAS-FILLED ENCLOSURE, Physics of fluids, 8(11), 1996, pp. 3179-3193
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.