FRICTION-WEIGHTED SELF-CONSISTENT EFFECTIVE BINARY DIFFUSION-APPROXIMATION

The self-consistent effective binary diffusion (SCEBD) approximation f or multicomponent diffusion in gas mixtures is reconsidered and reform ulated. The new formulation is based on the fact that a suitable rearr angement of the Stefan-Maxwell equations provides an exact expression for the complementary mean velocity a(i) for species i as a weighted a verage of the velocities of all the other species. The coefficients in a(i) are normalized friction coefficients which are simply related to the true binary diffusion coefficients. A simple factorized bilinear approximation to the friction coefficients then yields approximate spe cies diffusion fluxes identical in form to those of a previous intuiti ve treatment [4], together with a new relation between the previously ambiguous weighting factors w(i) and the friction coefficients. This r elation places the SCEBD approximation on a firm foundation by providi ng a rational basis for determining the w(i). A simple further approxi mation based on the known form of the friction coefficients for hard s pheres yields w(i) = (const.) rho(i)/root M(i), where rho(i) and M(i) are respectively the mass density and molecular weight of species i. T hese weighting factors are shown to produce considerably more accurate diffusion velocities than the conventional choice w(i) = (const.)rho( i)/M(i).