On Diffusion-Induced Non-Constant Composition Profiles in the Boundary Layer of Inert Multicomponent Mixtures

In the boundary layer of multicomponent fluid mixtures, the species-specific mass flux in the wall-normal direction is determined by the combination of turbulent-diffusiophoretic diffusion due to composition gradients, and diffusion due to gradients in other scalar fields (e.g. thermophoresis, barophoresis, and forced diffusion). For inert mixtures, a balance must exist between all the diffusive transport mechanisms so that the net diffusive mass flux normal to the wall is zero everywhere. This paper discusses under which conditions non-constant composition profiles are necessary to obtain physico-chemical equilibrium, hence vanishing transport, in the wall-normal direction. Mathematical modeling is employed to demonstrate how this may affect fluid property profiles, wall heat flux, and wall shear stress in an ideal, ternary gas mixture ($H_2 + N_2 + CO_2$) subject to a temperature gradient. It is concluded that competing diffusive transport mechanisms, under certain circumstances, may result in non-constant composition profiles and significantly different wall and bulk compositions, for inert multicomponent mixtures. Hence, wall-normal diffusion in the boundary layer must be accounted for to correctly describe or interpret multicomponent flow systems sensitive to wall friction and heat exchange.