  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.
