The mixture fraction and mixture fraction variance equations do not appear under Fluids Models in CFX-Pre, but they are solved as Normal Transport Equations. Thus, they require boundary conditions to be set just like any other equation. The mf and mf variance equations use the laminar Thermal diffusivity as the mass diffusion coefficient if the default Lewis no. = 1 assumption is used. If Lewis no. is not equal to 1 (under Solver Control -> Advanced Options -> Multicomponent Energy Diffusion), then the bulk viscosity is used. The diffusivity of both equations is dominated by the turbulent diffusion, so it is not usually necessary to change the laminar diffusivity. There is no way to set an arbitrary laminar diffusivity in the mf and mf variance equations.