KR348: Mixture Fraction and Mixture Fraction Variance variables are an important part of Flamelet and Partially Premixed Combustion Models. Species are stored in the Library as a function of Mixture Fraction.
Why cannot I see these variables in CFX-Pre under FLUID MODELS? Can a user set an arbitrary diffusivity for these variables?

The mixture fraction and mixture fraction variance equations DO NOT appear under FLUID MODELS in CFX-Pre but they are solved as NORMAL TRANSPORT EQUATIONS and so require boundary conditions to be set as for 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.

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