In the code shown above, diffusity is multiplied by density and volume fraction. However, in the MFIX Documentation Numerical Technique by Madhava Syamlal, no terms as density and volume fraction appear in the diffusion term as shown below:
I can understand why volume fraction is multiplied by referring the MFIX Documentation Theory Guide, in which heat flux is multiplied by volume fraction, and energy equation is similar to species equation.
But where does the density come from?
Thank you for your reply!
You are referencing old material. The equation you share is simply the generic scalar equation from the numerical guide. If you were to look at the theory guide more closely it would indicate no diffusive flux is included in the species conservation equation. MFIX does include this term. A simply comparison of units of each term indicates that the gamma term in your generic equation must include mass. So you have diffusion coefficient in units of length^2/time and mass comes in through density.
Thank you for your reply. Your answer is very enlightening!
