Variable density model requires inert species

For the variable density model, I did not find a specific explanation in the mfix help literature. Please let me know if there is specific help documentation. I’d also like to ask, do I have to set up an inert particle for the variable density model? If I don’t want to set an inert particle, can I use two of the same reactive particles?Is this reasonable?

Have you worked through the variable density Biomass gasification tutorial?

If you are using variable density, then there must be a non-reacting species within each particle phase. This is not the same as having an inert particle. It just means that not all of the chemical species within a particle can be reactive.

Hope this helps,
– Charles

Thank you very much for your answer, I see what you mean. But I would like to ask if you have a more detailed explanation of the variable density model, because in practice, particles do not have non-reactive components.

The variable density model as implemented in MFiX requires a non-reacting (inert) species. If this is not usable, you may attempt to change the code, but I have no idea how difficult this would be…

You could also try making the mass fraction of the non-reacting species very low.

Thanks for your reply. I’m still wondering if there are any specific instructions or formulas for the variable density model? Is it a uniform transformation model or a shrinking core model?

Please see

section of Madhava Syamlal, M. Musser, J., and Dietiker, J.-F., “The two-fluid model in the open-source code MFiX”, book chapter in CRC Multiphase Flow Handbook Computational Methods, CRC Press, October 2016.


section 3.5.1 of Theoretical Review of the MFIX Fluid and Two-Fluid Models (Technical Report) | OSTI.GOV

Thank you very much for your answer! I’d like to make sure, can I set the inert particle to the reactive particle as one component? And the ratio of the two is 0.01:0.99. Does that have any other effect?

Sorry I don’t understand your question. The solids phase chemical composition is defined with chemical species. If you want to use the variable density model, one of these species must be inert.

Yes, I meant to set both the inert component and the reactive component as one component:

If you are talking about CaCo3 and CaCO3_s, these are treated as different species (they have different names), although they can have the same properties (Cp, density, MW).