About the material in mfix

I’m new user .When i use mfix ,I don’t know Which species the properties belong to?

The properies in the top table (Density, viscosity etc) pertain to the fluid phase, which in this case is a misture of the two species. The properties belonging to the individual species are in the bottom table (Molecular weight, heat of formation). To edit the individual species properties, double-click on a row, or select a row and click on the ‘edit’ (pencil) icon.

– Charles

But the two fluid both has their own density and viscosity.So is there any mistake if the single properties contains the two fluid?
The above I have said may be unclear,so I put a new screenshot on below.

The question is that if I want initialize the entire flow field with air,its density and viscosity is not real.How could I solve this problem?

If you choose constant density, species’ information won’t be important for density. In this scenario, you have air that has 998 kg/m^3 density and 0.001 Pa.s viscosity. Mass fraction would be important for molecular weight, reaction rates, etc.

Look at the constant tabs near density and viscosity. You need to select the ideal law or write UDF for density calculation. Ideal gas law and Sutherland’s law is a good choices for the mixture.

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Set Density to “Ideal gas law”
Set Viscosity to “Sutherland’s Law”
Set Mol. Weight to “Mixture”.

Then the properties for the fluid phase will be computed from the species properties and mass fractions.

If you select “Constant” then the individual species properties are not used and a constant value is used for the phase. This is probably not what you want, if the mass fractions are changing.

– Charles

I upload my file.I want initialize the my model (a cuboid,40003006mm)with air and inject H2O_L and solid,could you help me set my file?(like the fluid and initial condition)
5.mfx (24.4 KB)

Hi. We are happy to answer any specific questions that you have, or address any bugs or difficulties that you come across. But please try to be more specific in your requests. Thanks.

– Charles

The geometry model goes like this:a cuboid(4×0.3×0.006m)which has three inlet on the left, the whole right bottom is an outlet and the other bottom is wall.
My physical problem goes like this: at first air fills my entire computational domain, and then a mixture of water(H2O_L) and particles is injected at a constant velocity at each entrance.
Could you help me to set my file?

Air and H2O_L are both fluids, so add these both to the Fluid table:

In Initial conditions, specify the composition for the fluid phase as 100% air, 0% water:

In the Boundary conditions for the inlets, use the Composition table to specify that the fluid phase contains only water. The balance between water and solids is controlled by the volume fraction control.


Thank you for your answers!
When I refer to your steps I notice that the fluid properties have only one which is belong to H2O_L.
So where are the density and viscosity of AIR?
Thanks again!

The “Properties” in the Fluid panel are bulk properties of the fluid phase, not of each individual species. So this is density and viscosity of the air/water mixture.

If the composition of the mixture is changing (starting with air and injecting water), then Constant density does not make sense

This is why you should use Ideal gas law / Sutherland’s law / Mixture for the fluid phase properties, this way the density, viscosity and molecular weight will be computed dynamically depending on the ratio of the air/water mix

Thank you very much!