[eldorado]

Hi everybody,

I'm currently trying to simulate a flow in a generic cylinder, where the mass fractions of the included species of a variable composition mixture change with respect to the local temperature in the numerical domain. I have the empirical dependency of the mass fractions and the temperature:

i.e. (generic)
1) Y_H2SO4 = x0 * T^3 + x1 * T^7, where T is the local (nodal) Temperature and Y is the mass fraction of H2SO4

2) Y_H2SO3 = 1 - Y_H2SO4

How can I include this expression, so that the mass fractions of each defined species change with respect the formula above? Is it necessary to add sink/source terms in a subdomain? Here I have the problem that the source term of H2SO4.mf has a unit of kg m^-3 s^-1 but how is it calculated? I’d suggest Density * Mass Fraction of the Specie and some time? Is there any way how this can be accomplished?

[ghorrocks]

If you are modelling chemistry then you should consider using the chemistry model in CFX, which is based on more realistic reaction rate equations such as Arrhenius. Then you can model this just using the built in models and not have to use unusual empirical relations.

But to answer your direct question: use a source term to specify the mass fractions as a function of temperature. You will need to use the specified value approach (source term = -C*(P-Pspec), source term coefficient = -C) for this.

It is highly unlikely that you have found the units of the source term to be wrong. These things are checked very thoroughly. What is far more likely is you have misinterpreted what the parameter you are looking at is. Is it a total source or specific? Which parameter are you referring to?

[Opaque]

I am not certain what kind of multicomponent fluid you are trying to model.

Hopefully, you are not bypassing the ANSYS CFX chemistry/reacting modeling capabilities.

Here is an idea for you to try, at you own risk:

1 - Create the multicomponent fluid, say two materials A, and B, named "mixtureAB"
2 - Select "mixtureAB" for the Fluid definition for the domains you have in your setup
3 - Define the model you want to use for each material within the mixture as "Algebraic Equation" except for one. That material will be your ballast/constraint
4 - Input the expression that defines the Mass Fraction as a function of Temperature as you described in your original post. Be mindful and careful with the units/dimensions

Run the simulation. Hopefully those expressions for Mass Fraction make sense, and the solution will converge.