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John August 1, 2006 11:28

Basic Multi-component flow Question
Hi! Unfortunately the user manuals confuse me. I hope someone here can help me.

I'm attempting to solve a ideal gas, CO2-H2O multicomponent flow, setting CO2-transport eqn H2-constraint eqn.

My questions are:

1) What is being solved for NS and Energy eqn? Is it the combined mixture or just a single component?

2) Is there a way to make sure the total pressure is the sum of the partial pressures (Dalton's Law). or is that already taken care of in the code? I know the velocity is considered the same in each component, I just hope the pressure isn't.

opaque August 1, 2006 11:38

Re: Basic Multi-component flow Question
Dear John,

1 - The NS and Energy equation are solved for the fluid (or mixture) that you have defined.

2 - Be careful with what you mean with Total Pressure in CFX, Total Pressure is the stagnation pressure.. I guess you are referring to Absolute Pressure and how it is used when evaluating the thermodynamics properties of your 2 components.

The multicomponent formulation does not make direct use of partial pressure.. I do not see where it fits since the mixture density is mass fraction harmonic weighted value of the density of each components. Each component is evaluated at the Absolute Pressure. These approach is equivalent to Dalton's Law if work out the algebra.

I hope this helps,


John August 1, 2006 12:36

Re: Basic Multi-component flow Question
Just need some clarification, (I'm a little slow...)

>>1 - The NS and Energy equation are solved for the fluid that you have defined.

I defined fluid A. I defined fluid B. I defined fluid A+B.

Is your response that the NS and energy solver, solves for the multicomponent mixture and not for each component?

My confusion arises with the use of "or mixture". Did you mean to say just "mixture".

Thanks for your help.

opaque August 1, 2006 13:05

Re: Basic Multi-component flow Question
Dear John,

ANYS CFX is very "picky" with the words. When you setup a Fluid domain, you select a fluid and the NS equation is solved for this fluid only. Because you are using the MCF approach the velocities of the components is the same (as you previously mentioned).

If you select 2 (or more) fluids (careful not materials), the solver will solve as many NS equations as fluids (assuming you are not using the homogeneous multiphase model).

Now, a fluid must be a Material which in your case happens to be a mixture of single component pure substances, ie. Multicomponent material.

Hope it is clear,


HekLeR August 8, 2006 22:48

Re: Basic Multi-component flow Question
You have used MCF.

The solver solves for one velocity field, one pressure (total pressure as you call it, which is different than total pressure output by the solver) & enthalpy (total say) & a mass fraction eqn for CO2 in your case.

Dalton's law is obeyed by assuming harmonic mass fraction weighted mixture density (eg: density).

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