[lordluan]

Hi there, I am new in CFD and I am studying it by myself. And I have some questions.

I know that I can look at the theory guide and try to relate the options to the equations. But Is there a simple way of doing it?

Also I am confused with adding source terms and others expressions. If I looked at the literature and saw a set of equations that I should use, how can I know make the relation between the variables which are already avalaible in CFX and expressions I need to add?

I'm reading the tutorials to see if I understand how to make the relations I mencioned above, but so far, I couldn't understand.

Thanks in Advance

Luan

[ghorrocks]

The documentation is the only place you will find what the underlying equations are.

Source terms - you just have to match up the variables and derive expressions for ones which are not available. Again, there is no shortcuts.

[lordluan]

Thanks a lot!

[lordluan]

Originally Posted by ghorrocks

The documentation is the only place you will find what the underlying equations are.

Source terms - you just have to match up the variables and derive expressions for ones which are not available. Again, there is no shortcuts.

Ghorrocks, the available mass transport equation available in CFX is the continuity equation? This is what a could understand from the documentation. If I want to simulate a mass transfer I ought to add mass source terms to the continuity equation? I am reading the documentation, but I am lost in adding the source terms.

[Opaque]

A description is key here. The word mass transport is used in many contexts; therefore, it is not clear what you are trying to achieve.

Since you already have a set of equations in mind from some publication, post the equations you would like to solve and here we can try to guide on how those equations are presented in ANSYS CFX.

[lordluan]

Originally Posted by Opaque

A description is key here. The word mass transport is used in many contexts; therefore, it is not clear what you are trying to achieve.

Since you already have a set of equations in mind from some publication, post the equations you would like to solve and here we can try to guide on how those equations are presented in ANSYS CFX.

Thanks very much for answering.

I am modelling a 3D adsorption which have different equations that could be used. In the adsorption I have a packed bed which is percolated by multicomponent fluid.
1. (see the attachment) The mass balance equation in CFX documentation doesn't have a source term (the whole equation is equal to 0). I created a variable composition mixture of gases to be my multicomponent fluid, the equations CFX will use to solve are similar to those attached? I will associate the specific aspects of adsorption using the source term of the mass balance equation. The equation for the momentum balance is clearer.

I am reading the documentation, but I couldn't figure out this aspect of the problem.

[Opaque]

Based on those equation, check the documentation and tutorials for "Multicomponent Flow".

Learn how to create a variable composition mixture material
Select that material as your fluid at the domain level
add the diffusion coefficient for each material component (recall diffusion in the mixture)
Set the proper boundary conditions for the material components at inlets if any.

Unfortunately, ANSYS CFX only supports zero flux boundary condition at walls. Your description does not show with boundary conditions you require.

If the material components are just transported by the flow, and diffused, you should be fine at this point. If material components are generated, or consumed, the you need to set a reaction mechanism describing such process.

Hope you can move forward with the above information. Let us know how it goes.

[lordluan]

Thanks for the help!

With adsorption one of the components will be "consumed" by the surface of the porous solid as the flow percolates it. I read the tutorial reacting flow in a mixture tube. But tua tutorial doesn't explain the connection between the sources created to the equations that are available by default um CFX.
If CFX has the governing equation that I want to add. Are there a variable name by default which I have to use (for momentum, mass and energy governe equations)? I've already looked at CFX-pre guide.

[Opaque]

The active equations are presented in the Domain panel:

For a fluid/porous domain:
- the continuity and momentum are active by default.
- you must decide if the energy equation is needed (I guess it is for your case), and select the appropriate model, Thermal or Total (read the documentation guidelines)
- The continuity for the species are presented as Component Details, and you must select a ballast/constrain material. See the panel for details, and set Diffusion coefficient appropriately.

By now, you should be set with the equations in the interior.

For the adsorption at the boundary, open the boundary panel and set a boundary mass source/sink using the continuity equation listed there. The panel will guide you through what data is required. You should know how the adsorption depends on other parameters and determine the adsorption rate.

[lordluan]

Originally Posted by Opaque

The active equations are presented in the Domain panel:

For a fluid/porous domain:
- the continuity and momentum are active by default.
- you must decide if the energy equation is needed (I guess it is for your case), and select the appropriate model, Thermal or Total (read the documentation guidelines)
- The continuity for the species are presented as Component Details, and you must select a ballast/constrain material. See the panel for details, and set Diffusion coefficient appropriately.

By now, you should be set with the equations in the interior.

For the adsorption at the boundary, open the boundary panel and set a boundary mass source/sink using the continuity equation listed there. The panel will guide you through what data is required. You should know how the adsorption depends on other parameters and determine the adsorption rate.

Opaque, thank you very much for the incredible help.
Soon, I'll post my results.

[lordluan]

Is it possible to add a source term to the continuity equation in ANSYS CFX? In solver theory guide the continuity equation is set to be equal to 0, but in some papers related to phenomena I want to model, the continuity equation is set to be equal to a mass source term.

[Opaque]

Check the documentation for sources

There is a way to add sources to continuity equation, which automatically adds the contribution to the other equations as well.

[lordluan]

Originally Posted by Opaque

Check the documentation for sources

There is a way to add sources to continuity equation, which automatically adds the contribution to the other equations as well.

Hello, so far I've created a subdomain which covers all my domain. In the subdomains options I went to Sources and Toggled on the continuity equation. But, as stated in the documentation I have to add not only my mass source equation, but also other info like velocity, mass fraction, turbulence eddy dissipation and others, what got me really confused. But I'm working on it.

Thanks in advance