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Old   January 17, 2017, 20:31
Post Convection-Diffusion Modeling
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Ryan
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Hi All,

Overall model: I am modeling a flow (chemical A entering as a component with water as the solvent) over a porous media, where a chemical reaction (A->B) occurs within the porous media. I want to have only diffusion of the chemical species within the porous media. I have diffusion coefficients and an overall reaction rate.

My question: I believe I need to implement equations derived from the convection-diffusion equation within the porous media. This would cover the diffusion of the chemical species and the reaction as well. I want know how to input a differential equation (or how to convert the equation into a form valid for CFX to handle it) into CFX to handle these physics.

I have looked through the tutorials, and chapter 15 seems to be the only one that deals with chemical reactions, but mine is far from what is covered in that model. This is because for my reaction, I am holding it at neutral pH and the reaction is isothermal.

Any help would be greatly appreciated!

Last edited by minnesota123; January 17, 2017 at 21:43.
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Old   January 18, 2017, 04:42
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If you want to implement a convection-diffusion equation, then in CFX you want to define an additional variable. Then define the variable to be a convection equation and give it a diffusion coefficient.

Just a thought: Would it be simpler to use the CFX built-in chemical reaction stuff, but use a momentum source term to define your own porous material model? Then you can put source terms in any equation you like. Not sure if this helps, but I thought I would mention it.
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Old   January 18, 2017, 10:05
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The overall convection-diffusion is a differential equation and it is my understanding that I cannot input the differential equation, as simulations need numerical approximation forms of the equations. This is where I am stuck because I cannot find anything online on adding these types of equations to CFX.
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Old   January 19, 2017, 01:33
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As stated in my previous post, you implement this equation using the additional variables in CFX.

In CFX you cannot directly enter a differential equation (like you can in matlab or FREEFEM). You are limited to the built-in differential equations and fortunately the convection-diffusion equation is one which is built in.
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Old   January 19, 2017, 10:36
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If possible, can you post the equation you want to solve ? Then, some of us here can point out how it matches the built-in implementations in ANSYS CFX and advise you on the next steps if possible.

For porous media, you must account for volume porosity and interactions between solid and fluid region within the media. It would be good to see how the equation includes them in the model, and what is possible using the software.
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Old   January 19, 2017, 10:44
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removed duplicate

Last edited by Opaque; January 19, 2017 at 10:45. Reason: Duplicate
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Old   January 19, 2017, 11:54
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The equation I would like to implement is:

dC/dt = ( Grad dotprod D*Grad(C) ) + k*C

Where Grad is the gradient, dotprod is the dot product, C is the concentration of a species, D is the diffusion coefficient, and k is the reaction rate.

Any help on inputting this into CFX would be greatly appreciated!
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Old   January 19, 2017, 15:01
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ANSYS CFX solves the following equation for an additional variable

d (density * AV_specific)/dt = div (density * D_kinematic * grad (AV_specific)) + S_av

If your material density is constant, you should have no problem. If it is variable density, there will be a minor difference in the diffusion term.

You can Insert an Additional Variable, named as needed, select Specific, Scalar and set the units required.

Edit your Domain panel, Fluid Models section, activate the AV listed, select Transport Equation, set the Kinematic Diffusivity (D_kinematic) in the equation above, press Apply

Set boundary conditions for your AV (notice they must be C / density).

For the source term, insert a subdomain, and add the source term to the AV.

Read the documentation for fine details of what you are setting to get familiar what it really means.

Hope the above helps,
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