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Old   April 21, 2009, 03:20
Default Question for modelling flow in porous media
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Yang Xiaoguang
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My model contains two zones; the upper is gas channel contains hydrogen and water vapor, and the lower part is porous media. I want to model the diffusion of gases in porous zones but don't know how to set the boundary conditions in fluent. Here is my questions:

1. What is the difference between the two types of boundary conditions: porous-jump and porous zones? Can I set the lower zone to be the porous zone and set the interface between the two parts porous-jump condition?
2. What does the parameters mean in porous-jump condition. Can I set the face peameability the same as the lower porous zone? And what does the thickness here mean? Is it the thickness of the lower porous zone?
3. In porous-zone media, what does the parameter direction vector means?

I really hope someone can help me? Thank you very much!
Best wishes my friends.

legend
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Old   April 21, 2009, 04:19
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I believe its enough to just declare the lower part as a porous zone. You don´t need the porous jump boundary at all for your problem.
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Old   April 21, 2009, 08:36
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Andrew
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if I remember correctly, the porous jump is a pressure drop zone simulating the pressure drop across the porous media. As stated, that is not your case. I think your problem will be forcing flow through the porous media as opposed to all of it going through the upper channel. But, I am not too sure of your flow direction..west->east?..or does it go through the porous media first and then into the open zone (south->north)?
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Old   April 21, 2009, 10:26
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Yang Xiaoguang
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Thank you very much mettler.

The mixed gas come from the upper channel inlet and flows from west to east. As the lower part is the porous zone the gas may diffuse into the lower one (north ->south). How to deal with the boundary condition of the interface between the two fluid zone?

Sincerely
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Old   April 21, 2009, 10:39
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Don´t specify any boundary condition at the interface.
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Old   April 21, 2009, 10:39
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Thank u very much Hawimeddel!
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Old   April 21, 2009, 10:53
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exactly, if you set up the porous zone Fluent will take care of the rest. You need to check into how much diffusion you are getting. I didn't have much luck when I was trying something similar. You might need to make it extremely porous to get an significant flow.
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Old   April 21, 2009, 14:46
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Hey Yang, Are you working on a mixing problem. I had a similar question too and I'm not sure of how the diffusivity of two different gases are modeled. Shouldn't the porous media definition consist of some kind of diffusivitiy co-efficient too apart from the pressure resistances?
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Old   April 21, 2009, 22:13
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Hi Aroon, the mass diffusivity coefficient of two different gases in fluent is talked about in "section 8.9" of "fluent6.3 user's guide". I wish that section will give u some ideas. However, in porous media, the coefficient should be modified including the effect of porosity and tuotorsity , but I still don't konw how to set it in fluent.
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Old   April 21, 2009, 22:24
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the permeability and inertia coefficient have to be set for Fluent's model. The only way I was able to do that was to actually experimentally measure the pressure drop through the porous media as a function of velocity. After I plotted that out I curve fit it to a second order equation. That let me use the Forcheimer equation to get the permeability and the inertia coefficient. There are a lot of papers written that might have values you could use too..
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