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Two-phase flow though porous media - gas and liquid permeability

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Old   October 9, 2014, 10:39
Default Two-phase flow though porous media - gas and liquid permeability
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Hello,

I would like model a two-phase flow (air and water) through a porous media using the VOF method.

My question is do I have to set different permeability for each phase (liquid and gas)? Or the permeability is the same for both phases? If not, How can I related liquid and gas permeability?

Many articles just mentioned one value for the permeability when simulating two-phase flow through porous media. But in Fluent you have to set the permeability for each phase...

Can anyone give some insights about this? I would appreciate!
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Old   October 20, 2014, 12:44
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Quote:
Originally Posted by Rui_27 View Post
Hello,

I would like model a two-phase flow (air and water) through a porous media using the VOF method.

My question is do I have to set different permeability for each phase (liquid and gas)? Or the permeability is the same for both phases? If not, How can I related liquid and gas permeability?

Many articles just mentioned one value for the permeability when simulating two-phase flow through porous media. But in Fluent you have to set the permeability for each phase...

Can anyone give some insights about this? I would appreciate!
Hi Rui_27,
To simulate the flow in the porous media, I set the viscous/inertial resistance in fluent.
Acctually,I am facing a similar questions right now.
In my simulation, the porous wick is saturated with liquid water. However, evaporation occurs at the surface of the porous wick.
Should I set these parameters for water vapor?
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Old   October 21, 2014, 02:28
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Rui, You should use the same permeability in this case. Just change the inertial resistance factors if they are different for air and water through the medium that you are modelling.
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Old   October 22, 2014, 09:49
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Hi David, thank you for your answer!

I have now another question about porous media formulation in Fluent that i think you might be able to help me.

I did some preliminary simulations changing the porosity of the media, and i obtained some surprising (at least for me) results: increasing the porosity makes water to flow "slower" in the porous media, and i was expecting the opposite!

I found in Fluent's manual that porosity only affects heat transfer properties (which i am not considering) and the time-derivative in the continuity equation. However, I don't understand how can this lead to the results i obtained.

Can you give some insights about that? thanks!
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Old   October 22, 2014, 09:52
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Did you change any of the inertial or viscous loss coefficients?
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Old   October 22, 2014, 10:02
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No, just the porosity. Attached a picture with the results i mentioned.

Some details about the simulations:

I am modeling two-phase flow (water and air) through a channel that as a porous and a non-porous region.
Air is entering at the upper part of the channel (from left to right) while water is entering at the bottom part of the channel (the porous media) and through a porous directly into the upper part of the channel.
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File Type: jpg Sem título.jpg (44.4 KB, 104 views)
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Old   October 22, 2014, 10:35
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Do you have the "Relative Velocity Resistance Formulation" turned on?
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Old   October 22, 2014, 10:40
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Yes, Relative Velocity Resistance Formulation is on.

I just set 1E12 for the viscous resistance in all directions, and the porosity of the porous media.
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Old   October 22, 2014, 10:54
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A porosity greater than 1 makes no sense. If the porosity is 0 then it is solid. If the porosity is 1 then it is an open volume. Therefore setting it to 1E12 is not correct. Are you interested in viscous resistance in this simulation? 1E12 is very high if it is not based on anything physical.
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Old   October 22, 2014, 11:40
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Sorry, i was not very clear in my previous message. I set 1E12 just for the viscous resistance! For the porosity I tried 0, 0.1, 0.5 and 1. For porosity of 0, the solution diverged on the first iteration because as you said the porous media become a solid, and water cannot move through a solid. That i was expecting. But when I increase the porosity, water moved faster through the porous media with 0.1 of porosity and slower when the porosity was 1. That is what I can not understand... i was expecting that water would move more easily for larger values of porosity.
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Old   October 22, 2014, 12:46
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With a low porosity the physical velocity is higher and therefore the water would seem to flow faster. Essentially at low porosity there is less open volume and therefore for a given inlet volumetric flow the time to fill will reduce. At a porosity of 1 the volume is at its maximum and therefore the time to fill is longest.
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Old   October 22, 2014, 12:52
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David, thanks a lot for your help and patience!

I now understand the results i obtained.

best regards
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