Diffusion in Porous Media

kdep · March 27, 2016, 18:29

Hi everyone, I am new to this forum and looking for some help with respect to diffusion in porous media.

I am using Fluent 17.0. I have a freestream region (an inner box) from which mass is transferred into an catalyst bed (outer box) where a reaction occurs.

My question has four parts. The first two are less questions and more just me trying to put my mind at ease.

1. If I use fixed values under the Cell Zone Conditions tab and set my x, y, and z velocities in my porous region to zero do I still end up with diffusion? I would figure the momentum equation's source term would result in diffusion from the freestream region into the catalyst region, but I am not sure.

2. Going over to diffusion, I saw in the user guide that the continuity and momentum equations for porous media multiplies the diffusion coefficient by porosity, hence reducing it. However, the anisotropic diffusion equation also includes porosity. Will the mass flux vector calculated using anisotropic species diffusion be again multiplied by porosity when it is put into the continuity equation? And I'm assuming that the Schmidt number is irrelevant for laminar flow correct?

3. The relative viscosity value in the porous zone seems to affect my velocity magnitude in the porous region, however I am not sure where and how it is involved in Fluent's calculations.

4. My last question is a little off topic with regards to the other three, but the diffusion term in reaction-diffusion balance for surface chemistry for surface reactions (equation 7.50 in the theory guide) does not have much information about it and I was wondering if anyone would be able to fill me in on where it comes from and whether it accounts for the difference in diffusion between two compounds, e.g. hydrogen and carbon dioxide.

Sorry for the long post, but I have been sitting on these questions for a while and scouring the forum and the user and theory guides has not produced much.

Kurtis

kdep · March 28, 2016, 17:50

My primary concern is turning off convective mass transfer in the porous region, leaving it with only diffusion as a form of mass transport. I have tried using fixed values to set my velocities to zero however my contour plots still show velocity in the porous region. If somebody could quickly help me with that it would be great.

March 27, 2016, 18:29	Diffusion in Porous Media	#1
kdep New Member Ontario Join Date: Mar 2016 Posts: 5 Rep Power: 10	Hi everyone, I am new to this forum and looking for some help with respect to diffusion in porous media. I am using Fluent 17.0. I have a freestream region (an inner box) from which mass is transferred into an catalyst bed (outer box) where a reaction occurs. My question has four parts. The first two are less questions and more just me trying to put my mind at ease. 1. If I use fixed values under the Cell Zone Conditions tab and set my x, y, and z velocities in my porous region to zero do I still end up with diffusion? I would figure the momentum equation's source term would result in diffusion from the freestream region into the catalyst region, but I am not sure. 2. Going over to diffusion, I saw in the user guide that the continuity and momentum equations for porous media multiplies the diffusion coefficient by porosity, hence reducing it. However, the anisotropic diffusion equation also includes porosity. Will the mass flux vector calculated using anisotropic species diffusion be again multiplied by porosity when it is put into the continuity equation? And I'm assuming that the Schmidt number is irrelevant for laminar flow correct? 3. The relative viscosity value in the porous zone seems to affect my velocity magnitude in the porous region, however I am not sure where and how it is involved in Fluent's calculations. 4. My last question is a little off topic with regards to the other three, but the diffusion term in reaction-diffusion balance for surface chemistry for surface reactions (equation 7.50 in the theory guide) does not have much information about it and I was wondering if anyone would be able to fill me in on where it comes from and whether it accounts for the difference in diffusion between two compounds, e.g. hydrogen and carbon dioxide. Sorry for the long post, but I have been sitting on these questions for a while and scouring the forum and the user and theory guides has not produced much. Kurtis

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March 28, 2016, 17:50		#2
kdep New Member Ontario Join Date: Mar 2016 Posts: 5 Rep Power: 10	My primary concern is turning off convective mass transfer in the porous region, leaving it with only diffusion as a form of mass transport. I have tried using fixed values to set my velocities to zero however my contour plots still show velocity in the porous region. If somebody could quickly help me with that it would be great.