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Again, mass conservation with MS equations
Posted 25 nov 2010, 07:58 GMT-5 2 Replies
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Hello,
This question has to do with another topic I started last week but with which, unfortunately, nobody could help me.
In the file I have attached, I created a pipe through which oxygen and water (gas phase) circulate. Navier-Stokes and Maxwell-Stefan equations are used to describe momentum and mass transport. The lower boundary (boundary 3) is supposed to be a reactive surface and therefore an oxygen flux is set as boundary condition. If I calculate the oxygen mass flow (kg/s) that is going into the system (using the boundary integration tool, integrating the flux) and subtract the oxygen mass flow that is going out, I approximately obtain the same amount of oxygen that is disappearing through the reactive boundary, which means that mass balance for oxygen meets satisfactorily. However, if I do the same to check the water mass balance, the result is really not good. Calculating the difference between the water mass flux at the inlet and at the outlet, I obtain a noticeably different amount than the water that is being generated on the reactive boundary, which is the same (in mass terms) as the oxygen that is being consumed. The minumum mesh element quality is 1.000, so I guess that I cannot blame this on truncation errors or similar.
Can anybody give me an explanation for this?
Thank you very much for your help!
This question has to do with another topic I started last week but with which, unfortunately, nobody could help me.
In the file I have attached, I created a pipe through which oxygen and water (gas phase) circulate. Navier-Stokes and Maxwell-Stefan equations are used to describe momentum and mass transport. The lower boundary (boundary 3) is supposed to be a reactive surface and therefore an oxygen flux is set as boundary condition. If I calculate the oxygen mass flow (kg/s) that is going into the system (using the boundary integration tool, integrating the flux) and subtract the oxygen mass flow that is going out, I approximately obtain the same amount of oxygen that is disappearing through the reactive boundary, which means that mass balance for oxygen meets satisfactorily. However, if I do the same to check the water mass balance, the result is really not good. Calculating the difference between the water mass flux at the inlet and at the outlet, I obtain a noticeably different amount than the water that is being generated on the reactive boundary, which is the same (in mass terms) as the oxygen that is being consumed. The minumum mesh element quality is 1.000, so I guess that I cannot blame this on truncation errors or similar.
Can anybody give me an explanation for this?
Thank you very much for your help!
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2 Replies Last Post 30 nov 2010, 05:02 GMT-5
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Posted:
1 decade ago
Hi Diego,
I got your file in order to try to see where is the problem.
Looking at your boundary condition for the NS equation I suspect there
is something strange on the bottom boundary (ie: N3). Here you have set
a no-slip condition which is valid for a standard wall. But on this boundary
I think that due to chemical reactions you have a mass flux through the boundary.
In such a situation the no-slip condition is not valid. You must introduce a
condition on the normal velocity (which is this case is not 0 !) on the boundary.
One simple way to do that may be:
- compute the mass flux (mPhi_w) through the boundary (-1e-3*200[kg/(m^2*s)]*w_O2, I guess)
- compute the volumic mass at the wall Rho_w (=rho_MSc I guess).
- on the boundary set V= mPhi_w/Rho_w=-1e-3*200[kg/(m^2*s)]*w_O2/rho_MSc
You may use Weak constraint (Non-ideal) for the NS system. (from my experience, if we
not use them when a non zero velocity is set on boundaries the behavior of solution
may become erratic). You may find the relevant entries into the Properties submenu
of Physics for NS.
I hope this will help you.
Bye. J.M.
I got your file in order to try to see where is the problem.
Looking at your boundary condition for the NS equation I suspect there
is something strange on the bottom boundary (ie: N3). Here you have set
a no-slip condition which is valid for a standard wall. But on this boundary
I think that due to chemical reactions you have a mass flux through the boundary.
In such a situation the no-slip condition is not valid. You must introduce a
condition on the normal velocity (which is this case is not 0 !) on the boundary.
One simple way to do that may be:
- compute the mass flux (mPhi_w) through the boundary (-1e-3*200[kg/(m^2*s)]*w_O2, I guess)
- compute the volumic mass at the wall Rho_w (=rho_MSc I guess).
- on the boundary set V= mPhi_w/Rho_w=-1e-3*200[kg/(m^2*s)]*w_O2/rho_MSc
You may use Weak constraint (Non-ideal) for the NS system. (from my experience, if we
not use them when a non zero velocity is set on boundaries the behavior of solution
may become erratic). You may find the relevant entries into the Properties submenu
of Physics for NS.
I hope this will help you.
Bye. J.M.
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Posted:
1 decade ago
Hi!
Thank you very much for your help! However, despite I followed your indications I still have the problem. Water mass balance does not match, so I will have to study this carefully. Anyway, I owe you one because your suggestions about using the Weak constraint and setting the velocity on the lower boundary were really useful.
Thanks!
Thank you very much for your help! However, despite I followed your indications I still have the problem. Water mass balance does not match, so I will have to study this carefully. Anyway, I owe you one because your suggestions about using the Weak constraint and setting the velocity on the lower boundary were really useful.
Thanks!