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Dilute species transport concentration condition

Posted Nov 16, 2012, 4:21 p.m. EST Chemical Reaction Engineering Version 4.3a 3 Replies

Martin Hediger
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Hi
I wonder why the "Concentration" condition in the dilute species transport model is of type "boundary" and not "domain", since its unit is in "Mole/Volume". Why does this make sense? How should this be regarded?

Thanks for any input on this.
3 Replies Last Post Nov 19, 2012, 2:10 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 17, 2012, 5:12 a.m. EST
Hi

I agree a concentration density on a boundary seems strange (I'm not by ma COMSOL WS so I cannot check) but my first thoughts would e that on boundaries we defne fluxes or constant values.

Now it happens that a flux is ((myvalue/distance)/surface) which happens to give the same units as a (myvalue/volume) but it is in fact a flux per surface variable

Could tha explain your issue ?

--
Good luck
Ivar
Hi I agree a concentration density on a boundary seems strange (I'm not by ma COMSOL WS so I cannot check) but my first thoughts would e that on boundaries we defne fluxes or constant values. Now it happens that a flux is ((myvalue/distance)/surface) which happens to give the same units as a (myvalue/volume) but it is in fact a flux per surface variable Could tha explain your issue ? -- Good luck Ivar
Martin Hediger
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Posted: 1 decade ago Nov 18, 2012, 2:10 p.m. EST
Ok, so instead of thinking of what concentration to assign to that boundary, i should rather consider the flux at the boundary?
Ok, so instead of thinking of what concentration to assign to that boundary, i should rather consider the flux at the boundary?
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 19, 2012, 2:10 a.m. EST
Hi

Now back on my WS and checking the help file, I understand it not as a constant flux (the per second is missing to), but a infinite reservoir of given concentration, this applies for inflow, as for open boundary

if you check the boundary equations: inflow fixed the concentration on the boundary to the one of a fictitious reservoir on the external side. Outflow, defines a normal flux, but does not fix it in absolute, while open boundary allows for a normal concentration to leave, but a constant concentration to enter

The Flux BC set a flux per sec

--
Good luck
Ivar
Hi Now back on my WS and checking the help file, I understand it not as a constant flux (the per second is missing to), but a infinite reservoir of given concentration, this applies for inflow, as for open boundary if you check the boundary equations: inflow fixed the concentration on the boundary to the one of a fictitious reservoir on the external side. Outflow, defines a normal flux, but does not fix it in absolute, while open boundary allows for a normal concentration to leave, but a constant concentration to enter The Flux BC set a flux per sec -- Good luck Ivar

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