Hello Dumux, I am back with the same question and some more infos. The modeling approach for diffusion in Dumux is (in my opinion) wrong and gives unphysical results. This is most pronounced for diffusion in a gas phase and when the intrinsic permeability is low.
Consider the model setup depicted in Figure1.jpg. In this setup the only
transport mechanism for H2O from the right boundary is diffusion because the
sink of N2 is high and the advective flow is from left to right. In this setup,
the pressure will drop to the right while the gradient of x_g^H2O is vice
versa. However, the concentration of H2O will be lower at the right boundary
and diffusion will occur from a lower to a higher concentration!
This is completely unphysical. There is no reason why the component should flow
against its concentration gradient. The reason for this lies in the formulation
of the diffusive fluxes:
[cid:[email protected]]
Where
[cid:[email protected]]
In the Dumux formulation, the second (pressure dependent) term on the very
right is neglected resulting in the possibility that species diffuse against
their concentration gradient.
The density gradient is proportional to the pressure gradient from left to
right while the molfraction gradient is vice versa.
I hope this made things more clear.
I would recommend to use grad(rho*x) instead of grad(x) for the calculation of
the diffusive fluxes.
I am always open for discussion.
Kind regards
Georg
Von: Dumux [mailto:[email protected]] Im Auftrag von
[email protected]
Gesendet: Mittwoch, 25. November 2015 14:30
An: [email protected]
Betreff: [DuMuX] A fundamental question concerning diffusion in Dumux
Hello Dumuxers,
I was wondering why the diffusive fluxes in Dumux are defined as
D*rho*grad(mol-or-massfraction). Typically one would use D*grad(c) (e.g. Ficks
law) where c=rho*mol-or-massfraction. Using the Dumux equation means that local
differences in the density are neglected for diffusive fluxes. Is there any
reason/justification for this? Or is my thinking just wrong?
Best regards
Georg Futter
--------------------------
German Aerospace Center (DLR)
Institute of Engineering Thermodynamics | Computational Electrochemistry |
Pfaffenwaldring 38-40 | 70569 Stuttgart
Dipl.-Ing. Georg Futter | Ph.D. student
Telefon 0711/6862-8135 | [email protected]<mailto:[email protected]>
www.DLR.de<http://www.dlr.de/>
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