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Hi,

I have a 2-D diffusion problem. Actually it is about drying. In the fick's law I use moisture content (kg h2o/kg d.m.) instead of concentration (mol/m3) as defined in Comsol.

At the boundary conditions, I also use a different term: I use the concentration difference between surface and drying air as driving force. But, here the surface concentration is defined in (kg/m3) converted from the moisture content. In principle, with the boundary conditions, the process will final reach an equlibirum with the surrounding air conditions.

But now the problem is the process never stoped...there could be some point that the surface concentration will be equal to the air concentration but in my system, after reaching the equilibrium the process still continued...so I could get some minus values...

I am wondering maybe this problem is caused by the changed boundary conditons? Because I defiened a diffferent term for the boudary flux...

I am thinking maybe one of the solutions could be to set a logical conditions to the boundary conditions. For example, 'if the surface concentration equals to the air concentration then the value for surface concentration will remian the same for the rest running time?'

Is there anyone knows how to solve this problem?

Thanks a lot!

Xin