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-   -   using source terms instead of defining reaction ( June 18, 2013 05:22

using source terms instead of defining reaction
Hi all,I have faced a big problem about my project that is adsorption of vapors onto the porous surface,Would you tell me kindly please when do we define reactions and when we use SOURCE TERMS in species transport?

I defined surface reaction in my problem but it doesn't work.

I hope your helps to progress my tez.

A CFD free user June 25, 2013 03:35

Default reaction system in FLUENT are volumetric reactions based on Arrhenus relation which in most cases is sufficient but in some cases such as a PEM fuel cell which reactions are not Arrhenus but are electrochemical, you need to define them using source terms, something I'm getting into at the moment. June 26, 2013 04:57

Hello A-A Azarafza
Thank you so much for your reply,Do you know also that how can we determine the destination of mass transfer in our mass source term UDF?I mean that for example how did you determine the destination of your species in their transfer onto the poles?
Is it necessary to write a separate UDF for this purpose?

thanks alot

A CFD free user June 27, 2013 02:47

Well, if I want to describe my case, as I'm not going to involve liquid phase in my model(I consider all species in gas phase), so I don't need to define a mass transfer source term, but I need to define a reaction source term where takes place inside catalyst layers. Actually, I add the source term to continuity and species transport equations. But, let me tell you about the water transport through membrane, there are three different mechanisms that transport water cross the membrane, for instance, water transport due to Fick's law which is sort of mass transfer. For this, I just get the water concentration in anode and cathode layers, respectively and then obtain the rate of mass transfer using a ADJUST macro. As far as I figure you're kind of involving an adsorption process, in your case, I think you should define your reaction (sink) on solid zone (source term in species transport equation) and get the mass transfer rate using a Adjust macro something like my case. I hope it helps. June 29, 2013 06:23

sorry,I wrote an udf for mass transfer sink for my species,but I'm new in fluent & udf and unfortunately I have basic questions yet, do you mean that I should write ONE udf with both DEFINE_SOURCE macro & ADJUST macro?Or SEPARATE 2 UDF'S?
Also,how should I specify that the path of this species mass transfer(species No. 0) is from gas phase to porous medium?I mean that what should I write for this purpose in the udf?Would you kindly help me please?
thank you very very much for your help and attention.

A CFD free user July 1, 2013 05:47

Hello again
Sorry for a little late response, actually I was in a work trip.
Yes, as you mentioned, I suppose you should define the two macro in one UDF. Using source macro, define the reaction on desire zone (porous zone) and then define your mass transfer equation using general applicable UDF (adjust macro). I think your problem would be figured out. Respecting to the species flow direction, as far as I know there's no necessity to define it, coz, species are transfer depending on their chemical potential and sort of concentration.
I hope it helps.

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