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 Pete January 12, 2004 00:14

Please help: How to set BC for the mass diffusion

HI,

I'm trying to solve a simple mass diffusion problem. Two reservoirs separated by a thin membrane. One reservoir fills with NaOH, the other just distrilled water. And I want to measure the concentration of OH at the other reservor for a certain time. I just want to do it in 2D ( and only x-dir flow). So the configuration would be very simple, a rectangular (is this correct? or does it need to be 3 rectangulars i.e. left reservoir, membrane (solid), and right reservoir???).

Question?

1. What type of the Boundary condition for left and right side of the membrane? 2. I believe I need to input the mass flow rate (or mass flux), but since the OH is in the reservoir and just diffuses by the concentration gradient, velocity should be zero, shouldn't it? 3. is there any example of diffusion problem in Fluent help?

Thank you very much for your help in advance. Pete

 tyw January 13, 2004 04:27

Re: Please help: How to set BC for the mass diffus

for you first question, I think u can try the porous jump bc to act as the membrane. havent use it, but the fluent manual describe it can be use as a performable plate. I suppose it also can be use to act as a thin membrane??

2q, are you try to simulate the diffusion of the NAOH or just the OH ion itself?? in the case of diffusion of the ion, i think you need to create the two ions, i.e. NA and OH. then you must know the diffusivity of both ion. If I remember correctly we need to input the D by ourself.

hope this help tyw

 Pete January 13, 2004 10:47

Re: Please help: How to set BC for the mass diffus

Thanks for your suggestion tyw,

I'm looking for only OH ion, not Na ion.

What about the mass flow rate? The diffusion occurs because of the concentration gradient, no flow? ( Can I assume to be a laminar flow with very low velocity?)

Thanks, Pete

 tyw January 15, 2004 21:23

Re: Please help: How to set BC for the mass diffus

are you simulating a closed system, if closed system then i dont think we need to input the mass flow rate. even if you want to give the mass flow, where do you think the mass flow b.c. are?? Since you are tring to simulate a concentration gradient driven diffusion, i dont think we need to difine the mass flow. Just provide the gradient and diffusion should be triggered

for diffusion of different ions, i think you need to create three different species: NA,OH and the third species in the other chamber.

May be you can try a more simple setting first??: just two connected chamber with out the present of the membrane??

I suggest you refer to the species transport tutorial provided in FLUENT. inside the tutorial you can learn how to setup a diffusion simulation.

hope this help.

tyw

 kayne91 August 7, 2013 00:53

hi there,

i am also trying to simulate a pure ion diffusion process across a membrane (without any flow). is species transport the right model to start with?

Regards,
kayne

 A CFD free user August 8, 2013 11:47

Hi
Yeah, you can do it by species transport or even you can define a UDS. The only thing you need to care about is the right model for diffusivity or already have it experimentally.

 kayne91 August 8, 2013 20:58

Quote:
 Originally Posted by A CFD free user (Post 444606) Hi Yeah, you can do it by species transport or even you can define a UDS. The only thing you need to care about is the right model for diffusivity or already have it experimentally.
alright. thank you so much for your advice. now i am confident to continue with my model research.

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