Menu

Discussion Forum

New Discussion
Filtra

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Transport of diluted species physics for diffusion through two layers

Posted 15 ott 2014, 18:56 GMT-4 Chemical Reaction Engineering Version 4.4 3 Replies

Shailesh Konda
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Hi,

I have two layers of diffusive material, each having different diffusion co-efficient. There is a concentration gradient between bottom and top layer. So, i know the top and lower concentration for the layers. I have incorporated transport of diluted species physics in the two domain. I am not sure what boundary condition should i apply on the interface between two layers. Any heads up on this problem?

-Shailesh
3 Replies Last Post 17 ott 2014, 02:06 GMT-4
Lasse Murtomäki
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago 16 ott 2014, 07:53 GMT-4
You need not to define a boundary condition in the middle interface, if you don't particularly want that. Comsol assumes the flux continuity at the interface, i.e. D1*grad(c1) = D2*grad(c2). But if there is a thin barrier between the layers use that. The only parameter you need is the permeability of the barrier (cm/s). Funny enough, in Comsol you define it as D/d where D is the diffusivity in the barrier and d its thickness, but the only thing that matters is their ratio.

Lasse
You need not to define a boundary condition in the middle interface, if you don't particularly want that. Comsol assumes the flux continuity at the interface, i.e. D1*grad(c1) = D2*grad(c2). But if there is a thin barrier between the layers use that. The only parameter you need is the permeability of the barrier (cm/s). Funny enough, in Comsol you define it as D/d where D is the diffusivity in the barrier and d its thickness, but the only thing that matters is their ratio. Lasse
Shailesh Konda
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago 16 ott 2014, 16:59 GMT-4
I thought the same. But the problem with that approach is , the interface between the two layers is kept by default at No flux boundary condition in the physics. So i was not sure, how should i define that interface boundary condition between the two domains.
I thought the same. But the problem with that approach is , the interface between the two layers is kept by default at No flux boundary condition in the physics. So i was not sure, how should i define that interface boundary condition between the two domains.
Lasse Murtomäki
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 10 years ago 17 ott 2014, 02:06 GMT-4
OK, you have also two physics nodes, i.e. twice "Transport of Diluted Species". Let's say that in domain 1 your variable is c1 and in domain c2. Then make a flux boundary condition in domain 1

N = -M*(c1-c2)

and in domain 2

N = M*(c1-c2)

if the flux is from domain 1 to domain 2. M is a mass transfer coefficient at the boundary, ( ) m/s. If there is no transport barrier between the domains, put M = 1 m/s, for example. This makes sure that the flux towards the boundary remains under diffusion control in the bulk of the domain.

But if you have only one physics node with two diffusion coefficients in two different domains, there should not be "No flux" BC.
OK, you have also two physics nodes, i.e. twice "Transport of Diluted Species". Let's say that in domain 1 your variable is c1 and in domain c2. Then make a flux boundary condition in domain 1 N = -M*(c1-c2) and in domain 2 N = M*(c1-c2) if the flux is from domain 1 to domain 2. M is a mass transfer coefficient at the boundary, ( ) m/s. If there is no transport barrier between the domains, put M = 1 m/s, for example. This makes sure that the flux towards the boundary remains under diffusion control in the bulk of the domain. But if you have only one physics node with two diffusion coefficients in two different domains, there should not be "No flux" BC.

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content