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Transport of diluted species physics for diffusion through two layers

Posted 15 oct. 2014, 18:56 UTC−4 Chemical Reaction Engineering Version 4.4 3 Replies

Shailesh Konda
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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 oct. 2014, 02:06 UTC−4
Lasse Murtomäki
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Posted: 1 decade ago 16 oct. 2014, 07:53 UTC−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
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Posted: 1 decade ago 16 oct. 2014, 16:59 UTC−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
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Posted: 1 decade ago 17 oct. 2014, 02:06 UTC−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.

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