Boundary condition question (diffusive flux)
 

Hello everyone,

I'm currently modelling an ethanol fuel cell anode, and I need to add a boundary condition to an interface. This interface is between a porous region, where an ethanol solution permeates through, and a catalyst layer. Once the ethanol solution reaches the catalyst, a reaction happens.

This boundary condition will simply state that (ethanol diffusive flux)=(reactive flux) within the interface region.

How can I write an expression that includes this ethanol diffusive flux? I've only found the variables for the velocities, but those are for the bulk I presume.

Thanks in advance.

I presume your ethanol is a mass fraction equation. So you just add a mass fraction source term at the interface.

Hi Glenn, thanks for the reply.

I'm aware I need to create a source for the ethanol mass fraction at the interface, but my question is how can I write an expression that would be equivalent to the stated BC (diffusive flux=reactive flux)?

I tried writing a source like this: -massFlowEth/areaInterface

But didn't quite work as intended.

Are you modelling the chemistry to get the reactive flux? Or does it come from something else?

It is an electrochemical reaction. Put it simply, the reactive flux depends on the ethanol local concentration at the interface. So I guess I just need to declare the kinetics expression as the source term?

Yes, that's right.