Mass Transfer - Forced Convection Boundary Condition

I am trying to apply a forced convection boundary conditon to a time-dependent PDE. The BC I want to apply is:

The mass transfer coefficient, density, diffusion coefficient, and Xa are all assumed to be constant. What is the best way to go about doing this?

I set up a Neumann BC with

I was able to get a solution but wasn't sure if I set this up appropiately.

I also tried to set up a Dirichlet BC with

applyBoundaryCondition(model,'dirichlet','Edge',4,'r',@fun1,'h',1);

function BC = fun1(~,state)
kc = 0.0021;
Xa = 3*10^-6;
rho_s = 1500;
Ds = 4.1*10^-4;
BC = Xa - (rho_s*Ds/kc).*state.uy;
end

but I wasn't able to get a solution. Does anyone have any comments if either of these approaches are appropiate or, if not, how should I approach this probelm?

Answer 1

[Ravi Kumar]

Your Neumann BC approach is the right option. But be aware that you can only model mass diffusion within domain, forced convection at the boundary is fine.

Regards,

Ravu