Hello,

I want to calculate a 2D Problem for a reaction-diffusion equation:

with the two concentrations c_f and c_b, the Diffusioncoefficient D, two rate constants lambda and mu.

I would like to implement something in analogy to plane-strain/stress but for plane concentration, so that I still have a Diffusionflux in the z-axis but my concentrations are only diffrent from zero in the xy-plane.

For my problem the Flux in the z-axis would also be sufficient because the ones in the x and y plane are much smaller in comparrison.

Can you help me/explain me how I can implement this in fenics?

Here are the 2 equations in fenics format:

```
> Diff_eqn = dc_f*(c_f-c0_f)/dt *dx + D*inner(nabla_grad(c_f),nabla_grad(dc_f))*dx +\
> phi*dc_f*(c_b-c0_b)/dt *dx
> Reac_egn = dc_b*(c_b-c0_b)/dt *dx - dc_b*lam*c_f*dx + dc_b*mu*c_b*dx
```

Thanks in advance!

Martin