Discussion Forum

Hi all,

The equations I am solving are simple set of equations just to get a feel for the software as I build more complicated systems. Here, I have a simple diffusion-reaction equation of Zn+2OH->ZnO+2H2O I am solving using equation based modelling since thats what I will be using in the later part of my modelling. Here, in domain 1(centre square) the initial conc of Zn is given and all other species are set to zero. The idea is that OH diffuses from outside (outer annular square) into the inner square, reacts with Zn to give ZnO and H2O. ZnO does not diffues and H2O must diffuse outside. I have formed a union of the system and enforced conituinty at the edges. Now my question is, 1) Although I have given the diffusion coefficient in domain 1 and 2 for Zn and ZnO to be zero, I still see both these species diffusing out of the central square which should not occur? 2) The solution produced negative results for ZnO inside the centre square which can never be the case since ZnO is the species being produced. After being produced the ZnO is not supposed to diffuse since its diffusion coefficient is set to zero. So is it due to numerical diffusion that I see the artifacts here? And how can I avoid them or am I making a mistake in the model? I appreciate any help I can get. I am new to COMSOL and the learning curve is quite high.