The diffusion-advection equation above is what I'm trying to simulate which is just the species transport equation with an extra term that is not accounted for in the species model in ansys. The C is the concentration kmol/m3, λ is the decay constant in s^-1, D is the diffusion coefficient m2/s, S is a constant source term in kg/m3s and u is the velocity m/s. I'm trying to include the term -λC which is the missing term in the species model it's called the decay term. I wanted to add it as a source term defined as E= -λC. The concentration building in the domain decays/sink by the non-constant value -λC which also depends on the value of that concentration. The gas described by this equation will be introduced in the near-wall cells to simulate the gas emanating from the wall. There is also air entering the domain providing natural ventilation which further decreases the gas emanating from the wall.