Species concentration loss

N

Dear Friends, I have a geometry (basically a box) where I am trying to solve species transport. There are two species introduced at the inlet located on top of the box. A fan is also placed in the box. The species are introduced after solving the flow equations and thus only equations for species are solved. The dilute aproximation theory is used to specify the diffusion coefficients. My problem is that the the solution seems to be time step dependent. That is, if I increase the time step size, the flow seems to lose species when compared with experimental values. I would like to what would be a sensible time step and based on what parameters. Thanks in advance.