mass imbalance

co2

hi folks,

I am solving a 3D unsteady problem of gas diffusion in a cylindrical tank with conical top -- A vent at top.

The grain at the bottom of tank is modeled as porous zone and has co2 gas source term.

the top vent is modeled as pressure outlet. I am using mixture template (species model) ( CO2 + O2 + N2) -- This gives me a good approximation for co2 diffusing into air in the tank.

But at the start of my problem due to mass imbalance, continuity residual is not going below 0.001

what should i do? What are some of the tricks that you can suggest to get rid of the initial convergence problem in my 3d unsteady model run.

1. reduce time step size? 2. increase co2 source term?

I am out of clues -- PLEASE HELP!

Thanks, CO2