Unsteady Solution Diverges

Prateep Chatterjee

I have a laminar premix combustion problem going. the steady solution converges easily (the energy part converges slowly though). I start the unsteady simulation with the following paramters and I get recirculation at the exit:

second order pressure; SIMPLE pressure-velocity coupling; second order upwind for energy, velocities (2D), species, density and DO (discrete ordinates) radiation

the recirculation continues to grow (number of faces at the exit) and some of the residuals start going up (energy, y-velocity, methane) and every time-step some more faces have recirculation. I've tried adaption, first order schemes, lower time steps, lots of iterations per time step, but everything fails eventually.

guys, any suggestions? it's kind of urgent as my advisor is breathing down my neck. I've absolutely no idea what the problem is. I've shortened the combustor by 3 inches and nothing else (boundary conditions, flow conditions etc) has changed. the original grid gives me near perfect (!) results, but this modified grid refuses to yield any solution at all. the steady solution seems to be good because it matches the steady solution part for the first grid.

HELP!