I'm trying to solve an unsteady, compressible, axi-symmetric problem with species transport in FLUENT. The problem is that I'm not able to get a converged solution. The solution just diverges or reports floating-point error in the 1st time step. Can anyone tell me the techniques to get a converged solution.

Saad

if coupled solver, and if you are already using coupled solver switch to FAS, but before that make sure that you are not making anymistake in setting up model and boundary condisitons, in compressible flows usually u face lot of probs if boundaries are not set up right.

Yes, belonging to boundaries, you should use inlet / pressure. Initialize the interior of the flow field slightly with a grater pressure than the pressure boundary. In high speed flow ( high mach number), it is possible that you get inflow at the pressure geometry outlet. From this numerical instabilities arise. In that case the mass conservation is saved, because the density is dependend on both temperatur AND pressure. A remedy is to use "total pressure boundaries". The static pressure is reduced of the value of 0.5*density*vel². That will force the flow outwards. Another point is to use stagnation boundaries.