Fluent 6.4 courant number
 

I simulate a CFBC. I have three Euler phases. Gas(g), Inert(s) and Fuel(s). There are homogeneous and heterogeneous reactions. I'm using Phase Coupling and Explicit formulation (unsteady). Fluent 12 prints the courant number in each iteration.The problem is that the courant number increases simultaneously untill 250 and then it crashes. The velocity field isn't diverging. The maximum velocities are OK. Moreover the case is converging in each iteration. Furthermore when i save a case with a courant number of 100 when i close fluent and reopen and reread the case courant number is 0.3 What is happening? Any suggestions?