When using the Finite Rate Chemistry model, using a timestep that is too large at the beginning may cause the solver to fail due to reaction rates not converging. Under these circumstances, the timestep must be reduced.

It may help to increase time scales for energy and mass fraction equation classes after running the simulation for a while. You should always use same time scale for energy and mass fraction equation classes.

Overshooting temperatures during the first few iterations may cause convergence difficulties. Try running first few iterations with the Eddy Dissipation Model, then restart with Finite Rate Chemistry.

Enabling temperature damping may also help, but may slow down convergence (this is set using the Expert Parameter temperature damping=t).