Timestep choice may have a significant effect on the convergence of a flow calculation. Combustion introduces large density gradients in the flow field that can increase the sensitivity of the continuity equation to the timestep. If you are having difficulty converging a combusting flow, particularly during an early stage of convergence, try reducing the timestep to improve solution of the continuity equation. For details, see Eddy Dissipation Model.

If the solution of a combusting flow appears to stall with mass not well conserved, use a timestep appropriate to the fluid flow for a few iterations to push fluid through the system.