For many cases, such as the Combustion Tutorial, the run can be started using automatic initial guesses for most fields and the choice of model in the first part of the tutorial, (EDM) is the most robust of the combustion models. If you are having trouble getting a complex problem to converge, the following steps may aid convergence:
Begin by specifying a single step reaction using the Eddy Dissipation Model.
For initial conditions, specify Oxygen with a mass fraction of 0.232 and set each of the other species to
Automatic.Allow the problem to converge to a reasonable level.
Restart the problem with Finite Rate Chemistry or Finite Rate Chemistry/Eddy Dissipation Model using the results from the previous problem as an initial guess.
Restarting the flamelet model from EDM does not work well because of the different model approach. This is because a previous EDM run would not provide mixture fraction mean and variance fields. Instead, it would be better to reduce the timestep for a few iterations.
Combustion introduces a strong coupling between scalars, energy and momentum. In difficult problems, you may find it helps to calculate a cold flow solution before turning on combustion.
In general for the Finite Rate Chemistry Model, the time scale of the combustion
process is smaller, so you may have to continue with a smaller timestep to force the
mass fraction and energy equations to converge. The time scale for the Eddy
Dissipation Model is equal to the turbulence time scale (
).
In some cases, it may be appropriate to use a larger time scale for the mass fraction equations. This is particularly true when there are regions of small flow velocity in the domain. In these cases, the time needed to convect the components into all areas of the domain may be relatively large.