The Equation Class Settings tab in CFX-Pre can be used to set different advection schemes and time scales for each class of equation you are solving. For multi-step Eddy Dissipation reactions, convergence can be improved by temporarily increasing the mass fraction time scale by a factor of about 5-10.

For the Eddy Dissipation Model, multistep convergence can be aided by first running a simplified single-step simulation and using the results from the run as an initial values file for a multi-step run.

You may restart a Flamelet model from a cold solution. You should avoid restarting with the Flamelet model from an EDM solution. You may restart an EDM case from a Flamelet model solution.

The High Resolution advection scheme is always recommended for combustion simulations because it is bounded and prevents over/undershoots. Care must be taken, however, to provide a mesh of sufficient quality to resolve most of the flow features. A very poor mesh will result in the scheme using a blend factor close to zero (therefore not providing a solution as accurate as expected).

For simulations that include Finite Rate Chemistry, small temperature variations can result in large changes in reaction rate. When a solution is converging, temperature values may change sufficiently to make the solution unstable. To aid convergence, add a TEMPERATURE DAMPING CCL structure within a SOLVER CONTROL block, as follows:

FLOW: 
  SOLVER CONTROL: 
    TEMPERATURE DAMPING: 
       Option = Temperature Damping 
       Temperature Damping Limit = <Real number> 
       Under Relaxation Factor = <Real number> 
    END 
  END 
END 

Depending on the location of the SOLVER CONTROL block, the temperature damping may be applied to a particular domain or phase. Set the Temperature Damping Limit to 0 so that positive damping is always applied. The Under Relaxation Factor can be set to multiply changes in temperature by a value between 0 and 1. You should try a factor of 0.2 if you are having trouble converging a solution.