Hi, we are modelling a methane burner to detect the reason for unburned methane in the outflow.According to measurements there should be a low fraction (a few ppm) of CH4 in the outflow. The flue gas is mixed with a secondary stream of cold air directly behind the burner. Therefore the residence time of the methane in the hot zones might be to short. We have tried PDF and Eddy-Dissipation, but we cannot get reasonable solutions. The fraction of unburned methane should depend on the temperature of the secondary stream (cold = more unburned methane, hot = less unburned methane). Fluent predicts a complete combustion, even with very low temperatures of the secondary stream. Does anybody have experience with modelling very low fractions of fuel? Which is the best method to use? Which model will show reasonable solutions according to the temperature dependency? We are using Fluent 5.5. Thanks!

What is the speed of flow in your burner. Is it laminar or turbulent flow. This is important when you choose chemistry: Laminar Finite-Rate or Eddy-Dissipation

The inlet velocity of the fuel is 34m/s and for the oxidizer it is 19m/s. It is turbulent flow.

For turbulent flow the best option will be: Finite-Rate/Eddy-Dissipation.

Thanks for the answer! It seems, that the flame is always extinguished if we include the finate rate with the default pre-exponential factor. We tried single step and 2-step Methane-Air combustion. With Eddy Disipation we always have complete combustion and it does not depend on the temperature of the secondary stream.