Mixture fraction and Rich flammability for methane
 

I am applying non-premixed RFL model. Stiochiometric Mixture fraction for CH4 is 0.055. and Rich flamability Limit will also be 0.055 for this case.
In my case Q-CH4= 0.03m3/s and Q-air=0.37128 m3/s (air excess no.= 1.3)
So what will be mixture fraction of this stream?
I am getting confused because when I reduced the RFL value from 0.055 (lets say 0.045) I got higher flame length and vice versa. But when I fixed RFL value and increase the air flow rate on the boundaries there is only negligible decease in flame length. While increasing RFL value(lets say =0.1) even at stoichiometric values (air excess number=1) flame length decreases significantly. This thing I observed in Non-premixed model, but in eddy dissipation flame length is solely depended on Air flow rate.
Any help will be highly appreciated. :confused: :)