16.2.5 Enabling the Rich Flammability Limit (RFL) Option

You can define a rich limit on the mixture fraction when the equilibrium chemistry option is used. Input of the rich limit is accomplished by specifying a value of the Rich Flammability Limit for the appropriate Fuel Stream, Secondary Stream, or both. You will not be allowed to specify the Rich Flammability Limit if you have used the empirical definition option for fuel composition.

ANSYS FLUENT will compute the composition at the rich limit using equilibrium. For mixture fraction values above this limit, ANSYS FLUENT will suspend the equilibrium chemistry calculation and will compute the composition based on mixing, but not burning, of the fuel with the composition at the rich limit. A value of 1.0 for the rich limit implies that equilibrium calculations will be performed over the full range of mixture fraction. When you use a rich limit that is less than 1.0, equilibrium calculations are suspended whenever $f$, $f_{\rm fuel}$, or $f_{\rm sec}$ exceeds the limit. This RFL model is often more accurate than the assumption of chemical equilibrium for rich mixtures, and also avoids complex equilibrium calculations, speeding up the preparation of the look-up tables. An RFL value of approximately twice the stoichiometric mixture fraction is appropriate.

For the Secondary Stream, the rich flammability limit controls the equilibrium calculation for the secondary mixture fraction. If your secondary stream is not a fuel, you should use an RFL value of 1. A value of 1.0 for the rich limit implies that equilibrium calculations will be performed over the full range of mixture fraction. When you input a rich limit that is less than 1.0, equilibrium calculations are suspended whenever $f_{\rm sec}$ exceeds the limit. (Note that it is the secondary mixture fraction $f_{\rm sec}$ and not the partial fraction $p_{\rm sec}$ that is used here.)

Experimental studies and reviews [ 11, 72] have shown that although the fuel-lean flame region approximates thermodynamic equilibrium, non-equilibrium kinetics will prevail under fuel-rich conditions. Therefore, for non-empirically defined fuels, the RFL model is strongly recommended.