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15.7.5 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.

FLUENT will compute the composition at the rich limit using equilibrium. For mixture fraction values above this limit, 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 a useful approach in hydrocarbon combustion calculations, allowing you to bypass complex equilibrium calculations in the fuel-rich region. The efficiency of RFL will be especially important when your model is non-adiabatic, speeding up the preparation of the look-up tables.

For the Secondary Stream, the rich flammability limit controls the equilibrium calculation for the secondary mixture fraction. For fuel streams, an RFL value of approximately twice the stoichiometric mixture fraction is appropriate. If your secondary stream is not a fuel, you should use an RFL value of 1. For a secondary fuel stream, you can consider modifying the value to use the RFL model. 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.)

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Experimental studies and reviews [ 38, 336] 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.


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