## 15.7.4 Choosing to Define the Fuel Stream(s) Empirically

The empirical fuel option provides an alternative method for defining the composition of the fuel or secondary stream when the individual species components of the fuel are unknown. When you do not select this option, you will define which chemical species are present in each stream and the mass or mole fraction of each species, as described in Section  15.9. The option for defining an empirical fuel stream is particularly useful for coal combustion simulations (see Section  15.9.5) or for simulations involving other complex hydrocarbon mixtures.

To define a fuel or secondary stream empirically

1.   Turn on the Empirical Fuel Stream option under Options in the Chemistry tab of the Species Model panel. If you have a secondary stream, enable the Empirical Secondary Stream option, or both as appropriate.

2.   Specify the appropriate lower heating value (e.g., Fuel Lower Caloric Value, Secondary Lower Caloric Value) and specific heat (e.g., Fuel Specific Heat, Secondary Specific Heat) for each empirically-defined stream.

 The empirical definition option is available only with the full equilibrium chemistry model. It cannot be used with the rich flammability limit (RFL) option or the steady and unsteady laminar flamelet models, since equilibrium calculations are required for the determination of the fuel composition. If your empirically-defined fuel is entering the domain via an inlet boundary, you will need to additionally define its molecular weight using the text command: define models species nonpremixed-combustion-expert If you change the molecular weight of the empirically-defined fuel or secondary stream, you will need to rebuild the PDF table.

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