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15.17 Postprocessing the Non-Premixed Model Results

The final step in the non-premixed combustion modeling process is the postprocessing of species concentrations and temperature data from the mixture fraction and flow-field solution data. The following variables are of particular interest:

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For the unsteady laminar flamelet model, mean species mass fractions are displayed for the first fifty species in the flamelet kinetic mechanism.

These quantities can be selected for display in the indicated category of the variable-selection drop-down list that appears in postprocessing panels. See Chapter  30 for their definitions.

If there are species that you want to have available for postprocessing that would be ignored by FLUENT due to their low concentration (e.g., CH, ${\rm CH}_2$, ${\rm CH}_3$ for the NOx calculation), you can force FLUENT to include them using the text interface.

define $\rightarrow$ models $\rightarrow$ species $\rightarrow$ non-premixed-combustion

When the console window prompts you with Force Equilibrium Species to Include..., specify the appropriate species by entering the chemical formula(s) in double quotes (e.g., "ch", "ch2").

Note that you will have to first set up the inputs for the fuel and oxidizer before you are given the option to include the species.

In all cases, the species concentrations are derived from the mixture fraction/variance field using the look-up tables. Note that temperature and enthalpy can be postprocessed even when your FLUENT model is an adiabatic non-premixed combustion simulation in which you have not solved the energy equation. In both the adiabatic and non-adiabatic cases, the temperature is derived from the look-up table.

Figures  15.17.1 and  15.17.2 illustrate typical results for a methane diffusion flame modeled using the non-premixed approach.

Figure 15.17.1: Predicted Contours of Mixture Fraction in a Methane Diffusion Flame
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Figure 15.17.2: Predicted Contours of CO $_2$ Mass Fraction Using the Non-Premixed Combustion Model
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