16.5.1 Forcing the Exclusion and Inclusion of Equilibrium Species

Because ANSYS FLUENT calculates all intermediate and product species automatically during the equilibrium calculation, certain species will be included that are generally not in chemical equilibrium. Principal among these are the NOx species. Specifically, the NOx reaction rates are slow and should not be treated using an equilibrium assumption. Instead, the NOx concentration is predicted most accurately using the ANSYS FLUENT NOx postprocessor, where finite-rate kinetics are included (see Section  21.1). The NOx species can be safely excluded from the equilibrium calculation since they are present at low concentrations and have little impact on the density, temperature, and other species.

To force the exclusion of a species from the equilibrium calculation, click the Control tab in the Species Model dialog box (Figure  16.5.1).

Figure 16.5.1: The Species Model Dialog Box ( Control Tab)

Under Species Excluded From Equilibrium, enter the chemical formula for the desired species in the Add/Remove Species field. Next, click Add to add the species to the Species list or Remove to remove an existing species from the Species list.

If there are species that you want to include in your PDF table that would be ignored by ANSYS FLUENT due to their low concentration (e.g., CH, ${\rm CH}_2$, ${\rm CH}_3$ for the NOx calculation), you can force ANSYS 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.