6.2.12 Hooking DEFINE_NOX_RATE UDFs

After you have compiled (Chapter  5) your DEFINE_NOX_RATE UDF in ANSYS FLUENT, the function name you supplied in the DEFINE macro argument will become visible and selectable in the NOx Rate drop-down list in the Formation tab of the NOx Model dialog box (Figure  6.2.14).

Models NOx Edit...

Figure 6.2.14: The NOx Model Dialog Box

Recall that a single UDF is used to define custom rates for the thermal NOx, prompt NOx, fuel NOx, and N $_2$O NOx pathways. By default, the custom NOx rate of your UDF is added to the rate calculated internally by ANSYS FLUENT for each pathway. The UDF rate will be added to the forward rate if it is assigned to the POLLUT_FRATE macro, or the reverse rate if it is assigned to the POLLUT_RRATE macro. If you would rather entirely replace the internally calculated NOx rate with your custom rate, click the desired NOx pathway tab ( Thermal, Prompt, Fuel, or N2O Path) under Formation Model Parameters, select Replace FLUENT Rate in the UDF Rate group box for that pathway, and then click Apply. Repeat this process for all of the remaining NOx pathways.

Unless specifically defined in your NOx rate UDF, data and parameter settings for each individual NOx pathway will be derived from the settings in the NOx Model dialog box. Therefore, it is good practice to make the appropriate settings in the NOx Model dialog box, even though you may use a UDF to replace the default rates with user-specified rates. There is no computational penalty for doing this because the default rate calculations will be ignored when Replace FLUENT Rate is selected.

To specify a custom maximum limit ( $T_{\rm max}$) for the integration of the temperature PDF for each cell, you must first select the UDF name (e.g., user_nox::libudf) from the NOx Rate drop-down list, as described previously. Then, click the Turbulence Interaction Mode tab and select either temperature or temperature/species from the PDF Mode drop-down list. Finally, select user-defined from the Tmax Option drop-down list and click Apply.

See Section  2.3.12 for details about defining DEFINE_NOX_RATE functions.