ANSYS FLUENT 12.0 User's Guide - 21.1.1 Using the NOx Model

21.1.1 Using the NOx Model

Decoupled Analysis: Overview

NOx concentrations generated in combustion systems are generally low. As a result, NOx chemistry has negligible influence on the predicted flow field, temperature, and major combustion product concentrations. It follows that the most efficient way to use the NOx model is as a postprocessor to the main combustion calculation.

The recommended procedure is as follows:

1.   Calculate your combustion problem using ANSYS FLUENT as usual.



The premixed combustion model is not compatible with the NOx model.



If you plan to use the ANSYS FLUENT SNCR model for NOx reduction, you will first need to include ammonia or urea (depending upon which reagent is employed) as a fluid species in the main combustion calculation and define appropriate ammonia injections, as described later in this section. See Section 15.1.3 for details about adding species to your model and Section 23.3 for details about creating injections.
2. Enable the desired NOx models (thermal, prompt, fuel, and/or N O intermediate NOx, with or without reburn), define the fuel streams (for prompt NOx and fuel NOx only), and set the appropriate parameters, as described in this section.
Models NOx Edit...
3. Define the boundary conditions for NO (and HCN, NH , or N O if necessary) at flow inlets.
Boundary Conditions
4. In the Equations dialog box, turn off the solution of all variables except species NO (and HCN, NH , or N O, based on the model selected).
Solution Controls Equations...
5. Perform calculations until convergence (i.e., until the NO (and HCN, NH , or N O, if solved) species residuals are below $10^{-6}$ ) to ensure that the NO and HCN or NH concentration fields are no longer evolving.
Run Calculation
6. Review the mass fractions of NO (and HCN, NH , or N O) with alphanumerics and/or graphics tools in the usual way.
7. Save a new set of case and data files, if desired.
File $\rightarrow$ Write $\rightarrow$ Case & Data...

Inputs specific to the calculation of NOx formation are explained in the remainder of this section.

Enabling the NOx Models

To enable the NOx models and set related parameters, you will use the NOx Model dialog box (e.g., Figure 21.1.1).

Models NOx Edit...

**Figure 21.1.1:** The **NOx Model** Dialog Box

In the Formation tab, select the NOx models under Pathways to be used in the calculation of the NO and HCN, NH , or N O concentrations:

To enable thermal NOx, turn on the Thermal NOx option.
To enable prompt NOx, turn on the Prompt NOx option.
To enable fuel NOx, turn on the Fuel NOx option.

When using the non-premixed combustion model, the Fuel NOx option is only available if the DPM model is also enabled.
To enable the formation of NOx through an N O intermediate, turn on the N2O Intermediate option. (Note that the N2O Intermediate option will not appear until you have activated one of the other NO models listed above.)

Your selection(s) under Pathways will activate the calculation of thermal, prompt, fuel, and/or N O-intermediate NOx in accordance with the chemical kinetic models described in this section through this section in the separate Theory Guide. Mean NO formation rates will be computed directly from mean concentrations and temperature in the flow field.

Defining the Fuel Streams

ANSYS FLUENT allows you to define multiple fuel streams when you are modeling prompt or fuel NOx formation. If either Prompt NOx or Fuel NOx is enabled in the Pathways group box in the Formation tab, perform the following steps:

1. Specify the Number of Fuel Streams in the Fuel Streams group box. You are allowed up to three separate fuel streams.
2.   Define the first fuel stream.

(a)   Select the fuel stream to be defined by using the arrow keys of the Fuel Stream ID text box.

(b)   When modeling fuel NOx formation in conjunction with the non-premixed combustion model (which requires that the discrete phase model be enabled as well), make a selection from the PDF Stream drop-down list (Figure 21.1.2) to define the species for this stream. You can select either the primary or secondary fuel stream species, as defined in the PDF table.



Note that the PDF Stream drop-down list defines the species for the fuel NOx calculations only.

(c)   When modeling prompt NOx formation or when using a combustion model other than non-premixed combustion, select the fuel species from the Fuel Species list. You cannot select more than 5 fuel species for each fuel stream, and the total number of fuel species selected for all the fuel streams combined cannot exceed 10.



Note that the Fuel Species selections define the species for the prompt NOx calculations only when modeling non-premixed combustion.

(d)   Set the other parameters associated with your selected pathway(s) in the Prompt and/or Fuel tabs under Formation Model Parameters. See Section 21.1.1 and Section 21.1.1 for details.
3. Repeat steps 2.(a)-2.(c) for each additional fuel stream.

**Figure 21.1.2:** The **NOx Model** Dialog Box with Non-Premixed Combustion

Note that the following limitations apply when you are modeling fuel NOx formation with multiple fuel streams, if more than one fuel stream has the same fuel type (as defined in the Fuel Type group box in the Fuel tab):

For multiple liquid fuel streams or multiple solid (coal) fuel streams, the injectors associated with the fuel streams should have different destination species, as defined in the Devolatilizing Species drop-down list in the Set Injection Properties dialog box (see Section 23.3.15 for details). The NOx calculations will be erroneous if the destination species are the same.
For multiple solid (coal) fuel streams, the fuel streams should have the same char-related parameter values in the Fuel tab--i.e., the Char N Mass Fraction, the Partition Fractions (for char N), and the BET Surface Area values. Note that even if different values are set for these char-related parameters, ANSYS FLUENT will only recognize those specified for the solid fuel stream with the lowest ID number, and then apply them to all of the other solid fuel streams.

For more information about the limitations associated with multiple fuel streams with the same fuel type, contact your ANSYS FLUENT support engineer.

Note that if you read a case file with NOx settings that was set up in a version of ANSYS FLUENT previous to 12, you may need to make a selection for the fuel species. This step is only necessary when all of the following conditions are met:

Fuel NOx is enabled in the Pathways group box.

Prompt NOx is not enabled in the Pathways group box.

Solid or Liquid is selected for Fuel Type in the Fuel tab.

Your fuel species selection should be made either in the PDF Stream drop-down list for non-premixed combustion, or in the Fuel Species list for all other combustion models.

Specifying a User-Defined Function for the NOx Rate

You can choose to specify a user-defined function for the rate of NOx production. By default, the rate returned from the UDF is added to the rate returned from the standard NOx production options, if any are selected. You also have the option of replacing any or all of ANSYS FLUENT's NOx rate calculations with your own user-defined NOx rate.

In addition to or instead of using the UDF to specify the NOx rate, you can use it to specify custom values for the maximum limit ( $T_{\rm max}$ ) that is used for the integration of the temperature PDF (when temperature is accounted for in the turbulence interaction modeling).

To use a UDF to add a rate to ANSYS FLUENT's NOx rate calculations, you must compile and load the desired function, and then select it from the NOx Rate drop-down list in the User-Defined Functions group box in the Formation tab. After you have selected the UDF, you have the following options:

You can specify that your custom rate is added to the ANSYS FLUENT NOx rate calculations, by retaining the default selection of Add to the FLUENT Rate in the UDF Rate group box for the appropriate NOx formation pathway(s) (e.g., in the Fuel tab).
You can replace the ANSYS FLUENT NOx rate calculations with your custom rate, by selecting Replace FLUENT Rate in the UDF Rate group box for the appropriate NOx formation pathway(s) (e.g., in the Fuel tab).
You can specify custom values for $T_{\rm max}$ , by selecting user-defined from the Tmax Option drop-down list in the Turbulence Interaction Mode tab.

See the separate UDF Manual for details about user-defined functions.

Setting Thermal NOx Parameters

The NOx routines employ three methods for calculation of thermal NOx (as described in this section in the separate Theory Guide). You will specify the method to be used in the Thermal tab, under Formation Model Parameters in the NOx Model dialog box:

To choose the equilibrium method, select equilibrium in the [O] Model drop-down list.
To choose the partial equilibrium method, select partial-equilibrium in the [O] Model or [OH] Model drop-down list.
To use the predicted O and/or OH concentration, select instantaneous in the [O] Model or [OH] Model drop-down list.

Note that the urea model uses the [OH] model.

If you hooked a UDF in the Formation tab, you can make a selection in the UDF Rate group box to specify the treatment of the user-defined NOx rate:

Select Replace FLUENT Rate to replace ANSYS FLUENT's thermal NOx rate calculations with the custom NOx rate produced by your UDF.
Select Add to FLUENT Rate to add the custom NOx rate produced by your UDF to ANSYS FLUENT's thermal NOx rate calculations.

Setting Prompt NOx Parameters

Prompt NOx formation is predicted using this equation and this equation in the separate Theory Guide. For each fuel stream specified in the Fuel Stream ID text box in the Formation tab, set the parameters in the Prompt tab under Formation Model Parameters in the NOx Model dialog box in the following manner:

Set the Fuel Carbon Number to specify the number of carbon atoms per fuel molecule.
Set the Equivalence Ratio as follows:

$\mbox{Equivalence Ratio} = \frac{\mbox{actual fuel-to-air ratio}} {\mbox{stoichiometric fuel-to-air ratio}}$

For any carbon number, C , the limits of the Equivalence Ratio are such that, if it is greater than 1.57, then limit the Equivalence Ratio to 1.57. If C is less than or equal to 4, then an additional limit is applied. When the Equivalence Ratio is between 0.365 and 0.685, the midpoint value is computed, which is 0.525. Thus for Equivalence Ratio values below the midpoint value, set the value to the lower limit and for an Equivalence Ratio above the midpoint value, set the value to the upper limit). These limits are purely mathematical and only guarantee positive prompt NOx rates.

If you hooked a UDF in the Formation tab, you can make a selection in the UDF Rate group box to specify the treatment of the user-defined NOx rate:

Select Replace FLUENT Rate to replace ANSYS FLUENT's prompt NOx rate calculations with the custom NOx rate produced by your UDF.
Select Add to FLUENT Rate to add the custom NOx rate produced by your UDF to ANSYS FLUENT's prompt NOx rate calculations.

Setting Fuel NOx Parameters

When using the fuel NOx model, you must set the parameters in the Fuel tab under Formation Model Parameters for each fuel stream specified in the Fuel Stream ID text box in the Formation tab.

If you hooked a UDF in the Formation tab, you can make a selection in the UDF Rate group box to specify the treatment of the user-defined NOx rate:

Select Replace FLUENT Rate to replace ANSYS FLUENT's fuel NOx rate calculations with the custom NOx rate produced by your UDF.
Select Add to FLUENT Rate to add the custom NOx rate produced by your UDF to ANSYS FLUENT's fuel NOx rate calculations.

If there is no NOx rate UDF or if you selected Add to FLUENT Rate, you must define fuel parameters. To begin, specify the fuel type in the following manner:

For solid fuel NOx, select Solid under Fuel Type.
For liquid fuel NOx, select Liquid under Fuel Type.
For gaseous fuel NOx, select Gas under Fuel Type.

Note that you can use only one of the fuel types for a given fuel stream. The Gas option is available only when the Species Transport model is enabled (see Section 15.1.2).

Setting Gaseous and Liquid Fuel NOx Parameters

If you have selected Gas or Liquid as the Fuel Type, you will also need to specify the following:

Select the intermediate species ( hcn, nh3, or hcn/nh3/no) in the N Intermediate drop-down list.
Set the correct mass fraction of nitrogen in the fuel (kg nitrogen per kg fuel) in the Fuel N Mass Fraction field.
Specify the overall fraction of the fuel N, by mass, that will be converted to the intermediate species and/or product NO in the Conversion Fraction field. The Conversion Fraction for the N Intermediate has a default value of 1. Thus, any remaining N will not contribute to NOx formation. This is based on the assumption that the remaining volatile N will convert to gas phase nitrogen. However, this has very little effect on the overall mass fraction of gas phase nitrogen. Therefore, you do not have to solve for nitrogen species when solving pollutant transport equations.
If you selected hcn/nh3/no as the intermediate, specify the fraction of the converted fuel N, by mass, that will become hcn and nh3 under Partition Fractions. The fraction of fuel N that will become NO will be calculated by the remainder.
Note that setting a partition fraction of 0 for both HCN and NH is equivalent to assuming that all fuel N is converted to the final product NO, whereas a partition fraction of 0 for HCN and 1 for NH is the same as selecting nh3 as the intermediate.

ANSYS FLUENT will use this equation and this equation (in the separate Theory Guide) (for HCN) or this equation and this equation (in the separate Theory Guide) (for NH ) to predict NO formation for a gaseous or liquid fuel.

Note that there is a limitation that must be considered when defining more than one liquid fuel stream. See Section 21.1.1 for details.

Setting Solid (Coal) Fuel NOx Parameters

For solid (coal) fuel, ANSYS FLUENT will use this equation and this equation (in the separate Theory Guide) (for HCN) or this equation and this equation (in the separate Theory Guide) (for NH ) to predict NO formation. Several inputs are required for the coal fuel NOx model as follows:

Select the intermediate species ( hcn, nh3, or hcn/nh3/no) in the N Intermediate drop-down list.
Specify the mass fraction of nitrogen in the volatiles in the Volatile N Mass Fraction field.
Specify the overall fraction of the volatile N, by mass, that will be converted to the intermediate species and/or product NO in the Conversion Fraction field.
If you selected hcn/nh3/no as the volatile N intermediate, specify the fraction of the converted volatile N, by mass, that will become hcn and nh3 under Partition Fractions. The fraction of volatile N that will become NO will be calculated by the remainder.
Select the char N conversion path from the Char N Conversion drop-down list as no, hcn, nh3, or hcn/nh3/no. Note that hcn or nh3 can be selected only if the same species has been selected as the intermediate species in the N Intermediate drop-down list.
Specify the mass fraction of nitrogen in the char in the Char N Mass Fraction field.
Specify the overall fraction of the char N, by mass, that will be converted to the intermediate species and/or product NO in the Conversion Fraction field.
If you selected hcn/nh3/no as the char N conversion, specify the fraction of the converted char N, by mass, that will become hcn and nh3 under Partition Fractions. The fraction of char N that will become NO will be calculated by the remainder.
Define the BET internal pore surface area (see this section in the separate Theory Guide for details) of the particles in the BET Surface Area field.

Note that there are limitations that must be considered when defining more than one solid fuel stream. See Section 21.1.1 for details.

The following equations are used to determine the mass fraction of nitrogen in the volatiles and char:

$\dot{m}_{N_{v/c}} = \dot{m}_{v/c} * mf_{N_{v/c}}$

(21.1-1)

where
	$\dot{m}_{N_{v/c}}$	= rate of release of fuel nitrogen in kg/s
	$\dot{m}_{v/c}$	= rate of release of volatiles (v) or char (c) in kg/s
	$mf_{N_{v/c}}$	= mass fraction of nitrogen in volatiles or char

Let
	$TN_{fuel}$	= total nitrogen mass fraction in daf coal (i.e., from daf ultimate analysis)
	$N_{split}$	= char nitrogen as a fraction of total nitrogen
	$F_{vol}$	= mass fraction of volatiles in daf coal
	$F_{char}$	= mass fraction of char in daf coal

Then the following should hold:

$F_{vol} + F_{char} = 1$

(21.1-2)

$\frac{F_{char} * mf_{N_{c}}}{TN_{fuel}} = N_{split}$

(21.1-3)

$F_{vol} * mf_{N_{v}} + F_{char} * mf_{N_{c}} = TN_{fuel}$

(21.1-4)

$mf_{N_{v}} = \left(1 - N_{split} \right) * \frac{TN_{fuel}}{F_{vol}}$

(21.1-5)

$mf_{N_{c}} = N_{split} * \frac{TN_{fuel}}{F_{char}}$

(21.1-6)

Note that if water is assumed to release at the same rate as volatiles, the above calculation has to be slightly modified.

Setting N O Pathway Parameters

The formation of NO through an N O intermediate can be predicted by two methods. You will specify the method to be used in the N2O Path tab.

To choose the quasi-steady state method, select quasi-steady in the N2O Model drop-down list.

The transport equation for the species N O will not be solved for N O, however, N O will be updated at every iteration. Therefore, the residual values that appear for N O are always zero. Do not be alarmed if the solver keeps printing zero at each iteration.

To choose the simplified form of the N O-intermediate mechanism, select transported-simple in the N2O Model drop-down list. Here, the species N O is added to the list of pollutant species, and its mass fraction is solved via a transport equation.

The atomic O concentration will be calculated according to the thermal NOx [O] Model that you have specified previously. If you have not selected the Thermal NOx pathway, then you will be given the option to specify an [O] Model for the N O pathway calculation. The same three options for the thermal NOx [O] Model will be the available options.

If you hooked a UDF in the Formation tab, you can make a selection in the UDF Rate group box to specify the treatment of the user-defined NOx rate:

Select Replace FLUENT Rate to replace the NOx rate calculated by ANSYS FLUENT using N O intermediates with the custom NOx rate produced by your UDF.
Select Add to FLUENT Rate to add the custom NOx rate produced by your UDF to the NOx rate calculated by ANSYS FLUENT using N O intermediates.

Setting Parameters for NOx Reburn

To enable NOx reduction by reburning, click the Reduction tab in the NOx Model dialog box and enable the Reburn option under Methods. In the expanded portion of the dialog box, as shown in Figure 21.1.3, click the Reburn tab under Reduction Method Parameters, where you can choose from the following options:

**Figure 21.1.3:** The NOx Dialog Box Displaying the Reburn Reduction Method

To choose the instantaneous method, select instantaneous [CH] in the Reburn Model drop-down list.

When you use this method, you must be sure to include the species CH, CH , and CH in your problem definition. See this section in the separate Theory Guide for details.

To choose the partial equilibrium method, select partial-equilibrium in the Reburn Model drop-down list. You will then need to select the Reburn Fuel Species from the list of available species. ANSYS FLUENT will allow you select up to 5 reburn fuel species. Specify the Equivalent Fuel Type ( ch4, ch3, ch2, or ch). For example, if you choose methane as the reburn fuel, then the Equivalent Fuel Type would be ch4. If you choose a reburn fuel such as hv_vol (a volatile component of coal), then you must specify the most appropriate equivalent hydrocarbon fuel type so that the partial equilibrium model will be activated correctly.
Due to coal volatiles behaving very differently, it is important to select the correct equivalent fuel type. You must first consider the volatile fuel composition, then check the C/H ratio to find the fuel which most closely matches CH, CH , CH , or CH [ 43]. How the equivalent fuel is determined is still debatable, however, considering the C/H ratio of the fuel itself is a reasonable indicator.

Setting SNCR Parameters

Prior to enabling reduction by SNCR, make sure that you have included in the species list nh3 (for reduction by ammonia injection) and co nh2 2 (for reduction by urea injection). See this section in the separate Theory Guide for detailed information about SNCR theory.

To enable NOx reduction by SNCR, click the Reduction tab in the NOx Model dialog box and enable the SNCR option under Methods, as shown in Figure 21.1.4.

**Figure 21.1.4:** The NOx Dialog Box Displaying the SNCR Reduction Method

Then click the SNCR tab under Reduction Method Parameters, where you can choose from the following options:

To have ammonia or urea included as a gas-phase pollutant species from the injection locations, select gaseous in the Injection Method drop-down list.
If you plan to select this option for NOx postprocessing, then you must also include ammonia or urea as a gas-phase species. Additionally, you will need to specify the mass fraction of ammonia or urea at the respective inlet for the SNCR injection. You must include this set of inputs prior to the main ANSYS FLUENT combustion calculation.
To have ammonia included as a liquid-phase pollutant species from the injection locations, select liquid in the Injection Method drop-down list. If urea is injected as a liquid solution, then select liquid in the Injection Method drop-down list. Note that you must activate the DPM model with urea or ammonia included as a material.
If you plan to select this option for NOx postprocessing, then you must include NH as both a gas-phase and liquid-phase species. Additionally, you will need to specify injection locations for liquid droplet ammonia particles and set gaseous ammonia as the evaporation species. You need to include this set of inputs in conjunction with the main ANSYS FLUENT combustion calculation.
Since urea is a subliming solid, and usually is injected as a solution, mixed in water, you have to define solid properties for urea under the Create/Edit Materials dialog box. We assume that the water evaporates before urea begins its subliming process. The sublimation process is modeled similar to the single rate devolatilization model of coal. You will supply the value for the sublimation rate ( $s^{-1}$ ). You must specify the water content while defining the injection properties.
Specify the SNCR Reagent Species as nh3 (ammonia) or co nh2 2 (urea) in the drop-down list.
When using the non-premixed combustion model with a liquid-phase reagent injection, enter a value in the Reagent Fraction in Stream to specify the mass fraction of the reagent in the reagent stream. The remaining mass fraction is assumed to be water. If you enabled a secondary stream in your PDF calculation, by default the secondary stream will act as the reagent stream. You can assign the primary stream as the reagent stream by using the text command that follows (enter 0 in response to the PDF Stream ID prompt that follows the Injection Method prompt):
define/models/nox-parameters/nox-chemistry
If the Reagent Species selected is co nh2 2, then you will either accept the rate-limiting option for Urea Decomposition, or specify the NH3 Conversion value when selecting a user-specified Urea Decomposition.
You will use the urea decomposition under the SNCR tab to define which of the two decomposition models is to be used. The first model (which is the default) is the rate-limiting decomposition model, as given in this table in the separate Theory Guide. ANSYS FLUENT will then calculate the source terms according to the rates given in this table in the separate Theory Guide. The second model is for the user who assumes urea decomposes instantly into ammonia and HNCO at a given proportion. In this case, you will specify the molar conversion fraction for ammonia, assuming that the rest of the urea is converted to HNCO. An example value is given above.
The value for user-specified NH conversion is the mole fraction of NH in the mixture of NH and HNCO that is instantly created from the reagent injection. In this case, there is no urea source because all of reagent is assumed to convert to both NH and HNCO, instantly.

Setting Turbulence Parameters

If you want to take into account turbulent fluctuations (as described in this section in the separate Theory Guide) when you compute the specified NOx formation (thermal, prompt, and/or fuel, with or without reburn), define the turbulence parameters in the Turbulence Interaction Mode tab.

**Figure 21.1.5:** The **NOx Model** Dialog Box and the **Turbulence Interaction Mode** Tab

Select one of the options in the PDF Mode drop-down list in the Turbulence Interaction Mode tab:

Select temperature to take into account fluctuations of temperature.
Select temperature/species to take into account fluctuations of temperature and mass fraction of the species selected in the Species drop-down list (which appears when you select this option).
(non-premixed and partially premixed combustion calculations only) Select mixture fraction to take into account fluctuation in the mixture fraction(s).

When modeling the formation of other pollutants along with NOx, you should compare the selections made in the PDF Mode drop-down lists in the Turbulence Interaction Mode group boxes of the NOx Model dialog box and the Turbulence Interaction Mode group boxes of the SOx Model and Soot Model dialog boxes. If mixture fraction is selected in any of these dialog boxes, then it must be selected in all of the others as well.

The mixture fraction option is available only if you are using either the non-premixed or partially premixed combustion model to model the reacting system. If you use the mixture fraction option, the instantaneous temperatures and species concentrations are taken from the PDF look-up table as a function of mixture fraction and enthalpy and the instantaneous NOx rates are calculated at each cell. The PDF used for convoluting the instantaneous NOx rates is the same as the one used to compute the mean flow-field properties. For example, for single-mixture fraction models the beta PDF is used, and for two-mixture fraction models, the beta or the double delta PDF can be used. The PDF for mixture fraction is calculated from the values of mean mixture fraction and variance at each cell, and the instantaneous NOx rates are convoluted with the mixture fraction PDF to yield the mean rates in turbulent flow.

If you selected temperature or temperature/species for the PDF Mode, you should define the following parameters in the Turbulence Interaction Mode tab:

PDF Type allows you to specify the shape of the PDF, which is then integrated to obtain mean rates for the temperature and (if you selected temperature/species for the PDF Mode) the species. If you select beta, the PDF will be modeled using this equation in the separate Theory Guide. If you select gaussian, the PDF will be modeled using this equation in the separate Theory Guide.
PDF Points allows you to specify the number of points used to integrate the beta or Gaussian function in this equation or this equation in the separate Theory Guide on a histogram basis. The default value of 10 will yield an accurate solution with reasonable computation time. Increasing this value may improve accuracy, but will also increase the computation time.
Temperature Variance allows you to specify the form of transport equation that is solved to calculate the temperature variance. The default selection is algebraic, which is an approximate form of the transport equation (see this equation in the separate Theory Guide). You have the option of selecting transported to instead solve this equation in the separate Theory Guide. Though the transported form is more exact, it is also more expensive computationally.
Tmax Option provides various options for determining the maximum limit(s) for the integration of the PDF used to calculate the temperature:

The default selection is global-tmax, which sets the limit as the maximum temperature in the flow field.

You can select local-tmax if you would rather obtain cell-based maximum temperature limits by multiplying the local cell mean temperature by the value entered in Tmax Factor.

You can select specified-tmax to set the limit for each cell to be the value entered in Tmax.

If you have selected a user-defined function in the NOx Rate drop-down list in the Formation tab, then you can select user-defined so that the temperature limit is specified by a UDF. See the separate UDF Manual for details about user-defined functions.
Species only appears if you have selected temperature/species for the PDF Mode. Your selection in this drop-down menu determines which species' mass fraction is included in the NOx formation calculations.

Note that the species variance will always be calculated using the algebraic form of the transport equation ( this equation in the separate Theory Guide).

Defining Boundary Conditions for the NOx Model

At flow inlet boundaries, you will need to specify the Pollutant NO Mass Fraction, and if necessary, the Pollutant HCN Mass Fraction, Pollutant NH3 Mass Fraction, and Pollutant N2O Mass Fraction.

Boundary Conditions

You can retain the default inlet values of zero for these quantities or you can input nonzero numbers as appropriate for your combustion system.

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