33.3.26 NOx Model Dialog Box

The NOx Model dialog box allows you to set parameters related to the NOx postprocessor. See Section  21.1.1 for details about the items below.

Controls

Models   contains tabs for defining the models used to calculate the NOx production.

Formation   contains the parameters to define the NOx model formation.

Pathways   contains toggle buttons for activating the NOx models to be used for the calculation of NO and HCN concentrations.

Thermal NOx   enables calculation of thermal NOx.

Prompt NOx   enables the calculation of prompt NOx.

Fuel NOx   enables the calculation of fuel NOx. When using the non-premixed combustion model, the Fuel NOx option is only available if the DPM model is also enabled.

N2O Intermediate   enables the formation of NOx through an N $_2$O intermediate. This option will only appear if one of the previously listed NOx models is enabled.

Fuel Streams   allows you to define multiple fuel streams for prompt NOx and fuel NOx formation.

Number of Fuel Streams   sets the number of fuel streams. You are allowed up to three fuel streams.

Fuel Stream ID   specifies the fuel stream you are defining in the PDF Stream drop-down list, the Fuel Species selection list, the Prompt tab, and the Fuel tab.

PDF Stream   specifies the PDF stream species associated with a particular Fuel Stream ID, when calculating fuel NOx formation in conjunction with the non-premixed combustion model. You can select either the primary or secondary fuel streams, as defined in the PDF table.

Fuel Species   is a list containing all of the defined species, which allows you to specify the species that is the fuel associated with a particular Fuel Stream ID. 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 when the non-premixed combustion model is enabled, your selection in the Fuel Species list only applies to prompt NOx calculations.

User-Defined Functions   contains the NOx Rate drop-down list, which allows you to use a user-defined function (UDF) to contribute to the rate of NOx production. See the separate UDF Manual for details. Note that you may also use a UDF 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).

Reduction   allows you to specify the reduction methods.

Methods   contains the list of reduction methods.

Reburn   enables the calculation of NOx reburning effects.

SNCR   enables the calculation of NOx reduction by the SNCR method.

Turbulence Interaction Mode   contains parameters related to the effect of turbulent fluctuations on the NOx formation. See this section in the separate Theory Guide for details.

PDF Mode   is a drop-down list containing options that take into account turbulent fluctuations when you compute the specified NOx formation. See Section  21.1.1 for details.

none   specifies the use of laminar NOx rate calculations, so that the effects of turbulence are ignored.

temperature   includes fluctuations of temperature.

temperature/species   includes fluctuations of the temperature and the mass fraction of the species selected in the Species drop-down list (which appears when you select this option).

mixture fraction   includes fluctuations of the mixture fraction(s). This is available for non-premixed combustion calculations only.

PDF Type   allows you to specify the shape of the PDF.

beta   models the PDF using this equation in the separate Theory Guide.

gaussian   models the PDF using this equation in the separate Theory Guide.

PDF Points   controls the number of points at which the beta function in this equation or this equation in the separate Theory Guide will be integrated. The default value of 10, which indicates that the beta function will be integrated at 10 points on a histogram basis. The default value should yield an accurate solution with a reasonable computation time. Increasing this value may improve accuracy, but will also increase the computation time. This text box is only available when temperature or temperature/species is selected from the PDF Mode drop-down list.

Temperature Variance   allows you to specify the form of the transport equation that is solved to calculate the temperature variance.

algebraic   is an approximate form of the transport equation (see this equation in the separate Theory Guide).

transported   solves this equation in the separate Theory Guide.

Tmax Option   provides various options for determining the maximum limit(s) for the integration of the PDF used to calculate the temperature.

global-tmax   sets the limit as the maximum temperature in the flow field.

local-tmax-factor   yields cell-based maximum temperature limits by multiplying the local cell mean temperature by the value entered in Tmax Factor.

specified-tmax   sets the limit for each cell to be the value entered in Tmax.

user-defined   allows you to hook a user-defined function that specifies custom values for the maximum limit ( $T_{\rm max}$), which is used for the integration of the temperature PDF. This option is only available if you have already compiled a UDF and selected it in the Formation tab.

Species   is a drop-down list which appears when temperature/species is selected from the PDF Mode drop-down list. Here you will select the species whose mass fraction fluctuations will be factored into the NOx rate calculations.

Formation Model Parameters   contains the tabs used to define the NOx pathways.

Thermal   contains parameters for modeling thermal NOx. (The contents of this tab will appear only if Thermal NOx is enabled in the Formation tab.)

[O] Model   is a drop-down list in which you can select the method to be used for calculation of thermal NOx. To choose the equilibrium method, select equilibrium. To choose the partial equilibrium method, select partial-equilibrium. To choose the predicted O concentration method, select instantaneous. See this section , this section , and this section in the separate Theory Guide for details.

[OH] Model   is a drop-down list in which you can select the method to be used for calculation of thermal NOx. To exclude OH, select none. To choose the partial equilibrium method, select partial-equilibrium. To choose the predicted OH concentration method, select instantaneous. See this section , this section , and this section in the separate Theory Guide for details.

UDF Rate   provides options for the treatment of the NOx production specified by the UDF selected in the Formation tab.

Replace FLUENT Rate   replaces ANSYS FLUENT's thermal NOx rate calculations with the custom NOx rate produced by your UDF.

Add to FLUENT Rate   adds the custom NOx rate produced by your UDF to ANSYS FLUENT's thermal NOx rate calculations.

Prompt   contains parameters for modeling prompt NOx. (The contents of this tab will appear only if Prompt NOx is enabled in the Formation tab.) The settings made in this tab will be associated with a particular fuel stream, specified in the Fuel Stream ID text box in the Formation tab.

Fuel Carbon Number   specifies the number of carbon atoms per fuel molecule.

Equivalence Ratio   is the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio.

UDF Rate   provides options for the treatment of the NOx production specified by the UDF selected in the Formation tab.

Replace FLUENT Rate   replaces ANSYS FLUENT's prompt NOx rate calculations with the custom NOx rate produced by your UDF.

Add to FLUENT Rate   adds the custom NOx rate produced by your UDF to ANSYS FLUENT's prompt NOx rate calculations.

Fuel   contains parameters for modeling fuel NOx. (The contents of this tab will appear only if Fuel NOx is enabled in the Formation tab.) The settings made in this tab will be associated with a particular fuel stream, specified in the Fuel Stream ID text box in the Formation tab.

Fuel Type   specifies the type of fuel NOx to be calculated.

Solid   enables the calculation of solid fuel NOx.

Liquid   enables the calculation of liquid fuel NOx.

Gas   enables the calculation of gas fuel NOx.

N Intermediate   allows you to specify any one of the hcn, nh3, or
hcn/nh3/no as the intermediate species. See this section in the separate Theory Guide for details.

Volatile N Mass Fraction   specifies the mass fraction of nitrogen in the volatiles. This parameter appears only for Solid fuel NOx calculations.

Fuel N Mass Fraction   specifies the mass fraction of nitrogen in the fuel. This parameter appears only for Gas or Liquid fuel NOx calculations.

Conversion Fraction   specifies the overall mass fraction of fuel N (for gas and liquid fuels), or volatile N or char N (for solid fuels), that will be converted to intermediate species and/or product NO.

Partition Fractions   specifies the mass fraction of the converted fuel N (for gas and liquid fuels), or volatile N or char N (for solid fuels), that will become hcn and nh3. The fraction that will become NO will be calculated by the remainder. This option will appear only if you have selected hcn/nh3/no for the N Intermediate or Char N Conversion drop-down lists.

Char N Conversion   is a drop-down list in which you can select no, hcn, nh3, or hcn/nh3/no as the species to which the char N is converted (when you are calculating solid fuel NOx). This parameter appears only for Solid fuel NOx calculations. See Section  21.1.1 for details.

Char N Mass Fraction   specifies the mass fraction of nitrogen in the char. This parameter appears only for Solid fuel NOx calculations.

BET Surface Area   sets the BET internal pore surface area (see
this section in the separate Theory Guide for details) of the particles. This parameter appears only for Solid fuel NOx calculations.

UDF Rate   provides options for the treatment of the NOx production specified by the UDF selected in the Formation tab.

Replace FLUENT Rate   replaces ANSYS FLUENT's fuel NOx rate calculations with the custom NOx rate produced by your UDF.

Add to FLUENT Rate   adds the custom NOx rate produced by your UDF to ANSYS FLUENT's fuel NOx rate calculations.

N2O Path   contains the method to be used for formation of NO through an $N_2$O intermediate. (The contents of this tab will appear only if N20 Intermediate is enabled in the Formation tab.)

N2O Model   contains the drop-down list of available N2O models.

quasi-steady   enables the quasi-steady-state method of calculation (The transport equation for the species $N_2$O will not be solved).

transported-simple   enables the transported simple method of calculation (The pollutant species $N_2$O is added in the species list and it's mass fraction will be calculated using the transport equations).

UDF Rate   provides options for the treatment of the NOx production specified by the UDF selected in the Formation tab.

Replace FLUENT Rate   replaces the NOx rate calculated by ANSYS FLUENT using N $_2$O intermediates with the custom NOx rate produced by your UDF.

Add to FLUENT Rate   adds the custom NOx rate produced by your UDF to the NOx rate calculated by ANSYS FLUENT using N $_2$O intermediates.

Reduction Method Parameters   contains tabs that allow you to define the methods of reduction. (These tabs are do not appear unless a reduction method has been enabled in the Reduction tab.)

Reburn   allows you to define the NOx reduction when Reburn is enabled in the Reduction tab.

Reburn Model   contains the drop-down list of reburn methods.

instantaneous[CH]   activates instantaneous method in the Reburn Model. When you choose this method a warning to include CH, $CH_2$, and $CH_3$ will be displayed.

partial-equilibrium   activates partial method in the Reburn Model.

Reburn Fuel Species   contains reburn fuel species drop-down list.

Equivalent Fuel Type   contains equivalent fuel type drop-down list.

SNCR   allows you to define the NOx reduction when SNCR is enabled in the Reduction tab.

Injection Method   contains the parameters for NOx reduction by SNCR method.

gaseous   includes ammonia or urea as a gas-phase pollutant species from the injection locations.

liquid   includes ammonia or urea as a liquid-phase pollutant species from the injection locations.

Reagent Species   allows you to specify the reagent species as either ammonia ( nh3) or urea ( co $<$nh2 $>$2)

Reagent Fraction in Stream   allows you 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. Note that the Reagent Fraction in Stream text box is only available when using the non-premixed combustion model with a liquid-phase reagent injection.

Urea Decompostion   allows you to specify the decomposition model to use when the selected Reagent Species is co $<$nh2 $>$2.

rate-limiting   specifies that the source terms be calculated according to the rates given in this table in the separate Theory Guide.

user-specified   allows you to specify the molar conversion fraction for ammonia, assuming that the rest of the urea is converted to HNCO.

NH3 Conversion   is the mole fraction of NH $_3$ in the mixture of NH $_3$ and HNCO instantly created from the reagent injection. The NH3 Conversion text box only appears when user-specified is selected for Urea Decompostion.