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20.1.10 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 FLUENT as usual.

figure   

If you plan to use the 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  14.1.4 for details about adding species to your model and Section  22.12 for details about creating injections.

2.   Activate the desired NOx models (thermal, fuel, and/or prompt NOx, with or without reburn) and set the appropriate parameters, as described in this section.

Define $\rightarrow$ Models $\rightarrow$ Species $\rightarrow$ NOx...

3.   Define the boundary conditions for NO (and HCN, NH $_3$, or N $_2$O if necessary) at flow inlets.

Define $\rightarrow$ Boundary Conditions...

4.   In the Solution Controls panel, turn off the solution of all variables except species NO (and HCN, NH $_3$, or N $_2$O, based on the model selected).

Solve $\rightarrow$ Controls $\rightarrow$ Solution...

5.   Perform calculations until convergence (i.e., until the NO (and HCN, NH $_3$, or N $_2$O, if solved) species residuals are below $10^{-6}$) to ensure that the NO and HCN or NH $_3$ concentration fields are no longer evolving.

Solve $\rightarrow$ Iterate...

6.   Review the mass fractions of NO (and HCN, NH $_3$, or N $_2$O) with alphanumerics and/or graphics tools in the usual way.

7.   Save a new set of case and data files, if desired.

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



Activating the NOx Models


To activate the NOx models and set related parameters, you will use the NOx Model panel (e.g., Figure  20.1.3).

Define $\rightarrow$ Models $\rightarrow$ Species $\rightarrow$ NOx...

Figure 20.1.3: The NOx Model Panel
figure

In the Formation tab, select the NOx models under Pathways to be used in the calculation of the NO and HCN, NH $_3$, or N $_2$O concentrations:

Your selection(s) under Pathways will activate the calculation of thermal, prompt, fuel, and/or N $_2$O-intermediate NOx in accordance with the chemical kinetic models described in Sections  20.1.3- 20.1.6. Mean NO formation rates will be computed directly from mean concentrations and temperature in the flow field.



Setting Thermal NOx Parameters


The NOx routines employ three methods for calculation of thermal NOx (as described in Section  20.1.3). You will specify the method to be used in the Thermal tab, under Formation Model Parameters in the NOx Model panel:

figure   

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



Setting Prompt NOx Parameters


Prompt NO formation is predicted using Equations  20.1-25 and 20.1-27. The parameters are entered in the Prompt tab under Formation Model Parameters in the NOx Model panel in the following manner:



Setting Fuel NOx Parameters


For fuel NOx models, you will first need to specify the fuel type in the Fuel tab under Model Parameters in the following manner:

Note that you can use only one of the fuel models at a time. The Gas option is available only with the finite-rate and eddy break-up (EBU) chemistry models are active (see Section  14.1.3).

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:

FLUENT will use Equations  20.1-29 and 20.1-30 (for HCN) or Equations  20.1-40 and 20.1-41 (for NH $_3$) to predict NO formation for a gaseous or liquid fuel.

Setting Solid (Coal) Fuel NOx Parameters

For solid fuel, FLUENT will use Equations  20.1-54 and 20.1-55 (for HCN) or Equations  20.1-61 and 20.1-62 (for NH $_3$) to predict NO formation. Several inputs are required for the coal fuel NOx model as follows:

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} \cdot mf_{N_{v/c}} (20.1-112)


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    
  $P_{N_{v}}$ = percentage by mass rate of nitrogen in volatiles
  $P_{N_{c}}$ = percentage by mass rate of nitrogen in char
  $TN_{fuel}$ = percentage by mass of nitrogen in fuel (daf)
  $N_{split}$ = split of nitrogen between volatiles and char
  $F_{vol}$ = mass fraction of volatiles in coal (daf)
  $F_{char}$ = mass fraction of char in coal (daf)

Then the following should hold:


 F_{vol} + F_{char} = 1.0 (20.1-113)


 \frac{P_{N_{v}}}{P_{N_{c}}} = N_{split} (20.1-114)


 (F_{vol} \cdot P_{N_{v}}) + (F_{char} \cdot P_{N_{c}}) = TN_{fuel} (20.1-115)


 P_{N_{c}} = \frac{TN_{fuel}}{(F_{vol} \cdot N_{split}) + F_{char}} (20.1-116)

figure   

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



Setting N $_2$O Pathway Parameters


The formation of NO through an N $_2$O intermediate can be predicted by two methods. You will specify the method to be used in the N2O Path tab.

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 NO pathway, then you will be given the option to specify an [O] Model for the N $_2$O pathway calculation. The same three options for the thermal NOx [O] Model will be the available options.



Setting Parameters for NOx Reburn


To enable NOx reduction by reburning, click the Reduction tab in the NOx Model panel and enable the Reburn option under Methods. In the expanded portion of the panel, as shown in Figure  20.1.4, click the Reburn tab under Reduction Method Parameters, where you can choose from the following options:

Figure 20.1.4: The NOx Panel Displaying the Reburn Reduction Method
figure



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). Refer to Section  20.1.8 for detailed information about SNCR theory.

To enable NOx reduction by SNCR, click the Reduction tab in the NOx Model panel and enable the SNCR option under Methods, as shown in Figure  20.1.5. Click the SNCR tab under Reduction Method Parameters, where you can choose from the following options:

Figure 20.1.5: The NOx Panel Displaying the Reburn Reduction Method
figure



Setting Turbulence Parameters


If you want to take into account turbulent fluctuations (as described in Section  20.1.9) when you compute the specified NO formation (thermal, prompt, and/or fuel, with or without reburn), click the Turbulence Interaction tab and select one of the options in the PDF Mode drop-down list under Turbulence Interaction Mode.

Figure 20.1.6: The NOx Model Panel and the Turbulence Interaction Tab
figure

The mixture fraction option is available only if you are using the nonpremixed 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 flowfield 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 in 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.

Number of Beta Points

You can, optionally, adjust the number of Beta PDF Points. The default value of 10, indicating that the beta function in Equation  20.1-104 or Equation  20.1-105 will be integrated at 10 points on a histogram basis, will yield an accurate solution with reasonable computation time. Increasing this value may improve accuracy, but will also increase the computation time. This option is not available when you select mixture fraction in the PDF Mode drop-down list under Turbulence Interaction Mode. In this case, the mixture fraction points defined in the PDF table will be used.



Specifying a User-Defined Function for the NOx Rate


You can, optionally, choose to specify a user-defined function for the rate of NOx production. The rate returned from the UDF is added to the rate returned from the standard NOx production options, if any are selected. However, if you would like to replace any or all of FLUENT's NOx rate calculations with your own user-defined NOx rate, you can enable the Replace with UDF Rate option for the appropriate NOx formation pathway(s) after loading the UDF file.

In the Formation tab, select the desired function in the NOx Rate drop-down list under User-Defined Functions. See the separate UDF Manual. for details about user-defined functions.



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.

Define $\rightarrow$ 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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