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:
| 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.
Define Models Species NOx...
Define Boundary Conditions...
Solve Controls Solution...
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 Models Species NOx...
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:
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 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:
| 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:
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 NO rates.
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 ) 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 ) 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:
|= rate of release of fuel nitrogen in kg/s|
|= rate of release of volatiles (v) or char (c) in kg/s|
|= mass fraction of nitrogen in volatiles or char|
|= percentage by mass rate of nitrogen in volatiles|
|= percentage by mass rate of nitrogen in char|
|= percentage by mass of nitrogen in fuel (daf)|
|= split of nitrogen between volatiles and char|
|= mass fraction of volatiles in coal (daf)|
|= mass fraction of char in coal (daf)|
Then the following should hold:
| 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.
| 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.
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 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:
| When you use this method, you must be sure to include the species CH, CH
, and CH
in your problem definition. See Section
20.1.7 for details.
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:
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 FLUENT combustion calculation.
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 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 materials panel. 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 ( ). You must specify the water content while defining the injection properties.
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 Table 20.1.3. FLUENT will then calculates the source terms according to the rates given in Table 20.1.3. 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 nh3 conversion is the mole fraction of nh3 in the mixture of nh3/hnco instantly created from the reagent injection. In this case, there is no urea source because all of reagent is assumed to convert to both nh3 and hnco, instantly.
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.
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 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.