
To compute the soot formation, you will need to start from a converged fluidflow solution. The procedure for setting up and solving a soot formation model is outlined below, and described in detail on the pages that follow. Remember that only the steps that are pertinent to soot modeling are shown here. For information about inputs related to other models that you are using in conjunction with the soot formation model, see the appropriate sections for those models.
Define Models Species Soot...
Solve Controls Solution...
Solve Monitors Residual...
Define Boundary Conditions...
Selecting the Soot Model
You can enable the calculation of soot formation by selecting a soot model in the Soot Model panel (Figure 20.3.1).
Define Models Species Soot...
Under Model, select either the OneStep or the TwoStep model. The panel will expand to show the appropriate inputs for the selected model.
(If you want to include the effects of soot formation on the radiation absorption coefficient, turn on the Generalized Model option under SootRadiation Interaction.)
Setting the Combustion Process Parameters
For both soot models, you will next define the Process Parameters, which depend on the combustion process that you are modeling. These inputs include the stoichiometry of the fuel and soot combustion and (for the twostep model only) the average size and density of the soot particles:
These parameters will not appear when the onestep model is used.
Defining the Fuel and Oxidizing Species
In addition to defining the stoichiometry for the fuel and soot combustion, you need to tell FLUENT which chemical species in your model should be used as the fuel and oxidizer. In the Soot Model panel under Species Definition, select the fuel in the Fuel dropdown list and the oxidizer in the Oxidant dropdown list.
If you are using the nonpremixed model for the combustion calculation and your fuel stream consists of a mixture of components, you should choose the most appropriate species as the Fuel species for the soot formation model. Similarly, the most significant oxidizing component (e.g., O ) should be selected as the Oxidant.
Setting Model Parameters for the SingleStep Model
When you choose the OneStep model for soot formation, the modeling parameters to be defined are those used in Equations 20.33, 20.35, and 20.36:
Note that the default values for these parameters are for propane fuel [ 63, 388], and are considered to be valid for a wide range of hydrocarbon fuels.
Setting Model Parameters for the TwoStep Model
When you choose the TwoStep model for soot formation, the modeling parameters to be defined are those used in Equations 20.35, 20.36, 20.39, 20.311, and 20.312:
The default values for the twostep model are the same as in Magnussen and Hjertager [ 229] (for an acetylene flame), except for , which is assumed to have the original value from Tesner et al. [ 373]. If your model involves propane fuel rather that acetylene, it is recommended that you change the value of to [ 5]. For best results, you should modify both of these parameters, using empirically determined inputs for your specific combustion system.
Defining Boundary Conditions for the Soot Model
At flow inlet boundaries, you will need to specify the Soot Mass Fraction, , in Equation 20.31, and (for the twostep model only) the Nuclei mass concentration, , in Equation 20.37.
Define Boundary Conditions...
You can retain the default inlet values of zero for both quantities or you can input nonzero numbers as appropriate for your combustion system.
Calculating Coupled Soot Solutions
If you are calculating a coupled solution for the soot and the flow field, you will generally need to increase the convergence criteria for soot (and nuclei, for the twostep model) to . You may choose to keep the recommended value of used for the uncoupled soot calculation, but be aware that the coupled solution may not be able to converge to this stricter tolerance.
For coupled calculations you should also use a lower underrelaxation factor for soot (and nuclei, for the twostep model). A value of 0.2 will be suitable in most cases.
If you are calculating a coupled solution and you are modeling radiative heat transfer using a variable absorption coefficient, you should enable the Generalized Model for SootRadiation Interaction in the Soot Model panel. When this option is enabled, FLUENT will include the effect of soot on the variable radiation absorption coefficient, as described in Section 13.3.8.
Reporting Soot Quantities
FLUENT provides several additional reporting options when your model includes soot formation. You can generate graphical plots or alphanumeric reports of the following items:
Both of these parameters are contained in the Soot... category of the variable selection dropdown list that appears in postprocessing panels.