33.3.28 Soot Model Dialog Box

The Soot Model dialog box allows you to set parameters related to the soot model. See Section  21.3.1 for details about the items below.

Controls

Model   specifies which model should be used for computing soot formation.

Off   disables the calculation of soot formation.

One-Step   enables the one-step soot model described in this section in the separate Theory Guide.

Two-Step   enables the two-step soot model described in this section .

Moss-Brookes   enables the Moss-Brookes soot model described in this section in the separate Theory Guide.

Moss-Brookes-Hall   enables the Moss-Brookes-Hall soot model described in this section in the separate Theory Guide. This option is only available when C $_2$H $_2$, C $_6$H $_6$, C $_6$H $_5$, and H $_2$ are present in the gas phase species list.

Species Definition   contains inputs for specifying

the chemical species for your model.

Fuel   is a drop-down list containing all of the defined species. Here you will select the species that is the fuel for the One-Step and Two-Step models, as well as the Moss-Brookes model when a precursor species is not identified in the defined species list.

Oxidant   is a drop-down list containing all of the defined species. Here you will select the species that is the oxidizer for the One-Step and Two-Step models.

Precursor from   allows you to select from a list of species or enter the correlation values of species. This selection is available when using the Moss-Brookes and Moss-Brookes-Hall models.

Soot Precursor   is a selection list containing all of the possible precursor species found via a query of the defined species list. By default, ANSYS FLUENT only considers c2h2, c6h6, and c2h4 as possible precursor species. For information about including other species in the possible precursor species search, contact your ANSYS FLUENT support engineer. From this list you will select the species that are the soot precursor species for the Moss-Brookes and Moss-Brookes-Hall models.

Surface Growth   is a selection list containing all of the possible surface growth species, as explained previously for the Soot Precursor selection list. Here you will select the species that are the surface growth species for the Moss-Brookes and Moss-Brookes-Hall models.

Fuel Carbon Number   is the number of carbon atoms in the species selected in the Fuel drop-down list. This text box appears only for the Moss-Brookes and Moss-Brookes-Hall model, when user-correlation is selected in the Precursor from drop-down list.

Fuel Hydrogen Number   is the number of hydrogen atoms in the species selected in the Fuel drop-down list. This text box appears only for the Moss-Brookes and Moss-Brookes-Hall model, when user-correlation is selected in the Precursor from drop-down list.

Molecular Weight of Precursor   is the molecular weight of the precursor species. This text box appears only for the Moss-Brookes and Moss-Brookes-Hall model, when user-correlation is selected in the Precursor from drop-down list. The default value is the weight of acetylene.

Precursor Correlation   is a drop-down list you can use to define a laminar diffusion profile which relates mixture fraction to precursor mass fraction. This text box appears only for the Moss-Brookes and Moss-Brookes-Hall model, when user-correlation is selected in the Precursor from drop-down list.

piecewise-polynomial   specifies that the precursor mass fraction is a piecewise-polynomial function of mixture fraction. The default values used by ANSYS FLUENT correspond to a methane diffusion flame simulation, in which both the air and fuel initial temperatures are set to 290 K, and acetylene is assumed as the soot precursor. These values can be revised via the Edit... button.

constant   specifies that the precursor mass fraction is a constant function of mixture fraction, the value of which is specified in the text box below the Precursor Correlation drop-down list.

Edit...   opens the Piecewise-Polynomial Profile dialog box when piecewise-polynomial is selected from the Precursor Correlation drop-down list, thus allowing you to revise the default values.

Turbulence Interaction Mode   contains inputs that specify how turbulent fluctuations are accounted for in the soot formation calculations for the Moss-Brookes and Moss-Brookes-Hall models. For further details on these inputs, see Section  21.3.1.

PDF Mode   is a drop-down list that contains the options for addressing turbulent fluctuations in the soot rate calculations. Note that mixture fraction is the most accurate option, and should be used if it is available.

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

temperature   specifies that the soot rate calculations include the effect of temperature fluctuations.

temperature/species   specifies that the soot rate calculations include the effect of fluctuations of temperature, as well as fluctuations of the mass fraction of the species selected in the Species drop-down list (which appears when you select this option).

mixture fraction   is the most accurate option, specifying that the soot rate calculations include the effect of fluctuations of mixture fraction(s). Note that this option is not available if you are using the eddy-dissipation model.

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 will be integrated on a histogram basis. Increasing this number may improve accuracy, but will also increase compute 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 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.

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 soot rate calculations.

Process Parameters   contains parameters that control the combustion process modeling.

Mean Diameter of Soot Particle   is the assumed average diameter of the soot particles in the combustion system, used to compute the soot particle mass $m_p$ in this equation in the separate Theory Guide for the Two-Step model.

Mean Density of Soot Particle   is the assumed average density of the soot particles in the combustion system. For the Two-Step model, it is used to compute the soot particle mass $m_p$ in this equation in the separate Theory Guide. For the Moss-Brookes and Moss-Brookes-Hall models, it is $\rho_{\rm soot}$ in this equation and $\rho$ in this equation in the separate Theory Guide. The default value supplied by ANSYS FLUENT is 1800 kg/m $^3$ (as was used in the work of Brookes and Moss [ 12]).

Stoichiometry for Soot Combustion   is the mass stoichiometry $\nu_{\rm soot}$ in this equation in the separate Theory Guide , which computes the soot combustion rate in the One-Step and Two-Step models. The default value supplied by ANSYS FLUENT (2.6667) assumes that the soot is pure carbon and that the oxidizer is O $_2$.

Stoichiometry for Fuel Combustion   is the mass stoichiometry $\nu_{\rm fuel}$ in this equation in the separate Theory Guide , which computes the soot combustion rate in the One-Step and Two-Step models. The default value supplied by ANSYS FLUENT (3.6363) is for combustion of propane (C $_3$H $_8$) by oxygen (O $_2$).

Mass of Incipient Soot Particle   is $M_{\rm P}$ in this equation and this equation , which is used in the Moss-Brookes and Moss-Brookes-Hall model computations. The default value supplied by ANSYS FLUENT (144 kg/mol) is the mass of 12 carbon atoms. Note that for the original implementation of the Hall extension, the model assumed this mass to be 100 carbon atoms.

Soot Oxidation Model   contains model options that determine the form of the soot oxidation term in the calculations of the Moss-Brookes and Moss-Brookes-Hall models.

Fenimore-Jones   takes into account the soot oxidation due to the hydroxyl radical.

Lee   takes into account the soot oxidation due to the hydroxyl radical and molecular oxygen.

Model Parameters   contains parameters that control the soot formation model.

Soot Formation Constant   is the parameter $C_s$ in this equation in the separate Theory Guide. This item appears only for the One-Step soot model.

Equivalence Ratio Exponent   is the exponent $r$ in this equation in the separate Theory Guide. This item appears only for the One-Step soot model.

Equivalence Ratio Minimum   and Equivalence Ratio Maximum are the minimum and maximum values of the fuel equivalence ratio $\phi$ in this equation in the separate Theory Guide. this equation will be solved only if Equivalence Ratio Minimum $< \phi <$ Equivalence Ratio Maximum; if $\phi$ is outside of this range, there is no soot formation. This item appears only for the One-Step soot model.

Activation Temperature of Soot Formation Rate   is the term $E/R$ in this equation in the separate Theory Guide. This item appears only for the One-Step soot model.

Magnussen Constant for Soot Combustion   is the constant $A$ used in the rate expressions governing the soot combustion rate ( this equation and this equation ) in the separate Theory Guide. This item appears only for the One-Step and the Two-Step soot models. For the Two-Step model, this input will be called Magnussen Constant for Soot and Nuclei Combustion.

Limiting Nuclei Formation Rate   is the limiting value of the kinetic nuclei formation rate $\eta_0$ in this equation in the separate Theory Guide. Below this limiting value, the branching and termination term, ( $f-g$) in this equation in the separate Theory Guide , is not included. This item appears only for the Two-Step soot model.

Nuclei Branching-Termination Coefficient   is the term $(f - g)$ in this equation in the separate Theory Guide. This item appears only for the Two-Step soot model.

Nuclei Coefficient of Linear Termination on Soot   is the term $g_0$ in this equation in the separate Theory Guide. This item appears only for the Two-Step soot model.

Pre-Exponential Constant of Nuclei Formation   is the pre-exponential term $a_0$ in the kinetic nuclei formation term, this equation in the separate Theory Guide. This item appears only for the Two-Step soot model.

Activation Temperature of Nuclei Formation Rate   is the term $E/R$ in the kinetic nuclei formation term, this equation in the separate Theory Guide. This item appears only for the Two-Step soot model.

Alpha for Soot Formation Rate   is $\alpha$, the constant in the soot formation rate equation, this equation in the separate Theory Guide. This item appears only for the Two-Step soot model.

Beta for Soot Formation Rate   is $\beta$, the constant in the soot formation rate equation, this equation in the separate Theory Guide. This item appears only for the Two-Step soot model.

Magnussen Constant for Soot and Nuclei Combustion   is the constant $A$ used in the rate expressions governing the soot combustion rate ( this equation and this equation in the separate Theory Guide). This item appears only for the One-Step and the Two-Step soot models.

[OH] Model   is a drop-down list that allows you to specify the method by which the OH radical concentration is calculated, i.e., instantaneous or partial-equilibrium. This list appears only for the Moss-Brookes and the Moss-Brookes-Hall soot models.

[O] Model   is a drop-down list that specifies the method by which the O radical concentration is calculated, i.e., equilibrium, partial-equilibrium, or instantaneous. This list appears only for the Moss-Brookes and the Moss-Brookes-Hall soot models, when you have selected partial-equilibrium for the [OH] Model.

To use the concentration of OH or O predicted by the combustion model, select instantaneous for [OH] Model or [O] Model.

Options   contains an option for modeling the effect of soot on a variable radiation absorption coefficient. This group box will appear only when one of the radiation models in the Radiation Model dialog box is active.

Soot-Radiation Interaction   enables the soot-radiation interaction model described in this section in the separate Theory Guide.