13.3.1 Overview and Limitations

ANSYS FLUENT provides four models for the prediction of soot formation in combustion systems. In addition, the predicted soot concentration can be coupled with radiation. That is, you can include the effect of soot on radiation absorption when you use the P-1, discrete ordinates, or discrete transfer radiation model with a variable absorption coefficient.

Predicting Soot Formation

ANSYS FLUENT predicts soot concentrations in a combustion system using one of four available models:

• the one-step Khan and Greeves model [ 162], in which ANSYS FLUENT predicts the rate of soot formation based on a simple empirical rate

• the two-step Tesner model [ 216, 349], in which ANSYS FLUENT predicts the formation of nuclei particles, with soot formation on the nuclei

• the Moss-Brookes model [ 39], in which ANSYS FLUENT predicts soot formation for methane flames (and higher hydrocarbon species, if appropriately modified) by solving transport equations for normalized radical nuclei concentration and the soot mass fraction

• the Moss-Brookes-Hall model [ 120], which is an extension of the Moss-Brookes model and is applicable for higher hydrocarbon fuels (e.g., kerosene)

The Khan and Greeves model is the default model used by ANSYS FLUENT when you include soot formation.

In the Khan and Greeves model and the Tesner model, combustion of the soot (and particle nuclei) is assumed to be governed by the Magnussen combustion rate [ 216]. Note that this limits the use of these soot formation models to turbulent flows. Both models are empirically-based, approximate models of the soot formation process in combustion systems. The detailed chemistry and physics of soot formation are quite complex and are only approximated in these models. You should view the results of the Khan and Greeves model and the Tesner model as qualitative indicators of your system performance unless you can undertake experimental validation of the results.

The Moss-Brookes model has less impiricism and should theoretically provide superior accuracy than the Khan and Greeves and Tesner models. The Hall extension provides further options for modeling higher hydrocarbon fuels. Note that the Moss-Brookes-Hall model is only available when the required species are present in the gas phase species list.

Restrictions on Soot Modeling

The following restrictions apply to soot formation models:

• You must use the pressure-based solver. The soot models are not available with either of the density-based solvers.

• The Khan and Greeves model and the Tesner model can model soot formation only for turbulent flows (whereas the Moss-Brookes model and the Moss-Brookes-Hall model can be used with both laminar and turbulent flows).

• The soot model cannot be used in conjunction with the premixed combustion model.