Ansys Forte provides three methods for predicting soot/particulate properties:
Use Models > Soot Model to turn on the Soot model and access its options.
Note: The pseudo-gas model is automatically selected when a pseudo-gas mechanism is loaded at the Chemistry model step (Chemistry). The mechanism would include the soot as a pseudo-gas species, and the gas-phase mechanism would contain all the relevant soot nucleation, growth and oxidation pathways. As a result, Models > Soot Models check box can be unchecked when the pseudo-gas model is used. Soot particles cannot be tracked when this model is used; only the overall soot mass can be simulated with this method. Under the Ansys Forte data directory, one chemistry set each is provided for gasoline and diesel that uses the pseudo-gas approach.
Although detailed kinetics can be used to predict soot precursors in the gas phase, it is often expedient to use a semi-empirical model to get a qualitative but direct measure of the particulate matter generated under the simulation conditions of interest. For this reason, Ansys Forte includes a popular 2-step semi-empirical soot model with acetylene as the soot precursor that approximates the mass and the particle size of soot formed, based on local species concentrations throughout the simulation. However, for the soot model to be used, it is also necessary to have a species named "soot" appended to the species list with thermodynamic property entries in the chemistry input files within the chemistry set that is imported to Ansys Forte. All of the installed Ansys Forte chemistry sets in the system data directory include this species. If this model is chosen, no surface chemistry is required. Soot particles cannot be tracked when this model is used; only the overall soot mass can be simulated with this method.
The Soot Model Settings panel allows modification of the particle formation equations in the model by controlling key parameters in those equations. Options include:
Details regarding the semi-empirical soot model and these parameters are provided in the Ansys Forte Theory Manual. Some guidance is available in the tool tips for the individual options in the Editor panel.
The Method of Moments model (see Method of Moments, in the Ansys Forte Theory Manual) allows tracking of particle size and number density, in addition to soot mass and volume fraction, throughout the simulation. It requires more detailed kinetics in the gas-phase fuel combustion model as well as a surface kinetics description of reactions that occur on the particle surface to initiate the formation of dispersed-phase particles. The Method of Moments option requires that a chemistry set be populated with a dispersed-phase particle-inception reaction, where gas-phase precursors react to form a dispersed particulate bulk phase along with some proportion of occupied and unoccupied surface sites. From a Moment-equation representation of the particle size distribution, the particle number density, mass density, and volume fraction are derived, as well as a mean particle size. These variables are determined as a function of location and time throughout the simulation.
The Soot Model Settings panel has options for changing the number of moments, changing the scaling factors used in the calculation, the option to include particle coagulation, and particle coagulation efficiency.