Soot

The formation and emission of carbonaceous particles is a process that is often observed during the combustion of hydrocarbons. These particulates, called soot, are a carbon-based material comprising mainly of polycyclic aromatic hydrocarbons (PAH's) and are identified in flames and fires as a yellow luminescence.

In gas turbines, internal combustion engines, and other practical combustion devices, the formation of soot is mostly a product of incomplete combustion. Apart from the resulting loss in combustion efficiency, the associated health hazards are also a particularly serious effect of soot formation.

A typical diesel engine can emit soot at a rate of 0.2 to 0.6 g/km, driving the demand to produce cleaner engines. However, some situations require the presence or generation of soot, for example, in the production of automobile tyres where carbon black is needed. In furnaces for industrial application, or in heat generators, the intermediate formation of soot is required to augment the heat transfer by radiation. The soot then has to be oxidised before the exhaust is released into the environment. These processes require a detailed knowledge of the different mechanisms leading to soot formation.

It is widely accepted that the formation of soot is a complex process which consists of:

fuel pyrolysis and oxidation reactions
formation of polycyclic aromatic hydrocarbons (PAH)
the inception of first particles
the growth of soot particles due to reaction with gas phase species
the coagulation of particles
the oxidation of soot particles and intermediates

Simcenter STAR-CCM+ models soot particles as approximately spherical, with a diameter, $d$ . Since the soot particles have a range of diameters, Simcenter STAR-CCM+ models the soot diameters using a particle size distribution function (PSDF). Two options are provided for modeling this PSDF—soot moments, or soot sections. The Soot Two-Equation model solves transport equations for the number of soot particles and their size (or diameter), but assumes a single diameter at each spatial point.

Soot Absorption Coefficient

When the participating radiation model is active, soot can contribute to the overall gas absorption coefficient for the cell. You can set the absorption coefficient as a constant, as a field function, or as the Planck Mean Absorption Coefficient $k_{a}$ in units of 1/m.

Figure 1. EQUATION_DISPLAY

k_{a} = \frac{C_{0} f_{v} T}{0.0037535}

(3650)

where:

$C_{0}$ is a constant that you can set.
$f_{v}$ is the soot volume fraction from Eqn. (3675).
$T$ is the temperature.

When accounting for all participating media, the sum of the absorption coefficient is calculated by Eqn. (1723).