  Soot formation in Delft flame III, a pilot stabilized turbulent diffusion
flame burning natural gas/air, is investigated using ANSYS FLUENT by
considering two different approaches for soot inception. In the first approach
soot inception is based on the formation rate of acetylene, while the second
approach considers the formation rate of two and three-ringed aromatics to
describe the soot inception [1]. Transport equations are solved for soot mass
fraction and radical nuclei concentration to describe inception, coagulation,
surface growth, and oxidation processes. The turbulent-chemistry interactions
and soot precursors are described by the steady laminar flamelet model (SLFM).
Two chemical mechanisms GRI 3.0 [2] and POLIMI [3] are used to represent the
effect of species concentration on soot formation. The radiative properties of
the medium are included based on the non-gray modeling approach by considering
four factious gases; the weighted sum of gray gas (WSGGM) approach is used to
model the absorption coefficient. The effect of soot on radiative transfer is
modeled in terms of effective absorption coefficient of the medium. A beta
probability density function (\b{eta}-PDF) in terms of normalized temperature
is used to describe the effect of turbulence on soot formation. The results
clearly elucidate the strong effect of radiation and species concentration on
soot volume fraction predictions. Due to increase in radiative heat loss with
soot, flame temperature decreases slightly. The inclusion of ethylene has less
synergic effect than that of both benzene and ethylene. Both cases have less
impact on the nucleation of soot. The increase in soot volume fraction with
soot-turbulence interaction is in consistence with the DNS predictions.
