Impact of the primary particle polydispersity on the radiative properties of soot aggregates

Combustion generated soot appears as fractal aggregates formed by polydisperse nearly spherical primary particles. Knowledge of their radiative properties is a prerequisite for laser based diagnostics of soot. In this parametric study, the effect of primary particle polydispersity on soot aggregate absorption and scattering properties is investigated numerically. Two series of fractal aggregates formed by normal and lognormal distributed primary particles of different levels of standard deviation were numerically generated for typical flame soot with a fractal dimension and prefactor fixed to Df=1.73 and kf ≈ 1.5, respectively. Three aggregate sizes consisting of Np=15, 50 and 150 monomers per aggregate were investigated. Due to the uncertainty in soot refractive index, radiative properties were calculated by considering two different refractive indices at λ ≈ 532 nm recommended in the literature using the Discrete Dipoles Approximation and the Generalized Multiparticle Mie method. The results are interpreted in terms of correction factors to the Rayleigh–Debye–Gans theory for fractal aggregates (RDG-FA) for the forward scattering cross section A and for the absorption cross section h. It is shown that differential cross section for vertically polarized incident light, total scattering and absorption cross sections are well predicted by the RDG-FA theory for all considered aggregates formed by normally (σ/dp¯≤ 30%) and lognormally (σgeo ≤ 1.6) distributed primary particles. The refractive index is found to be of greater impact than primary particle polydispersity on the importance of multiple scattering. The radiative force per unit laser power experienced by the soot aggregates was found primarily determined by the aggregate volume, regardless of the level of primary particle polydispersity.