Cluster–Cluster Aggregation Kinetics and Primary Particle Growth of Soot Nanoparticles in Flame by Light Scattering and Numerical Simulations

Abstract

The agglomeration kinetics of growing soot generated in a diffusion atmospheric flame are here studied in situ by light scattering technique to infer cluster morphology and size (fractal dimension Df and radius of gyration Rg). SEM analysis is used as a standard reference to obtain primary particle size DP at different residence times. The number NP of primary particles per aggregate and the number concentration nA of clusters are evaluated on the basis of the measured angular patterns of the scattered light intensity. The major finding is that the kinetics of the coagulation process that yields to the formation of chain-like aggregates by soot primary particles (size 10 to 40 nm) can be described with a constant coagulation kernel βc,exp=2.37×10−9 cm3/s (coagulation constant τc≈0.28 ms). This result is in nice accord with the Smoluchowski coagulation equation in the free molecular regime, and, vice versa, it is in contrast with previous studies conducted by invasive (ex situ) techniques, which claimed the evidence in flames of coagulation rates much larger than the kinetic theory predictions. Thereafter, a number of numerical simulations is implemented to compare with the experimental results on primary particle growth rate and on the process of aggregate reshaping that is observed by light scattering at later residence times. The restructuring process is conjectured to occur, for not well understood reasons, as a direct consequence of the atomic rearrangement in the solid phase carbon due to the prolonged residence time within the flame. Thus, on one side, it is shown that the numerical simulations of primary size history compare well with the values of primary size from SEM experiment with a growth rate constant of primary diameter about 1 n m/s. On the other side, the evolution of aggregate morphology is found to be predictable by the numerical simulations when the onset of a first-order “thermal” restructuring mechanism is assumed to occur in the flame at about 20 ms residence time leading to aggregates with an asymptotic fractal dimension Df,∞≈2.5.