(49d) Coagulation of Polydisperse Primary Particles in the Transition Regime

Coagulation of polydisperse primary particles (PPs) in the transition regime is the dominant agglomerate growth mechanism in low-temperature regions of flame reactors and high-pressure environments of combustion engines. During flame synthesis of nanoparticles, small PPs coagulate in the free molecular regime to form fractal-like agglomerates. When agglomerates exit the flame high-temperature region, they start to coagulate in the transition regime, as their mean free path becomes comparable to their diameter of gyration, d_g, or mobility diameter, d_m. The estimation of the d_m of evolving aggregates is not trivial, as it depends on their structure and the number/size of their constituent PPs, n_p. Scaling laws describing the d_m of agglomerates consisting of monodisperse PPs have been developed in free molecular, transition and continuum regimes. However, these models overestimate the d_m of mature soot aggregates up to 37% as they neglect PP polydispersity.

Here, an event-driven method [1] for agglomeration of polydisperse PPs is used to investigate the agglomerate structure and size distribution in the transition regime. The d_m, increases with increasing geometric standard deviation of the PPs, σ_g,p, while the average d_g is hardly affected by σ_g,p for small compact agglomerates. The broad agglomerate size distributions formed in the free molecular regime narrow down by coagulation in the transition regime, attaining a quasi-self-preserving size distribution, regardless of σ_g,p. A scaling law for the n_p is derived as function of d_m and d_g which can be coupled with method of moments or population balance models to assist aerosol reactor design.

[1] Goudeli, E., Eggersdorfer, M. L,. & Pratsinis, S. E. (2015). Langmuir, 31, 1320-1327.