Self-preserving size distribution and collision frequency of flame-made nanoparticles in the transition regime

Abstract

Coagulation of polydisperse primary particles is the dominant agglomerate growth mechanism in flame reactors and in the low-temperature or high-pressure regions of combustion engines. Here, the evolution of agglomerate size distribution (SD) and collision frequency function of coagulating soot and silica primary particles is investigated with simultaneous surface growth or coalescence using discrete element modeling (DEM). The growth of polydisperse soot and silica nanoparticles is captured from the free molecular up to the transition regime in excellent agreement with microscopy and mobility measurements in diffusion flames and flame spray pyrolysis (FSP) reactors. The broad agglomerate size distributions of soot and silica formed in the free molecular regime narrow by coagulation in the transition regime, attaining their mobility-based quasi-self-preserving or signature SD with the same geometric standard deviation of 1.48 ± 0.03, in excellent agreement with soot or silica SDs measured from diffusion flames and the CAST generator or from a FSP reactor, respectively. The coagulation rate of both soot and silica agglomerates is about twice as much higher as that of monodisperse agglomerates in the transition regime, regardless of material or combustion conditions. The DEM-derived collision kernel enhanced by a factor of 1.82 ± 0.35 can be interfaced with method of moments and monodisperse population balance models to improve the design of aerosol flame reactors and combustion engines.