Structure and size of soot nanoparticles in laminar premixed flames at different equivalence ratios

In this work, the physicochemical properties of flame-formed soot particles are investigated in a set of ethylene/air premixed flames as function of the equivalent ratio. The objective of this study is to highlight the peculiar characteristics of the particles produced under quasi-clean combustion conditions in comparison with soot formed in richer flames. The size distribution of flame-formed soot nanoparticles was measured by differential mobility analysis; particle composition by Raman and UV-vis light absorption spectroscopy and morphology by atomic force microscopy. Our results evidence that the particle size distribution is strongly affected by the flame equivalence ratio. Slightly rich flames produce mono-modal PSD with very small particles of only few nanometers. Bi-modal size distribution are found in richer flames when larger particles, i.e., 5-20 nm, are formed. Change from a mono-modal to a bi-modal particle size distribution can be related to changes in particle graphitization as evidenced by a slight increase of the in plane average size of the polyaromatic units within the particles, La, and a change in particle optical properties.