Precursor formation and soot inception in premixed ethylene flames

Absorption, fluorescence and scattering measurements were applied both in the ultraviolet and visible to rich premixed C2H4/O2 flames in the C/O range from 0.5 to 0.8. The concentration of soot and condensed hydrocarbon species were also measured by direct sampling along a lightly-sooting (C/O=0.65) and sooting (C/O=0.75) flame. The early formation in the flames of "particles" with typical average size around 2 nm (2500 a.m.u.), which do not absorb in the visible and fluoresce in the ultraviolet, was followed. These "transparent" particles were considered soot precursors on the basis of their decreasing concentration profile in correspondence of soot inception, as evaluated by both optical and chemical measurements. The soot inception process is characterized by the progressive aromatization of the "transparent" particles as shown by the progressive shift toward the visible of absorption and fluorescence. The internal rearrangement of a partially aromatic polymeric structure, more than a progressive formation of very large PAH structures, is proposed as the mechanism of soot inception.