Soot inception in laminar coflow diffusion flames

Abstract

This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region.