On the effects of varying coflow oxygen on soot and precursor nanoparticles in ethylene laminar diffusion flames

Abstract

This paper reports an experimental investigation of the effects of varying the oxygen concentration in the oxidizer stream on the formation and evolution of soot and precursor nanoparticles in an axisymmetric ethylene laminar coflow diffusion flame. The optical diagnostics employ temporally and spectrally resolved point-measurements of 266 nm excited scattering and laser-induced fluorescence, combined with 1064 nm excited laser-induced incandescence. Previous studies have indicated that oxygen enhancement changes soot volume fraction in sooting flames. However, the effects of coflow oxygen concentration on soot precursors are not well understood, and this aspect is addressed explicitly here. A set of eight flames is studied where the oxygen concentration in the coflow stream is varied from 19 to 40%, by volume, while the chemical composition of the jet stream is kept constant at 60% C2H4 / 40% N2. Oxygen enhancement in the coflow leads to a change in flame temperatures, luminosity, and flame length. It is found that the concentration of precursor nanoparticles increases with coflow oxygen. A combination of higher temperatures, red-shifting, and increased decay-times suggest structural growth in precursor nanoparticles with increasing coflow oxygen. In all flames, a similarity of particle evolution is noted with axial location, and this includes the presence of an intermediate stage during the transformation of precursors to soot. Moreover, the structural transformation from aliphatic to aromatic features is shifted towards the beginning of the precursor region when coflow oxygen is increased. It is found that structural growth starts at a similar temperature, regardless of coflow oxygen concentration. Soot volume fraction and primary soot particle diameter increase with coflow oxygen enrichment up to a threshold oxygen volume fraction, after which both the soot volume fraction and diameter decrease with further oxygen enrichment.