Planar imaging of soot formation in turbulent ethylene diffusion flames: Fluctuations and integral scales

Abstract

Planar imaging of soot in a series of atomspheric ethylene diffusion flames ranging from laminar to turbulent is reported. From single shot images of 532 nm light scattered at right angles by soot, the fluctuations, intermittency, and integral scales of soot are determined. The soot in turbulent (Re = 5500) ethylene flames occurs in highly convolved regions, suggestive of roll-up vortices. The regions are thin and elongated, lying somewhat perferrentially in the axial direction. The soot integral scales are smaller than velocity integral scales in corresponding cold jet flows. The soot in the late oxidative stages of the flame occurs in a few, highly intermittent, high concentration regions. The regions of low concentration have been oxidized away. This suggests that the soot emissions of turbulent flame are determined by (1) the regions of highest soot concentration, which may avoid oxidation by radiative cooling, as suggested by Kent, or (2) a few regions of high soot concentration that, by stochastic chance, fail to mix adequately with hot oxidizing gases in the transit time to the cooler gases above the flame. At given axial distances from the flame, there is little radial variation in the integral scale and strength of scattering within soot-containing eddies, but the number density of soot eddies does vary. This suggests that the soot chemistry is fast only within selective eddies, presumably of high temperature, and that the radial profiles are determined by mixing.