Soot-flowfield interactions in turbulent non-premixed bluff-body flames of ethylene/nitrogen

Abstract

Simultaneous measurements of soot concentrations and the velocity flowfields are used to better understand soot evolution and its correlation with the strain rate and residence time in a series of turbulent non-premixed bluff-body flames. Laser-induced incandescence (LII) and planar Particle image velocimetry (PIV) were applied simultaneously to measure the soot volume fraction (SVF) and the velocity field, respectively. Three flames were stabilised on axisymmetric bluff-body burners with different bluff-body diameters (38, 50, and 64 mm) but which are otherwise identical in dimension. A mixture of ethylene/nitrogen (4:1 by volume) was issued from a 4.6 mm central round jet at a bulk Reynolds number of 15,000. The annular co-flowing air velocity was kept constant at 20 m/s for all cases. In agreement with previous work, the highest SVF was found in the recirculation zone within the outer vortex, adjacent to the co-flowing air. The maximum SVF almost doubled, from 140 ppb to 250 ppb, when using the 64 mm burner, as compared with the 38 mm burner. Relatively small amounts of soot, around 30 ppb, were observed in the highly-strained neck zone. This was deduced from the instantaneous images as having been transported there from the recirculation zone, mostly from the inner vortex. The SVF in the jet region decreased with the increase in bluff-body diameter, which was found to be related to the decrease in the estimated total volume of the flame of almost 9%. The instantaneous images reveal the roller vortices between the co-flow and the recirculation zone supress soot and cause it to oxidise.