Soot formation and flame structure in swirl-stabilized turbulent non-premixed methane combustion

The relationships between flow field, flame structure, and soot distribution were investigated experimentally in turbulent 4 kW non-premixed swirl flames of methane and air at three different air supply rates. Auto-compensating laser-induced incandescence, particle image velocimetry, OH planar laser induced fluorescence, and OH* chemiluminescence techniques were used to measure soot volume fractions and primary soot particle size, velocity fields, OH, and OH* fields, respectively. While the instantaneous values of soot volume fractions were similar among the flames, the time-averaged soot volume fraction decreased by nearly a factor of three for a 7% increase in the air flow rate due to increased soot intermittency. Peak soot volume fractions occurred between two shear layers caused by the central fuel jet injection, recirculation zone, and swirling air jet interfaces. The decrease in soot towards the air side corresponded to locations with elevated OH, demonstrating its role in soot oxidation. The velocity measurements showed that engulfing the jet through a stronger recirculation rate induced by the higher air velocity enhanced the turbulent mixing between the recirculating fluid and fuel jet, and led to a shorter fuel jet penetration length. The reduction in soot volume fraction with air flow rate therefore is associated with greater turbulent mixing and higher oxidation rate in the recirculation zone. In contrast, the primary soot particle size, whose range was between 30 and 60 nm, did not show a dependence on the air flow rate. These results highlight the sensitivity of soot processes to fluid mechanics under turbulent conditions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.