Effects of confinement length of the central toroidal recirculation zone partly confined by the small pilot stage chamber on ignition characteristics

Distributed Lean Direct Injection (DLDI) combustor is a new promising low NOx emission combustor. The ignition process independently occurs in the pilot stage chamber, which partly confines the swirled spray at the small confinement ratio. This study investigates ignition characteristics of the partly confined swirled spray experimentally, where effects of confinement length on flame propagation path and ignition probability are mainly considered. Particle image velocimetry, Mie scattering, phase Doppler anemometry and high-speed visualization were used to reveal the mechanisms of flame propagation and stabilization. The ignition probability was firstly counted in three chambers with the length varying from 18 mm to 27 mm, when pressure drop, Delta P, was 3000 Pa, and total equivalence ratio, phi(overall), varied from 0.67 to 0.73. Results show that reduction of the chamber length increases the ignition probability. To interpret this trend, aerodynamics, droplets characteristics and ignition process in three chambers were analyzed at Delta P = 3000 Pa and phi o(verall) = 0.73. The time-average flow field shows that the modification of chamber length results in changes of the confinement length of the central toroidal recirculation zone (CTRZ). Furthermore, the structure of CTRZ is directly impacted by changes of the confinement length. The reduction of the confinement length makes more air flow back, which means the greater recirculation capability of the CTRZ. More recirculating air can transport more droplets back into the CTRZ before sparking, which can also transport more high-temperature mixture there to help hold flame during ignition process. The high-speed visualization shows that the failed mode of ignition process is that the swirling flame can grow up at the beginning of Phase 3, but will blow out gradually after the extinction of the flame in corner recirculation zone (CRZ). According to characteristics of aerodynamics, droplets characteristics and ignition process, it can be concluded that since significant time is taken for flame to reach the fully burning state, the reduction of the confinement length improves the recirculation capability of the CTRZ which helps the partly confined flame get better stability. In addition, in Phase 2 of ignition process, greater recirculation capability makes the anchored position of the edge flame more upstream, which can make the propagation path shorter. (c) 2020 Elsevier Masson SAS. All rights reserved.