Phase resolved characterization of conical premixed flames near and far from blowoff

A harmonically excited conical bluff body stabilized flame was studied to determine its behavior under strong burning and near blowoff conditions. Chemiluminescence imaging was employed using a high speed camera to characterize the phase resolved flame characteristics for a range of excitation frequencies from 50 to 400 Hz. Phase-resolved measurements of strain rate along the flame front were utilized to provide important insights for understanding of flame behavior particularly near blowoff. Phase resolved particle image velocimetry (PIV) was utilized to map the different phases of the velocity field for the harmonically modulated flames. PIV data along the flame front identified from seed density change were utilized to determine the strain rate variations at different phases of the flame oscillation. Oscillations of recirculation zone length were also characterized at all harmonic excitation frequencies accompanied by a cyclically varying strain rate along the flame front. At certain phases of the cycle, the strain rate reaches a maximum just before the phase when the recirculation zone length is a minimum. At this point, flame pinching and vortex breakdown phenomena were observed. This vortex breakdown phenomenon accompanies flame blowoff when the ratio of the minimum recirculation zone length to the convective length scale is in the range between 0.35 and 0.5. Based on these results, a description of flame blowoff phenomenon under externally forced flow is proposed.