PLIF measurements of instantaneous flame structures and curvature of an acoustically excited turbulent premixed flame

In this work, we experimentally measured the instantaneous flame structure and curvature of a methane fueled premixed turbulent acoustically-excited flame in a longitudinal combustor. For this, we implemented simultaneous formaldehyde (CH2O) and hydroxyl (OH) planar laser-induced fluorescence (PLIF) and high-speed schlieren imaging techniques to quantify the dynamic response of this acoustically-excited flame, which was fixed at 260 Hz. The Reynold number of the jet flow was varied from 1216 to 8108. Both imaging techniques revealed that the vortex appeared at the flame top in the presence of acoustic excitations. As the jet velocity increased, the jet flame was squeezed dramatically along the axial direction. The CH2O-PLIF is found to successfully capture the evolution of vortex shedding process. The vortex appears as a sharp angle, when the acoustically-excited flame is squeezed. The vortex size is then increased gradually over an acoustic fluctuation period. To measure the heat release rate from the flame, we used the pixel-by-pixel product of OH x CH2O as a quantitative index. Based on the experimentally measured CH2O, OH and their product characterizing the heat release, it is shown that the more the flame is squeezed by the acoustic waves, the more CH2O is produced and the more unsteady heat is released. Finally, the characteristics of the jet flame curvature were investigated. The variation of the probability density function of the flame curvature revealed that the acoustic excitation with a proper phase could smooth the flame front. (C) 2020 Elsevier Masson SAS. All rights reserved.