Behavior of the lifted jet flame under acoustic excitation

The importance of large coherent vortical structures on lifted jet flame stability under acoustic excitation was extensively demonstrated in this investigation. By employing visualization and digital image processing methods, the most probable stabilization locations of a lifted flame base in the hysteresis region were found to be located at the roll-up and pairing positions of the coherent vortices. The flame stabilization mechanism in the hysteresis region was identified by phase-averaged concentration and velocity measurements. Phase-averaged results show that the strong entrainment induced by the large vortices during roll-up and pairing processes plays a key role in preventing the lifted flame from propagating upstream and causes the hysteresis phenomenon. Modification of lifted flame stability by acoustic excitation was demonstrated to be feasible. Acoustic excitation at frequencies in the turbulence amplification region of the cold jet is helpful in enhancing the flame stability in the high exit velocity region where the flame base zone is stabilized downstream of the end of the potential core; such excitation could extend the blowout limit by more than 25%. Excitation at frequencies in the turbulence suppression region is helpful when the flame is stabilized upstream of the end of the potential core.