Re-excitation of buoyancy-driven flickering in a coflow diffusion flame under acoustic forcing

Buoyancy-driven flickering is an intrinsic instability of diffusion flames and can be suppressed by sufficiently strong air coflow beyond a critical velocity ratio. However, this study shows that such stabilized flames can be re-excited under external acoustic perturbations. High-speed schlieren imaging, OH* chemiluminescence, and particle image velocimetry are employed to characterize the flame response over a range of forcing frequencies, amplitudes, and velocity ratios. Although the flame exhibits synchronization at the forcing frequency, once the forcing amplitude exceeds the onset threshold, the dominant large-scale dynamics remain governed by the intrinsic flickering mode, even when the forcing frequency is much higher than the natural frequency. External perturbations enhance shear–layer instability and vorticity generation, promoting large-scale coherent structures and triggering the reemergence of buoyancy-driven flickering. The re-excitation intensity depends strongly on forcing amplitude and velocity ratio, while the intrinsic flickering frequency remains largely unaffected, with the re-excited flickering frequency lying in the range of 14–16 Hz. These results suggest that external acoustic perturbation acts primarily as a trigger that amplifies the intrinsic buoyancy-driven mode rather than replacing it.