Observations of vortex lock-in in an acoustically excited flame

Understanding the characteristics of structures arising from vortex shedding can be of great importance in combustors, where avoiding thermoacoustic instabilities is primarily desired. An experimental study is conducted with focus on the interaction between vortex shedding off circular cylinders located upstream of a bluff body stabilised CH₄/H₂ flame, when excited by acoustic oscillations. Measurements of the flow and flame properties were carried out through hot-wire anemometry and planar PIV, and by photomultiplier tubes and high-speed imaging capturing OH*-chemiluminescence, respectively. These measurements reveal that the flame reacts to the cylinder vortex shedding with its response showing footprints of classical vortex lock-in as the heat release rate also locks into the sub-harmonic of the acoustic forcing frequency. The results highlight the potential in controlling the onset of thermoacoustic instabilities by leveraging/tuning the effect of interference between vortex shedding behind the cylinders and the acoustic excitation of the flame to suppress the global flame response.