Forced synchronization of globally stable and unstable flames

We experimentally investigate the forced synchronization of a turbulent, lean-premixed, bluff-body-stabilized flame undergoing self-excited oscillations due to global hydrodynamic instability. We acoustically force the flame at different amplitudes (α) and frequencies (ff) around its natural global frequency (fn), while measuring its heat release rate (HRR) response via time-resolved CH∗ chemiluminescence imaging. As α increases at a fixed ff, the flame initially oscillates quasiperiodically at both fn and ff, but synchronizes with the forcing above a critical α, consistent with the behavior of a canonical self-excited oscillator. The minimum α required for synchronization increases with the detuning (|ff-fn|), but not symmetrically around ff/fn=1, resulting in a skewed Arnold tongue. The HRR amplitude grows at small detuning due to resonant amplification but decays at large detuning due to asynchronous quenching. Comparing the globally unstable flame with an equivalent globally stable flame, we find that both exhibit a range of coupled states, including desynchronization, phase synchronization, and generalized synchronization. Crucially, the phase locking value varies spatially throughout the flame body, with the shear layers synchronizing more readily than the wake and recirculation zones. At higher detuning levels (i.e., when the forcing frequency is below 0.9 or above 1.1 times the natural frequency, ff/fn<0.9 or ff/fn>1.1), the HRR amplitude of the globally unstable flame is observed to be lower than that of the globally stable flame. This finding suggests that global hydrodynamic instability in a flame may serve as a passive mechanism to weaken self-excited thermoacoustic oscillations in combustion systems such as gas turbines and rocket engines.