Global and Local Synchronization During Thermoacoustic Instability in a Swirl Combustor

Synchronization, along with various other collective behaviors, is a widely studied phenomenon in systems of coupled oscillators across diverse fields of science and engineering. In such systems, the global dynamics are governed by complex local interactions among spatially distributed oscillators, nonlinearly coupled through different mechanisms. In this paper, we investigate both the global and local interactions between the acoustic pressure field and the unsteady heat release rate in the flame of a swirl-stabilized turbulent combustion system during a self-oscillatory state known as thermoacoustic instability. These instabilities are typically characterized by large-amplitude limit cycle oscillations in the acoustic field of the combustor. While the global characteristics of flame-acoustic interactions during thermoacoustic instability have been well understood over the years, the local interactions leading to the observed global dynamics remain elusive. Through experimental analyses of simultaneously measured acoustic pressure and heat release rate fluctuations (captured via CH* chemiluminescence imaging of the flame) in the combustor, we demonstrate that these fluctuations globally phase synchronize with a finite time delay during thermoacoustic instability. However, the local interaction of these coupled oscillations exhibits coherent patterns of phase-locking, resulting in the formation of space-invariant phase clusters along the flame surface. Thus, we observe that the local clustered synchronization of acoustic pressure and heat release rate fluctuations in the reaction field sustains the global order in the combustor during thermoacoustic instability. Finally, we propose a phenomenological phase oscillator model, based on Kuramoto’s model for coupled phase oscillators, to uncover the potential underlying network structure governing flame-acoustic interactions and driving synchronization transitions in the swirl combustion system.