Self-excited flame dynamics and mutual synchronization of the bluff-body-stabilized lean-premixed turbulent flame: on the effect of combustor length

This experimental study investigates the self-excited flame dynamics and mutual synchronization between heat release rate (HRR) fluctuations ( q ˙ ′ ) and acoustic pressure ( p' ) in a bluff-body-stabilized, lean-premixed turbulent flame, highlighting the effect of combustor length ( L c ). Systematic variation of L c significantly alters the thermoacoustic state and the corresponding hydrodynamic instability. At L c =1200 mm, p' and q ˙ ′ exhibit low-amplitude, chaotic oscillations, indicating stable combustion. At L c =2200 mm, a transition to thermoacoustic instability (TAI) occurs, characterized by high-amplitude quasi-periodic oscillations and strong lock-in of frequency and phase between p' and q ˙ ′ . Further increasing L c to 3200 mm leads to a weak-form synchronized state (|δ[∆ψ](t)|<45°) characterized by even higher amplitude oscillations, remaining within the TAI regime. Hydrodynamic analysis reveals that while the cold (non-reacting) flow recirculation zone and turbulence kinetic energy (TKE) remain quantitatively consistent across all L c , the hot (reacting) flow recirculation zone monotonically decreases and the TKE increases significantly. Comprehensive temporal and spatiotemporal analysis of flame dynamics and mutual synchronization elucidate the counter-rotating vortices pairing and merging downstream of bluff-body, i.e., the underlying vortex-driven thermoacoustic feedback mechanism. These findings establish the critical role of the combustor length as a governing acoustic-cavity parameter in driving and sustaining the self-excited thermoacoustic feedback coupling between acoustics, flame dynamics, and hydrodynamics.