Characterizing nonlinear dynamic features of self-sustained thermoacoustic oscillations in a premixed swirling combustor

To meet stringent NOx emission requirements, lean premixed swirling combustion technology is widely applied in power generation and propulsion systems. However, such combustion systems are more susceptible to nonlinear combustion instability due to the fluid flow-acoustics-combustion interaction. It is typically self-exited and characterized by large-amplitude pulsating oscillations. By applying experimental measurements and theoretical modelling, we explore the rich physics of how a methane-burnt swirling flame sustains periodic pulsating combustion oscillations, and its nonlinear dynamics features via recurrence plots (RP) and 0-1 chaotic test. The effects of (1) the equivalence ratio Phi, (2) the volume flow rate V-a of inlet air and (3) the swirling number S-N are examined. Hopf Supercritical and period-doubling bifurcation behaviours are experimentally observed with increased F from lean to rich combustion. Same nonlinear features are theoretically modelled by using 'kick-oscillator' model representing combustion pulses and to capture periodic limit cycle behaviours followed by period-doubling bifurcation and transition to chaos. The deterministic or chaotic nature of the swirling combustor could be experimentally identified using classical approaches such as probability density functions (PDFs) and acoustics power spectrum in presence of combustion-sustained periodic fluctuations and 0-1 chaotic test method. When the combustor is experimentally operated under either lean (Phi <= 0.6) or rich (Phi >= 1.1) conditions, no self-sustained periodic acoustic fluctuations are generated. Furthermore the combustor is found to be more chaotic. The flame shapes (M- or V-shaped), colour, brightness and volumes are found to depend on S-N strongly. The present findings are physically insightful on understanding nonlinear features of swirling flow-acoustics-flame interaction.