Nonlinear dynamic characteristics of self-excited thermoacoustic instabilities in premixed swirling flames.

• Nonlinear thermoacoustic instability is studied under broad conditions. • Five kinds of stable or unstable flame modes are observed. • Hopf bifurcations for the instability are captured either by changing U in or Φ. • Appearance of ORZ flame is expected to trigger the instability. • Stable combustion regime is wider with lower swirl number S. The nonlinear behavior of thermoacoustic oscillation for premixed swirl flames fueled with CH 4 /air mixtures is experimentally studied by changing the equivalence ratio Φ , combustor inlet bulk velocity U in and swirl number S. The nonlinear dynamic characteristics of the system are analyzed by the methods of phase space reconstruction, recurrence plot, Poincaré section and 0–1 test for chaos. Five kinds of flame modes are observed in the combustion system with the change of Φ , U in , and S , which are lean/rich blow-off, stable combustion, quasi-periodic oscillation, and limit cycle oscillation. Subcritical Hopf bifurcation of the oscillation is captured either by changing Φ or U in , and Φ and U in are found to have similar effects on the flame mode transition under specific operating conditions. It is observed that the appearance of flame in the outer circulation zone (ORZ) and the enhancement of its burning intensity are matched with the occurrence of quasi-periodic and limit cycle oscillations, respectively. It is examined that the lean/rich blow-off limits and oscillation range are extended with a larger S , and the stable combustion regime is wider with a smaller S. This study contributes to understanding the nonlinear thermoacoustic instability of swirl flames under broad flame conditions, which helps to develop effective instability prediction and control methods in stable, efficient, and low-emission gas turbines. [ABSTRACT FROM AUTHOR]