Effect of stratified ratio on combustion instability in a methane-fuelled centrally staged swirl flame for gas turbine applications

This study experimentally investigates combustion instability in a centrally-staged swirl flame under varying stratified ratios (SR). Here, SR is defined as the ratio of the pilot-stage equivalence ratio to the main-stage equivalence ratio. Experiments were performed at a global equivalence ratio of 0.6, with SR values set to 1.9, 2.2 and 2.5. Thermoacoustic oscillations were observed at SR = 1.9 and 2.2, with dominant frequencies of 180 Hz and 182 Hz, respectively, whereas no self-excited oscillation occurred at SR = 2.5. At SR = 2.5, the dominant pressure frequency (184 Hz) and heat-release fluctuation frequency (42 Hz) yielded a ratio of 4.4, indicating clear decoupling between acoustic and combustion responses. With increasing SR, the flame structure changed from a lifted mushroom-shaped flame to a stable V-shaped flame anchored near the swirler exit. Under unstable conditions, pressure oscillations propagated mainly in the longitudinal acoustic mode. POD analysis showed that flame motion at SR = 1.9 was dominated by Mode 1. In contrast, at SR = 2.2, the energy contribution of Mode 1 decreased from 13.8% at SR = 1.9 to 7.3% at SR = 2.2, indicating a transition from single-mode dominance to multi-mode interaction. Increasing pilot-stage enrichment enhances root flame stabilization, suppresses periodic flame oscillation, and promotes the transition from thermoacoustically unstable to stable combustion.