Dynamics of self-excited thermoacoustic instability during lean operation of an inverse diffusion methane–oxygen flame

Self-excited thermoacoustic instability in a methane–oxygen inverse diffusion flame stabilised over a shear-coaxial injector is investigated experimentally at three thermal power levels. Acoustic measurements, high-speed OH* and CH* chemiluminescence and schlieren imaging, and modal decomposition are applied as the oxygen flow rate is increased at fixed fuel flow rate. The flame passes from stable combustion through intermittency to limit-cycle oscillation, with onset at a consistent equivalence ratio and velocity ratio across the power levels. The instability occurs at the first longitudinal chamber mode, confirmed by eigenfrequency prediction and a shortened-chamber experiment. The heat release drives the oscillation, with a global Rayleigh index of ＋0.99, and is confined to the inner oxygen-methane shear layer, where it condenses into a single dominant coherent mode at the transition. Near the injector exit, vortex formation alternates between an active and a quiescent half-period once per acoustic cycle while the flame root remains anchored; this organisation is absent in the shortened chamber at the same injector conditions. These observations indicate that the instability is sustained by periodic vortex formation that depends on the chamber acoustics. Whether the acoustic oscillation triggers roll-up by modulating the methane injection velocity or by acting directly on the shear layer remains open.