Influence of Flame Macrostructures on the Thermoacoustic Instabilities of a High-Shear Injector

Highly turbulent flames in gas turbines are stabilized by swirl-induced vortex breakdown, but the intricate coupling between flow, flame, and acoustics often triggers and sustains thermoacoustic instabilities. This study investigates the influence of flame macrostructures resulting from flare expansion angle – a diverging section at the swirler exit – with 40°, 60°, and 90° on the thermoacoustic instabilities of a novel high-shear counter-rotating radial swirler. The swirler with 60% primary and 40% secondary airflow passages, along with central radial fuel injection is considered. Experiments spanned Reynolds numbers of 10,500–16,800 and thermal powers of 10–16.2 kW. Acoustic pressure, high-speed stereo PIV, and high-speed OH ∗ chemiluminescence measurements were captured to characterize the thermoacoustic instabilities. Flare geometry fundamentally alters the central recirculation zone (CRZ) and flame topology. The 40° flare produces a wide CRZ and compact flame, triggering large-amplitude oscillations characterized by pronounced vortex shedding and cyclic downstream heat-release convection. Conversely, the 90° flare yields an elongated, jet-like flame with a narrow CRZ, and low-amplitude acoustic fluctuations. Nonlinear time-series and phase-resolved analyses reveal the dynamic coupling sequence. Velocity fluctuations lead the phase cycle, followed by heat-release rate and acoustic pressure. Furthermore, phase-averaged Rayleigh index mapping pinpoints spatial regions of driving or damping. These findings provide critical insights into the effect of flare angle and its important role in the average flow fields and flame macrostructures. This study also shows passive control strategies that use flare angles in practical combustors to mitigate thermoacoustic instabilities.