Effect of axial and normal cavity air injection on thermoacoustic instability in a lean premixed trapped vortex combustor

This study investigates the impact of cavity air injection on the combustion dynamics of a trapped vortex combustor (TVC), with particular emphasis on mitigating thermoacoustic instabilities. Experiments are conducted using a laboratory-scale combustor operating at a lean equivalence ratio of ϕ = 0.72 , representative of a limit-cycle instability regime. Two micro air injection strategies are systematically examined: axial injection through the cavity sidewall and normal injection through the cavity floor. Axial micro air injection through the cavity sidewall was systematically varied to assess its effect on instability behavior across cavity aspect ratios ( L / D = 0.75 –2.4). In a smaller cavity (open cavity) size ( L / D = 0.75 − 1 ), axial injection exhibits a complex evolution, transitioning through mixed-mode oscillations and low-frequency regimes (mode switching) before reaching the silent state. Notably, an initial decrease in SPL is followed by an increase at intermediate injection rates—surpassing the baseline limit-cycle levels—before ultimately decaying into the silent regime, with a maximum SPL reduction of 58 dB. At larger cavity (closed cavity) sizes ( L / D = 1.1 − 2.4 ), the system transitions from mixed-mode oscillations to high-frequency cavity modes (trapped modes, confirmed by the Helmholtz solver) before ultimately transitioning into the silent regime. For normal injection at a cavity aspect ratio of L / D = 1 , the flow undergoes a transition from periodic oscillations to aperiodic (silent) behavior via intermittency, producing a maximum sound pressure level (SPL) reduction of 64 dB when the injection velocity ratio is V inj = 2.48 V 0 . This suppression is attributed to flame stabilization at the injection site as the microjet flow rate increases, effectively disrupting the vortex acoustic lock-in. The effectiveness of wall-normal injection was found to be strongly dependent on the injection location within the cavity, with downstream injection near the cavity trailing edge providing the most effective instability suppression, resulting in the most rapid attenuation of thermoacoustic oscillations and the largest reduction in sound pressure level. High-speed OH* chemiluminescence imaging reveals significant alterations in flame topology and anchoring behavior across injection conditions, elucidating the role of flame–flow interactions in instability suppression. These results demonstrate that cavity air injection is a viable passive control strategy, disrupting constructive coupling between heat-release fluctuations and acoustic pressure oscillations. The study offers new insights into the mechanisms governing the thermoacoustic instability control relevant to TVCs operating under lean premixed conditions. Novelty and significance statement While secondary air injection has previously been shown to suppress thermoacoustic instabilities, the dynamical pathways leading to suppression and the influence of cavity geometry and injection location remain poorly understood, particularly in trapped vortex combustors. This study provides the first systematic investigation of axial and wall-normal cavity micro-air injection for passive control of thermoacoustic instability in a trapped vortex combustor. The results show that suppression is not monotonic and can proceed through intermediate regimes, including mixed-mode oscillations, intermittency, mode switching, and cavity-induced trapped acoustic modes, some of which may be detrimental to combustor performance. The study further demonstrates that control effectiveness depends critically on cavity geometry, injection location, and injection ratio, while also providing insight into flame-holding characteristics in open- and closed-cavity configurations. These findings advance the understanding of cavity-flow–flame–acoustic interactions and provide practical guidelines for robust passive-control design in lean combustors.