Experimental investigation of cavity fuel–air injection on flame dynamics and stability in a trapped-vortex combustor

This study investigates the impact of injecting premixed fuel–air mixtures into the cavity of a trapped-vortex combustor on its thermoacoustic behavior and lean blowout characteristics. Two cavity injection conditions were examined: a lean mixture with an equivalence ratio of ϕc=0.8 and a rich mixture with ϕc=1.12. The analysis focuses on the influence of cavity injection on flame stabilization, combustion dynamics, and the extension of lean operating limits under different global equivalence ratios. Experiments were conducted over a wide range of cavity size ratios (L/D=0.75–2.4) while varying the equivalence ratio from stable operation to lean blowout. High-speed OH* chemiluminescence imaging was employed to characterize the spatiotemporal evolution of the flame and to examine the formation and stability of the cavity-anchored flame (trapped vortex) within the cavity. Statistical analysis based on the cavity-to-shear-layer intensity ratio and the normalized vertical flame centroid are employed to characterize the spatiotemporal evolution of the cavity-stabilized reaction zone, demonstrating how changes in the cavity equivalence ratio from lean to rich conditions shift the lean blowout limit. The new cavity injection measurements were compared with the previously established no-injection baseline to assess the influence of cavity injection on the combustor dynamics [Singh and Nair, “Experimental investigation of flame, flow, and acoustic dynamics in a laboratory-scale cavity-based combustor,” Combust. Flame 291, 115128 (2026)]. The resulting ϕ–L/D operating map reveals a rich sequence of nonlinear dynamical states, including stable limit-cycle oscillations, quasi-periodic motion, intermittent bursting, strange non-chaotic attractors, chaotic oscillations, and eventual blowout. A direct comparison between configurations with and without cavity injection demonstrates that stabilizing a vortex inside the cavity markedly alters the thermoacoustic response. Without cavity injection, the combustor exhibits strong limit-cycle oscillations at larger cavity sizes (L/D=2.2–2.4). Introducing a premixed stream into the cavity promotes a cavity-anchored flame, suppresses the high-amplitude oscillations, and significantly delays the onset of blowout, thereby extending the stable operating envelope. Quantitatively, cavity injection substantially extends the lean blowout (LBO) limits for both cavity configurations. For the L/D=1 configuration with a lean cavity injection condition (ϕc=0.80), the LBO limit shifts from ϕ=0.44 to ϕ=0.36. Similarly, for the L/D=2 configuration with a rich cavity injection condition (ϕc=1.12), the LBO limit extends from ϕ=0.54 to ϕ=0.42. Overall, the results demonstrate that cavity injection stabilizes the trapped vortex, modifies the nonlinear thermoacoustic dynamics, suppresses large-amplitude oscillations, and significantly improves lean blowout performance. These findings highlight the crucial role of cavity vortex–flame interactions in enhancing the operational robustness of practical trapped-vortex combustors.