Dynamic stability of a swirl-stabilized flame in a counter-rotating dual-radial swirler

This study investigates the influence of injector flare angle (\((\beta)\)) on the flow dynamics and dynamic stability of a counter-rotating dual-radial swirl injector. The objective is to elucidate how geometric variation modifies both hydrodynamic and thermo-acoustic instability characteristics, thereby shaping flame topology and global stability limits in swirl-stabilized combustors. A series of non-reacting and reacting experiments were conducted for flare angles \((\beta)\) = 0°, 30°, and 50°. High-speed \((OH^*)\) chemiluminescence, stereo-PIV, and dynamic pressure measurements were acquired simultaneously to resolve unsteady flow-flame interactions. Time-resolved and spectral analyses—including Spectral POD—were employed to extract coherent structures, instability modes, and coupling mechanisms between pressure and velocity oscillations. Under non-reacting conditions, increasing the flare angle enhances the interaction between primary and secondary swirl streams, leading to stronger recirculation, a larger central recirculation zone (CRZ), and intensified precessing vortex core (PVC) activity. In reacting flows, \fa profoundly affects both static and dynamic stability. The \fae{0} and \fae{30} cases sustain attached V-flames dominated by longitudinal thermo-acoustic oscillations, whereas \fae{50} exhibits a transition from bubble-type to conical vortex breakdown (BVB → CVB), yielding lifted flames and intermittent low-frequency oscillations. This transition weakens acoustic coupling and produces a dynamically quieter yet stable flame. The flare angle is identified as a critical geometric control parameter dictating the balance between hydrodynamic and thermo-acoustic dominance. Optimizing \fa improves fuel–air mixing, extends the rich blow-off limit, and mitigates high-amplitude oscillations. These findings provide fundamental guidance for designing high-shear swirl injectors in next-generation low-emission gas-turbine combustors with enhanced stability and reduced acoustic sensitivity.