Vortex Breakdown in a Swirl-Stabilized Combustor

Results of nonreacting and lean premixed reacting flow tests in a swirl-stabilized combustor with a premixing fuel injector show the complex dynamic interaction between the flame and the vortex breakdown (VB) zone and the appearance of oscillations in the position of both. Particle-imaging velocimetry (PIV) is used to measure the distribution of axial, radial, and circumferential velocity fields, from which the swirl ratio for a given swirler configuration is computed. The position and size of the VB are determined for nonreacting cold and preheated flows and also for reacting flows at various equivalence ratios. OH chemiluminescence snapshots, taken simultaneously with the PI V, are used to identify the location of the reaction zones and hence the flame anchoring point relative to the VB positions, for each reacting flow case. For the given setup with a fixed swirler and fixed upstream pressure and mass flux, it is found that the VB zone, which occurs near the expansion plane in the nonreacting cold flow case, is pushed downstream of the expansion plane for the nonreacting, preheated flow case. For low equivalence ratio reacting flows near the flammability limit (0.48-0.52), the VB is anchored at the expansion plane and the flame is oscillating inside the recirculation zone downstream of it. At higher equivalence ratios (0.55-0.65), the breakdown zone apex (leading edge of the VB) and flame leading edge are close to each other and oscillate together near the expansion plane. At even higher equivalence ratios (>0.7), the flame is anchored at the expansion plane and corner recirculation zones while the VB point and breakdown zone oscillate downstream of it. The measured swirl ratios near the expansion plane exhibit strong correlation with the position of the VB for both average and instantaneous reacting flow states and with theoretical formulation for the critical swirl level for the appearance of vortex breakdown in reacting flows. A theoretical discussion on the various mechanisms that govern VB in reacting flows is given.