Effect of swirl on combustion dynamics in a lean-premixed swirl-stabilized combustor

The effect of inlet swirl on the flow development and combustion dynamics in a lean-premixed swirl-stabilized combustor has been numerically investigated using a large-eddy-simulation (LES) technique along with a level-set flamelet library approach. Results indicate that when the inlet swirl number exceeds a critical value, a vortex-breakdown-induced central toroidal recirculation zone is established in the downstream region. As the swirl number increases further, the recirculation zone moves upstream and merges with the wake recirculation zone behind the centerbody. Excessive swirl may cause the central recirculating flow to penetrate into the inlet annulus and lead to the occurrence of flame flashback. A higher swirl number tends to increase the turbulence intensity, and consequently the flame speed. As a result, the flame surface area is reduced. The net heat release, however, remains almost unchanged because of the enhanced flame speed. Transverse acoustic oscillations often prevail under the effects of strong swirling flows, whereas longitudinal modes dominate the wave motions in cases with weak swirl. The ensuing effect on the flow/flame interactions in the chamber is substantial.