Swirling Flow Structures and Flame Characteristics in a Lean-Premixed Combustor

Abstract

The present work attempts to address unsteady flame dynamics in a lean-premixed swirl-stabilized combustor, with attention focused on the effect of swirl on the flow development and combustion dynamics. The numerical analysis treats the conservation equations in three-dimensions, and takes into account finite-rate chemical reactions and variable thermophysical properties. Turbulence closure is achieved using a large-eddy simulation (LES) technique. A level-set flamelet library approach is used to simulate premixed turbulent combustion. 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 further increases, 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, as opposed to cases with weak swirl in which longitudinal modes dominate the wave motions. The ensuing effect on the flow/flame interactions in the chamber is substantial.