Characterization of extinction events near blowout in swirl-dump combustors

Abstract

This work examines the evolution of the lean blowout (LBO) process in a swirl-dumpstabilized combustor with center body. Previous studies identified extinction-reignition events as precursors to LBO. These events are investigated in greater detail using simultaneous fiber optic-based chemiluminescence sensors and high speed imaging in an atmospheric pressure, premixed methane-air combustor. It is found that the flame, which is stabilized above the center body, at first partially detaches due to turbulent fluctuations. This detached region moves around the center body due to the swirl and the feedback mechanism of the inner recirculation zone. As the LBO limit is approached, the weaker flame detaches more often and over a larger extent of the center body. When the flame is detached beyond a certain extent, enough cold packets of unburned gases move around the inner recirculation zone to reduce the feedback needed for stabilization and the flame detaches completely from the center body. This is the source of the extinction events previously noted in the sensor data. The flow field responds to the decreased heat release with a larger recirculation zone and stronger swirl. The flame shape and flame stabilization change to a tornado mode. In this mode, the flame is stabilized much farther downstream of the combustor inlet, but an occasional packet of burning gases is convected back to the inlet. If this flame packet is sufficiently strong, the original flow field and flame shape are restored. This return of the original stable flame mode is the onset of the reignition events found in the sensor data. Several occurrences of these events in time gradually weaken the combustion process and eventually the convected flame packets are not strong enough to restabilize the combustor, i.e., the combustor blows out. This understanding of the process of flame stabilization loss provides the necessary insight for improved design and analysis of LBO sensors systems