Effects of the fresh mixture temperature on thermoacoustic instabilities in a lean premixed swirl-stabilized combustor

Abstract

This numerical study investigated the effects of the fresh mixture temperature on thermoacoustic instabilities in a lean premixed swirl-stabilized combustion chamber by utilizing high-fidelity, fully compressible large eddy simulations. At low fresh mixture temperatures, the side recirculation zone stabilized the premixed flame on the boundary of the burner rim, while the central part of the flame was detached from the burner due to the inability of the central recirculation zone to assist in flame stabilization. However, the central recirculation zone became stable enough to stabilize the central portion of the flame near the burner rim as the fresh mixture temperature increased. Moreover, the coherencies and penetration depths of the coherent structures and precessing vortex cores in the combustor increased with the fresh mixture temperature. Analyses showed that the limit cycle instabilities that occurred at low fresh mixture temperatures resulted from coupling between heat release fluctuations and the first tangential acoustic mode of the combustor. However, as the fresh mixture temperature increased, a combustor dynamics transition occurred, through which the coupling between heat release and pressure fluctuations shifted toward the mixed tangential and radial acoustical modes of the combustor. During this mode transition, limit cycle oscillations were replaced by burst oscillations. The results revealed that recirculation zones are the key features that trigger thermoacoustic instabilities at low fresh mixture temperatures, while coherent structures and precessing vortex cores are the main combustion instability drivers at high fresh mixture temperatures.