Excitation of Thermoacoustic Instabilities by Interaction of Acoustics and Unstable Swirling Flow

Abstract

Unstablethermoacousticmodeswereinvestigatedandcontrolledinanexperimentallow-emissionswirlstabilized combustor, in which theacousticboundary conditions weremodie ed to obtain combustion instability. Theacoustic boundary conditions of the exhaust system could be adjusted from almost anechoic (ree ection coefe cient jj rjj < 0:2) to open-end ree ection. Several axisymmetric and helical unstable modes were identie ed for fully premixed and diffusion-typecombustion. Theseunstablemodeswereassociated with e ow instabilities related to therecirculation wake-like region on the combustor axis and shear-layer instabilities at the sudden expansion (dump plane). The combustion structure associated with the different unstable modes was visualized by phase-locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, whereas the helical modes showed variations in the radial location of maximal heat release. The axisymmetric mode was the dominant one during unstable combustion. It was obtained by forcing a longitudinal low-frequency acoustic resonance. Helical modes could only be obtained when the axisymmetric mode was suppressed by using a nonree ectingboundarycondition.Aclosed-loopactivecontrolsystemwasemployedtosuppressthethermoacoustic pressure oscillations and to reduce NO x and CO emissions. Microphones were used to monitor the pressure oscillations during thecombustion process and provide input to thecontrol system. An acousticactuation was used to modulate the aire ow and thus affected the mixing process and the combustion. Upstream excitation modie ed the shear-layer structure and was shown to be superior to downstream excitation, which combined less effective shear-layer excitation with noise cancellation. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant 24% reduction of NO x emissions were obtained in premixed combustion using an acoustic power of less than 0.002% of the combustion power. The control of the diffusion e ame was less effective, and NO x emissions increased at the phase that was most effective in suppressing the pressure oscillations. The differences between the behavior of the control system in the two combustion modes was caused by different levels of interaction between the combustion process and the shear layer.