Excitation of thermoacoustic instabilities by interaction of acoustics and unstable swirling flow

Unstable thermoacoustic modes were investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. The acoustic boundary conditions of the exhaust system could be adjusted from almost anechoic (reflection coefficient \r\ < 0.2) to open-end reflection. Several axisymmetric and helical unstable modes were identified for fully premixed and diffusion-type combustion. These unstable modes were associated with Row instabilities related to the recirculation 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 nonreflecting boundary condition. A closed-loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx and CO emissions. Microphones were used to monitor the pressure oscillations during the combustion process and provide input to the control system. An acoustic actuation was used to modulate the airflow and thus affected the mixing process and the combustion. Upstream excitation modified 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 NOx emissions were obtained in premixed combustion using an acoustic power of less than 0.002% of the combustion power. The control of the diffusion flame was less effective, and NOx 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.