Structure and control of thermoacoustic instabilities in a gas-turbine combustor

Thermoacoustic instability was investigated and controlled in an experiment al low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. Several axisymme tric and helical unstable modes were identified for fully premixed and part ially premixed/diffusion combustion. These unstable modes were associated w ith flow instabilities related to the recirculation region on the combustor axis and shear layer instabilities at the sudden expansion (dump plane). T he spatial locations of the intense combustion regions associated with the different unstable modes were visualized by phase locked images of OH chemi luminescence. The axisymmetric mode showed large variation of the heat rele ase during one cycle, while the helical modes showed variations in the radi al location of maximal heat release. A closed loop active control system wa s employed to suppress the thermoacoustic pressure oscillations and to redu ce NOx emissions. Microphone and OH emission sensors were utilized to monit or the combustion process and provide input to the control system. Acoustic actuation was utilized to modulate the air flow to affect the mixing proce ss and the combustion. Suppression levels of up to 5 dB in the pressure osc illations and a concomitant reduction of NO, emissions were obtained using an acoustic power of less than 0.002% of the combustion power. The micropho ne based controller was slightly more efficient than the OH-based controlle r. This was due to the reduced coherence of the combusting, large-scale str uctures which resulted in a deterioration of the OH signal when the control ler became effective.