Role of Coherent Structures in Acoustic Combustion Control

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. Several axisymmetric and helical unstable modes were identified for fully premixed and diffusion flame combustion. These unstable modes were associated with flow 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, while the helical modes showed variations in the radial location of maximal heat release. Closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission detection sensors were utilized to monitor the combustion process and provide input to the control system. An acoustic actuation was utilized to modulate the airflow and thus affecting the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOx emissions were obtained using an acoustic power of less than 0.002% of the combustion power. At the optimal control conditions it was shown that the major effect of the control system was to reduce the coherence of the vortical structures which gave rise to the thermoacoustic instability.