Coherent structures in swirling flows and their role in acoustic combustion control

Interaction between flow instabilities and acoustic resonant modes and thei r effect on heat release were investigated and controlled in an experimenta l low-emission swirl stabilized combustor. Acoustic boundary conditions of the combustor were modified to excite combustion instability at various axi symmetric and helical unstable modes in a fully premixed combustion. The co mbustion unstable modes were related to flow instabilities in the recircula ting wakelike region on the combustor axis and the separating shear layer a t the sudden expansion (dump plane). Flow field measurements were performed in a water tunnel using a simulated combustor configuration. The water tun nel tests demonstrated the existence of several modes of flow instabilities in a highly swirling flow, modes which were shown to affect the combustion process. Mean and turbulent characteristics of the internal and external s wirling shear layers were measured and unstable flow modes were identified. Instability modes during combustion were visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of t he 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 a nd to reduce NOx emissions. Microphone and OH emission detection sensors mo nitored the combustion process and provided input to the control system. An acoustic source modulated the airflow and thus affected the mixing process and the combustion. Effective suppression of the pressure oscillations and the concomitant reduction of NOx emissions were associated with a reduced coherence of the flow structures which excited the thermoacoustic instabili ty. (C) 1999 American Institute of Physics. [S1070-6631(99)01109-5].