| Active control of combustion instability in a lean premixed dump combustor was studied by injecting secondary gaseous fuel into selected regions of the combustor. Two strong instabilities were sustained, for which main fuel was supplied partially premixed (case I) or fully premixed (case II), respectively. The structure of unstable flames was identified by taking phase-averaged images over one period of the instability and local Rayleigh index distributions were calculated using the images and pressure data. An examination of the local Rayleigh index distribution revealed that there are regions of driving and damping in unstable flames. It was suggested that to suppress the instability, secondary heat should be added in the local damping region to maximize the effect of control.; To investigate the effect of temporal and spatial secondary fuel distribution on the control effectiveness, both dump plane injection and upstream injection were applied to each of the two different instabilities. For case I instability, dump plane injection resulted in effective control and for case 11 instability upstream injection was effective.; For the case of controlling case I instability with dump plane injection, secondary heat was added in the damping region and the instability was suppressed at an optimal phase delay. For the case of controlling case II instability, instability was not suppressed since secondary heat was added out of phase with the pressure, but did not occur in the damping region. It was found that a control strategy such that secondary heat is added out of phase with the pressure, cannot always achieve successful suppression of instability unless it is accompanied by spatially optimized secondary heat release.; From the study of mode contribution of secondary heat release to damping, it was found that even though the injection is made at one of the sub-harmonics of the instability frequency, the greatest contribution is from a harmonic mode corresponding to the instability frequency. Therefore, tailoring the temporal heat release profile so that the harmonic component corresponding to the instability frequency was maximized resulted in performance improvement as shown in the case study. |
