Optimizing Combustion Instability Suppression Using Secondary Fuel Injection

Abstract

of 150 Hz and with a root-mean-squared amplitude of 1.7%. Combustion control experiments were conducted while investigating the effects of various parameters important for controller design. Three different controllers, including a closed-loop controller at 150Hz and two open-loop controllers at 0-Hz and 150-Hz, were utilized. Also, two different actuator locations and three different amounts of secondary fuel were systematically tested. Concerning the actuator location, the instability suppression performance was comparable for both secondary fuel injection into the shear layer and into the recirculation zone. However, the shear-layer injection was deemed more desirable as the recirculation-zone injection resulted in soot emission concerns. The amount of secondary fuel was fixed at 2%, 5%, and 10% of the total fuel flow rate. With a non-feedback controller, the suppression performance was strongly linked to the secondary fuel amount, which also affected the dominant mode of instability. With the closed-loop controller, however, the performance was rather insensitive to the amount of secondary fuel, as the instabilities were effectively suppressed in all three cases. Our results showed that the closed-loop controller was the most effective among the three controllers. With the closed-loop controller, spectral amplitude of the targeted mode at 150-Hz was suppressed by more than 90%. Interestingly, under certain conditions, the suppression of the 150-Hz instability gave rise to a higher harmonic near 300-Hz. Nevertheless, the total amount of suppression was still substantial. Comparing the amplitudes of the highest spectral peaks, up to 75% reduction was obtained with the properly phased closed-loop controller.