Feedback control of an unstable ducted flame

Active control of a naturally unstable ducted flame was realized using acoustic forcing of either the shear layer of the flame jet or the duct itself. The feedback signal was derived from either the duct pressure signal or the CH intensity (related to the flame heat release rate), and delayed in time to produce cancellation of the natural resonant oscillations of the system. Direct driving of the shear layer using the duct pressure signal feedback produced the best control with the lowest power requirements. The controller was able to reduce the acoustic power in the duct at the resonant frequency from 19 Pa to about 0.7 Pa, or nearly 30 dB. When operating in the cpntrolled mode, the driving speaker is producing a sound pressure level more than three orders of magnitude below the natural duct uncontrolled level (both measured in the duct), so the effect is clearly not just acoustic cancellation.