ACTIVE CONTROL OF UNSTEADY COMBUSTION-INDUCED OSCILLATIONS

A loudspeaker-based active control system has been used to study a longitudinal combustion instability mode and its active control in a small scale premixed combustor. A simple gain/phase-shift type controller is shown to successfully suppress oscillations in the combustor at all equivalence ratios at low flow rates. It is found, however, that as the flow rate through the combustor is increased, the performance of the controller deteriorates rapidly, both in terms of allowable gain and phase margin and reduced effect on dominant instability such that beyond certain flow rates the controller is unable to suppress the oscillations for any values of gain and phase-shift. Nyquist stability diagram obtained from the frequency response data are used to understand the performance of the controller and help design an advanced controller with somewhat improved performance. The physical model developed by Lang et al.8 is adapted to help understand important features of the combustion instability and its active control. Without the controller the model is found to predict the frequency of the dominant instability mode and its variation with equivalence ratio fairly well. For off-stoichiometric operation the model predicts an increase in the number of potential instability modes as is also observed in the measured power spectra. With the controller "on," the model shows that the controller interacts significantly with the combustion and in one case examined here, it is shown that the controller stabilizes the second longitudinal mode for one value of gain but at just a slightly higher value renders the previously stable third mode unstable. This offers plausible physical explanation for the observed limited gain and phase margin and the limited effectiveness of the controller at the higher flow rates. The model also predicts the stability boundary to have a reduced phase margin as observed experimentally.