Adaptive control of combustion oscillations

Self-excited oscillations of a ducted flame, burning in the wake of a bluff-body flame-holder, are considered. The non-linear kinematic model used here to describe these oscillations calculates the influence of the velocity fluctuations on the shape of the flame surface, and hence on the heat release rate. It is easy and quick to vary parameters in our model, and therefore a theoretical and numerical adaptive control strategy is developed, aiming at eliminating the flame instabilities. The control is based on an IIR filter, its coefficients being optimised by the LMS algorithm. Some system identification (SI) is necessary because the output of the adaptive filter does not interfere directly with the combustion oscillations, but only through a transfer function. The efficiency of an on-line SI is demonstrated, and compared with offline SI. Our control strategy gives very satisfactory results, over a wide range of combustion regimes, provided that an adequate starting condition for the filter coefficients is chosen. This appropriate initial guess is justified by analysing the influence of the controller on the flame in the frequency domain. Finally, the influence of noise is investigated: the adaptive controller is successful at eliminating combustion oscillations in the presence of substantial noise