A Model-Based Active Control Design for Thermoacoustic Instability

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Abstract
Active control has been pursued vigorously for combating thermoacoustic instabilities in combustion processes. Most experimental investigations employ empirical design procedure for determining the characteristic parameters of the filter and phase-shifter of the controller. Such procedure has been observed to result in resonance at frequencies which were not excited in the power spectrum of the uncontrolled combustor, though the dominant thermoacoustic instability was suppressed. In this paper, we present an alternative design methodology which is based on the underlying physical model of the combustor and modern control theory. We show that using this methodology, one can avoid the generation of secondary peaks and achieve short settling time using small control energy. The physical model takes into account multiple acoustic modes, the heat release dynamics of a premixed flame, and the effect of an actuator such as a loudspeaker, on the flow variables over a wide range of frequencies. The model-based control strategy consists of optimizing a quadratic cost function of the pressure response and the control input, and leads to a straight forward selection of the parameters of the active controller. The resulting controller is compared to those obtained using empirical designs and is shown to modulate the gain-phase characteristics of the combustor over a wide range or frequencies thereby leading to a better performance and no secondary resonances.