Active control in combustion systems

Over the past decade, active control has been investigated in combustion systems as a means of preventing thermoacoustic instabilities from degrading the system performance. More recently, attempts to control instability and emissions as well as to satisfy other performance criteria have also been pursued. In this article, the authors review control problems in continuous combustion processes, the need for active control, and current status of the field. Despite the success reported in experimental investigations, a systematic procedure by which active control can be designed and implemented is currently not available. In particular, a theoretical framework which includes an investigation of the resonant nature of the thermoacoustic instability, an understanding of the significance of the mixing of modes and the dynamic impact of acoustic drivers, and the incorporation of the subtle coupling among the different physical processes of heat, mass, and momentum addition, as well as acoustics, turbulence, and chemistry, has not been developed. As an example of what can be accomplished using such a theoretically based approach, in this article the authors also review the results of their efforts expended thus far in the area of active control of a premixed combustor. The authors discuss the highlights of a feedback model proposed in Annaswamy et al. (1995) which captures the dominant dynamic features of the combustor. The authors demonstrate that their model captures the effects of several interacting modes, the mean-flow and mean-heat release in the combustor, and the locations of the actuator-sensor pair, and provides guidelines for an active control design. The authors suggest a systematic procedure for designing an active controller that provides an appropriate compensating action and results in improved performance.