Time Delay and Noise Coupling in Limiting Control Effectiveness in Unstable Combustors

Combustion instabilities are a significant problem encountered in low NOx, premixed combustors used for ground based or aero-applications. Current capabilities to predict the conditions under which instabilities occur are immature, often resulting in the appearance of expensive and problematic instabilities late in the development stage of an engine. Consequently, there is significant interest in development of active instability control systems that can suppress an unexpected instability. While significant progress has been made in demonstrating that active instability control is feasible, the results of control implementation are highly variable. Much work remains in understanding the factors influencing how effective control will be for a given system. This paper describes a theoretical analysis of the statistics of a self excited combustor with closed loop feedback control. In particular, influence of the controller upon regions over which the system can be stabilized is investigated, as well as the limit cycle amplitude and degree of “amplitude breathing” of the controlled combustor. Additionally, the sensitivity of these control effectiveness figures of merit to variations in controller characteristics is explored. The model predictions are consistent with the experimental observation that the same control system can have dramatically different effects upon the controlled combustor amplitude. For example, this was demonstrated by showing that the effect of the same controller is quite different for different unstable combustors with different internal time delays – this can include the controller having essentially no useful effect at all.