Control of Acoustically Coupled Combustion Instabilities

Abstract

AbstractThis chapter reviews control methods for combustion instability. It specifically considers the rather general situation where instabilities are coupled by acoustic modes. The chapter begins with fundamental acoustic principles for reactive flows, leading to the acoustic energy balance equation. This equation is used to describe the three main control strategies that can be envisaged for combustion instabilities: (i) passive control methods (PCM); (ii) flame dynamic control methods (FDCM); (iii) active control methods (ACM). These schemes are considered successively. The survey begins with passive control methods (PCM) in which the dissipation of acoustic energy is augmented byad hocsystems (acoustic liners, Helmholtz resonators, quarter wave cavities, and various other devices), which enhance absorption of acoustic energy. It is shown that such devices can be effectively used to modify the impedance of a boundary and that this can change the resonance properties of the combustor. Control of injection and flame dynamics is then envisaged. For this second group of methods (FDCM) it is shown that the flame geometry has a profound effect on the combustion response to incoming perturbations. The modification of the flow and flame configurations can thus be used to control the system response. It is then indicated that the dynamics of the injection system can be designed to decouple the system from the manifold feeding reactants into the combustor. Recent work indicates that the flame dynamics can be modified so that the combustion region becomes less susceptible to perturbations impinging on the flame. Finally, active control methods (ACM) are considered, and their application to combustion instability reduction is described. Implementation in the form of a closed loop control aims at modifying the feedback mechanism to hinder the resonant coupling between unsteady combustion dynamics and system acoustics. ACMs have been developed using various strategies, ranging from the control of injection parameters (fuel flow, air flow, secondary jets, etc.) to the active modulation of acoustic boundary conditions up‐ and downstream of the combustion chamber. Under application of the active control system, instabilities can be totally suppressed and the instability‐related flame extinctions are eliminated.