Nonlinear Interactions Between Forced and Self-Excited Acoustic Oscillations in Premixed Combustor

Abstract

This paper describes an experimental investigation of the nonlinear interactions between self-excited and driven oscillations in a premixed swirl-stabilized combustor. This interaction is studied by externally forcing oscillations at some frequency, fdrive , in an unstable combustor oscillating at a frequency, fins . Above a certain amplitude of forcing, the self-excited oscillation amplitude decreases. In some cases, the self-excited oscillations can be eliminated almost completely, but in others the effect is slight. As such, in the former case, open-loop forcing is very effective in reducing the overall acoustic power. This paper characterizes a variety of features of these oscillations, such as the quenching amplitude, its dependence upon frequency spacing between the driven and natural modes, and the frequency shift of the unstable mode. Although not understood, it is observed that the key difference between cases where open loop forcing is and is not effective in reducing the overall acoustic power, is that the instability frequency stays roughly constant and changes with forcing amplitude, respectively.