Forced Nonlinear Acoustic Damping in a Rijke Tube and its Application to Combustion Instability Control

Abstract

Nonlinear unsteady energy transport theory predicts that when multiple coupled oscillatory acoustic modes are unstable, only one of these modes may dominate. This phenomenon produces unusual experimental results in propulsion systems as small changes may alter the stability of many modes, thereby altering their interactions as well. These nonlinear interactions create instances where unstable modes appear to switch, where, in fact, multiple modes are unstable. Understanding this phenomenon is critical in diagnosing combustion instability and in developing damping techniques in unstable systems. In order to verify this predicted phenomenon a forced Rijke tube experiment was performed. This experiment shows that the first mode instability caused by the Rijke tube is eliminated when higher modes are driven. This effect only occurs near the natural frequencies of the Rijke tube and is likely not the result of changes in the Rijke tube operation itself. The positive results of this experiment have applications in the development of new innovative combustion instability damping techniques in propulsion systems as well as validating the analytical methods applied. With this process, a fundamentally new way of damping is presented as it is possible to eliminate low frequency oscillations by either destabilizing higher frequency modes or by directly driving those higher modes.