Passive control of combustion instability through an acoustic valve

Abstract

Thermo-acoustic instability results in large amplitude oscillations in pressure and heat release. These are a potential hazard to both aero-engines and ground-based gas turbines, which operate in fuel-lean combustion. Unsteady heat release generates sound waves that can resonate with the unsteady flame dynamics. Active control can be used to suppress instability, but engineers invariably favour passive solutions with simple designs. One passive design technique involves the addition of helmholtz resonators to absorb the sound energy at a few chosen frequencies, but the resonance frequency can shift significantly with the engine working conditions. A semi-active tuning of resonator geometry may be necessary and that complicates the design. In this study, we investigate the effectiveness of an acoustic valve that relieves the sound pressure like a small opening and its performance is not strongly frequency dependent. The proposed valve may be constructed as a side-branch helmholtz resonator with flexible cavity walls that lets out part of the sound wave but not the fluid flow. Eigen-value solution demonstrates that the addition of such a valve alters the fundamental standing wave structure inside the combustion chamber, and unstable operation conditions may be eliminated for arbitrary linear flame properties provided that a valve of reasonable size can be established.