Control of combustion instability with a tunable Helmholtz resonator

Unwanted combustion instability occurs in many propulsion systems, such as rocket motors, and aero-engines. It is typically caused by a coupling between acoustic waves and unsteady heat release from unsteady combustion. Combustion instability can be eliminated by using acoustic dampers, such as Helmholtz resonators (HRs), which introduce extra acoustic damping into the propulsion system. However, such acoustic dampers are generally only effective over a narrow frequency range. In this work, the damping effect of a Helmholtz resonator is optimized by tuning its neck area in real-time to stabilize an unstable combustion system over a broad frequency range. This is based on the fact that the resonator damping performance is maximized at resonance and varying its geometry can lead to its resonant frequency changes. Numerical investigation of a combustion system with a tunable Helmholtz resonator attached is conducted first. To validate the numerical results, experimental measurements are then performed on a Rijke tube with a neck-area tunable Helmholtz resonator. It is found that the unstable Rijke system is successfully stabilized by reducing the sound pressure level by more than 50 dB. Finally, off-design assessment of the tunable resonator damping performance is conducted by varying the dominant oscillation frequency by 24%. It is shown that the tunable resonator is capable to stabilize the combustion system again. This confirms that tunable Helmholtz resonators have great potential to be applied in practical combustion systems.