Investigation of thermoacoustic oscillation attenuation by modified Helmholtz dampers with dual frequency bands

Abstract

Two modified Helmholtz dampers with multi-adjustable frequency bands are proposed to overcome the drawback of Helmholtz dampers and Quarter-lambda tubes by which multi-irregular eigenfrequencies in gas turbine combustors cannot be simultaneously treated via one interface. Theoretical impedance models of these dampers are established and experimentally validated in an impedance tube. Enhanced thermoacoustic instability attenuation by modified dampers is validated in a Rijke tube. A Helmholtz-based method is combined with damper impedance models to predict the limit cycle behavior of a Rijke tube equipped with dampers. Results indicate that the impedance models can effectively calculate reflection coefficients of modified and traditional dampers with cooling purge air. The amount of the secondary tube influences the reflection coefficients of modified dampers. Experimental results indicate that amplitudes of the overall pressure fluctuation and each order fluctuation at the limit cycle are further attenuated by modified dampers compared with those by traditional ones. Larger purge flow rates for dampers lead to more pulsation attenuation caused by vortex shedding dissipation. The numerical method can predict the eigenfrequencies, velocity fluctuation ratios, and mode shapes at the limit cycle after using different types of dampers. When activating modified dampers, weaker fluctuations are successfully captured by this numerical approach.