Prediction of thermoacoustic instability in combustion chamber equipped with passive dampers

The thermoacoustic mechanism is considered one of the causes of combustion instability in modern gas turbine combustion chambers, according to the basic mechanism proposed by Rayleigh. On this basis, several authors have proposed the use of acoustic passive dampers, in order to hinder the onset of instability or, at least, to reduce the amplitude of the pressure oscillations. The passive device will produce two main actions, cooperating to stabilize the system: dissipating the acoustic energy produced by thermal fluctuations and modifying the phase between acoustic pressure and heat release in order to reduce the production of acoustic energy. The Helmholtz resonators proposed as damping devices act as acoustic impedances whose characteristics can be modified, not only by means of the manipulation of their geometrical features (volume, length and section of the neck) but also by means of holes and cooling flow. The present paper aims at describing how a resonator connected to the combustion chamber is able not only to damp the pressure oscillation but also to modify the instability conditions and, under certain conditions, to convert an unstable condition in a stable one, in a relatively large operating range. The study has been carried for a cannular type of combustion chamber by means of a one-dimensional analytical model and modelling the Helmholtz resonator by means of the electroacoustic analogy. Two different flame-acoustic interaction model have been examined. As a result of the application of the model the optimal design characteristics of the resonator and its position within the chamber are obtained. Moreover, unlike what it was expected, it appears that the optimal resonance frequency of the resonator does not exactly meet the frequency of the acoustic mode to be damped.