THERMOACOUSTIC OSCILLATIONS IN COMBUSTION-CHAMBERS OF GAS-TURBINES

This paper presents an overview of thermoacoustic oscillations in modern gas turbine combustors with premixed combustion. In conventional combustors a substantial percentage of air enters downstream of the primary zone of combustion. As a consequence, the liners of conventional combustors are very powerful sound attenuators. In a modern premixing combustor effectively all of the air enters through the burners, and there is almost no sound attenuation downstream of the primary zone of combustion. The combination of this problem with the difficulties to stabilize premixed flames has led to a situation where thermoacoustic stability has become the key issue of modern combustion technology. The relative importance of a variety of self-excited oscillations and oscillations that are forced by aerodynamic instabilities Is investigated. It is found that the two leading excitation mechanisms are associated with periodic (lean) extinction and with vortex rollup in the primary zone of combustion. For the latter case a simple combustor model is used to study the amplification effects due to resonant wave motion and coincidence of mechanical eigenfrequencies of combustor walls with excitation frequencies.