A mechanism of combustion instability in lean premixed gas turbine combustors

There has been increased demand in recent years for gas turbines that opera te in a lean, premixed (LP) mode of combustion in an effort to meet stringe nt emissions goals. Unfortunately detrimental combustion instabilities are often excited within the combustor when it operates under lean conditions, degrading performance and reducing combustor life. To eliminate the onset o f these instabilities and develop effective approaches for their control, t he mechanisms responsible for their occurrence must be understood. This pap er describes the results of an investigation of the mechanisms responsible for these instabilities. These studies found that combustors operating in a LP mode of combustion are highly sensitive to variations in the equivalenc e ratio (phi) of the mixture that enters the combustor. Furthermore, it was found that such phi variations can be induced by interactions of the press ure and flow oscillations with the reactant supply rates. The phi perturbat ions formed in the inlet duct (near the fuel injector) are convected by the mean flow to the combustor where they produce large amplitude heat release oscillations that drive combustor pressure oscillations. It is shown that the dominant characteristic time associated with this mechanism is the conv ective time from the point of formation of the reactive mixture at the fuel injector to the point where it is consumed at the flame. Instabilities occ ur when the ratio of this convective time and the period of the oscillation s equals a specific constant, whose magnitude depends upon the combustor de sign. Significantly, these predictions are in good agreement with available experimental data, strongly suggesting that the proposed mechanism properl y accounts for the essential physics of the problem. The predictions of thi s study also indicate, however, that simple design changes (i.e., passive c ontrol approaches) may not, in general, provide a viable means for controll ing these instabilities, due to the multiple number of modes that may be ex cited by the combustion process.