Thermoacoustic Stability Chart for High-Intensity Gas Turbine Combustion Systems

Abstract

The operating range of heavy duty gas turbines that feature lean premixed combustion to achieve low NO, emissions is limited by thermoacoustic oscillations. T o extend the operational envelope of the gas turbine, passive means have to be developed to suppress thermoacoustic instabilities. In order to develop passive means the complex interaction between acoustics and thermal heat release has to be taken into account. A new stability chart applicable to the qualification of industrial design has been developed that accounts for the acoustic properties of the combustion system including its boundary conditions and the flame response data. The method has been validated using detailed measurements of the eigenmodes in an operating gas turbine as well as experimental data from component test rigs. An explanation is given of the significant extension of the operation envelope of the gas turbine a s an effect of cylindrical extensions to the burner nozzle.

An analytical model for azimuthal thermoacoustic modes in an annular chamber fed by an annular plenum

“…The second approach is to use a network discretization along the azimuthal direction [1,22]. An advantage of the first method is the capability of considering modes which are azimuthal but also involve an axial dependance, something which can not be done yet with the second approach.…”

“…PBC configurations (Plenum + N Burners + Chamber) (Fig.1 right) have been proved [1,20] to correctly reproduce the behavior of complex industrial annular combustors (Fig. 2).…”

“…Describing the unstable acoustic modes which appear in annular gas turbine combustion chambers and finding methods to control them is the topic of multiple present research activities [1,2,3,4,5,6,7,8,9]. The complexity of these phenomena and the difficulty of performing simple laboratory-scale experiments explain why progress in this field has been slow for a long time since.…”

“…Similarly, most large scale annular chambers exhibit multiple acoustic modes in the frequency range of interest (typically 10 to 30 acoustic modes can be identified in a large scale industrial chamber between 0 and 300 Hz) and classifying them into categories is the first step to control them. These categories are typically 'longitudinal vs azimuthal modes' or 'modes involving only a part of the chamber (decoupled modes) vs modes involving the whole system (coupled modes)' [1,3,20,21]. Knowing that a given unstable mode is controlled only by a certain part of the combustor is an obvious asset for any control strategy.…”

“…For example, standing and turning modes [1,24] are both observed [1,5,17,25] but predicting which mode type will appear in practice and whether they can be studied and controlled with the same method remains difficult [23]. Similarly, most large scale annular chambers exhibit multiple acoustic modes in the frequency range of interest (typically 10 to 30 acoustic modes can be identified in a large scale industrial chamber between 0 and 300 Hz) and classifying them into categories is the first step to control them.…”