Combustion instabilities in industrial gas turbines - Measurements on operating plant and thermoacoustic modeling

Measurements of vibration and combustion chamber dynamic pressures have bee n taken on a number of 150 MW industrial gas turbines operating on pre-mixe d natural gas, both during long periods of base-load operation and during s hort duration load-swings. The data has been analyzed in terms of the frequ ency and bandwidth of the principle peak in the vibration and pressure spec tra as a function of bad and other operating parameters. It is observed tha t bandwidth, which is a measure of the damping of the resonant mode of the combustion chamber's acoustic resonance, decreases towards zero as the mach ines approach their combustion stability limits. A theoretical model of the thermoacoustic behavior of the combustion system has been developed to see to what extent the observed behavior on the operational machines can be ex plained in terms of an acoustic model of the ductwork and a flame character ized simply by a time-delay. This rime delay is obtained from the frequency response function of the flame in response to unsteady perturbations in in let velocity and is calculated using computational fluid dynamics. The mode l has also been used to illustrate the importance of fuel supply system des ign in controlling combustion stability II is shown that stability can be a strong function of the acoustic impedance of the fuel supply and that this can lead to enhanced or reduced stability depending on the flame character istics.