Combustion Instabilities in Industrial Gas Turbines — Measurements on Operating Plant and Thermoacoustic Modelling

Abstract

Measurements of vibration and combustion chamber dynamic pressures have been taken on a number of 150MW industrial gas turbines operating on pre-mixed natural gas, both during long periods of base-load operation and during short duration load-swings. The data has been analysed in terms of the frequency and bandwidth of the principle peak in the vibration and pressure spectra as a function of load and other operating parameters. It is observed that bandwidth, which is a measure of the damping of the resonant mode of the combustion chamber’s acoustic resonance, decreases towards zero as the machines approach their combustion stability limits. A theoretical model of the thermoacoustic behaviour of the combustion system has been developed to see to what extent the observed behaviour on the operational machines can be explained in terms of an acoustic model of the ductwork and a flame characterised simply by a time-delay. This time delay is obtained from the frequency response function of the flame in response to unsteady perturbations in inlet velocity and is calculated using computational fluid dynamics. The model has also been used to illustrate the importance of fuel supply system design in controlling combustion stability. It 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 characteristics.

The thermo-acoustic response of a premixed swirl burner

“…While the convection-related time delay concept has been used in many thermo-acoustic calculations (see, for example, reference [8]) and is strongly endorsed by the present measurements, there is little real understanding of the nature of the convected disturbance. Lieuwen and Zinn [7] and Dowling and Hubbard [10] specifically Fig.…”

“…This must be the goal for more complex systems, and the mathematical framework has been set out in papers by Lieuwen and Zinn [7], Hobson et al [8], Abbott [9] and Dowling and Hubbard [10]. Sivasegaram and Whitelaw [11] have noted the complex effects that swirl can have on the regimes of excitation.…”

Low computational cost CFD analysis of thermoacoustic oscillations

“…The space dependence of pressure field oscillations is obtained from the characteristics of the acoustic modes involved, whereas the space dependence of the heat release oscillations is strictly related to the shape of the flame front and to the portions of the injected fuel that are oxidated in the different regions of it. The latter aspect in particular complicates the flame frequency response function and results in a distribution of time lags rather than a single value τ, as highlighted by many authors (see, e.g., [10,11]).…”

“…• numerical models with a network of acoustic elements characterized by transfer matrices [10][11][12][13][14][15]. These models use radical assumptions on flow and combustion in order to simulate the flow with its fluid dynamic losses and to obtain the frequencies and the growth rates of the acoustic modes of interest (the complexity of which may vary from one-dimensional longitudinal modes up to fully three-dimensional modes).…”

Vibro-accoustical instabilities induced by combustion dynamics in gas turbine combustors

“…and are the perturbations per second of the mean values. The main sources of the instabilities in a lean premixed turbulence flame are fluctuations in the equivalence ratio, see (Hobson, Fackrell, & Hewitt, 2000). The mass flow of air exceeds several times the mass flow of fuel during lean combustion, a small perturbation in the fuel flow makes a significant impact on the flame stability, where at the same time the total mass flow of the mixture is hardly perturbed.…”