Thermoacoustic instability and flame transfer function in a lean direct injection model gas turbine combustor

Abstract

This paper discusses the combustion dynamic characteristics of hydrogen/methane flames and the flame response to fuel flow fluctuation in a model lean-direct injection gas turbine combustor. The results show that a higher frequency longitudinal thermoacoustic instability is triggered as hydrogen/methane ratios increase. The mode shift is accompanied by shortening flame length and hence decreasing convection time delay between fuel injection and flame location. The flame transfer function (FTF) subject to fuel flow rate modulation is determined over a range of frequencies for hydrogen/methane volume ratios of 25:75, 50:50, and 75:25 at a fixed equivalence ratio of 0.55. The input and output of the FTF are the velocity fluctuation of fuel-mixture at the inlet of the burner and the heat release fluctuation of flame in the combustion zone, respectively. Because of the difficulty of measuring fuel flow velocity at the inlet of the combustor while the flame exists, the fuel transfer function (FLTF) is measured and used along with the intermediate flame transfer function (ITF) to determine the FTF. The transfer functions are measured at frequencies up to 600 Hz and the FTF above 600 Hz is determined from the flame response at higher harmonic frequencies. Results of the FLTF measurement show that an acoustic resonance is formed within the fuel injector and the resonance frequency changes with the hydrogen/methane ratio as well as the length of the fuel feedline. The gain of FTF at a given frequency decreases as the hydrogen content increases and the absolute value of the slope of the phase plot, which is related to the convection time delay between fuel injector tip and flame location, decreases as the hydrogen enrichment ratio increases. Using the measured FTF as input to an open-source simulator, the occurrence of instability is predicted and compared with experimental results. The modeling results agree very well with the experimental data in predicting the occurrence of unstable combustion, the frequency at which unstable combustion occurs, and the acoustic mode inside the combustor.