Combustion instability mechanisms in a lean premixed gas turbine combustor

An experimental study was conducted to identify the combustion instability mechanisms of flame-vortex interactions and equivalence ratio fluctuations and to characterize the combined effects of the two mechanisms on self-excited unstable combustion in a swirl-stabilized lean-premixed gas turbine combustor. The combustor was designed so that the fuel injector location and the combustion chamber length could be independently varied. In addition, the fuel and air could be mixed upstream of the choked inlet to the combustor, thereby eliminating the possibility of equivalence ratio fluctuations. Experiments were performed over a broad range of operating conditions and at each condition both the combustor length and the fuel injection location were varied. Dynamic pressure in the combustor, acoustic pressure and velocity in the mixing section, and the overall rate of heat release were simultaneously measured at all operating conditions, and two-dimensional flame images were taken to visualize stable and unstable flame structures.;Two distinct instability regimes were observed, one near 220 Hz and the other near 340 Hz. It was found that the lower frequency regime is closely related to the maximum gain frequency of the flame at forced response measurements, while the high frequency regime is related to the quarter-wave frequency of the mixing section. It was also found that the strength of the instability changed significantly as the fuel injection location was varied, while the phase of the acoustic pressure and velocity fluctuations in the mixing section did not change. A time series of pressure and CH* chemiluminescence signals suggested the constructive and/or destructive coupling of the two mechanisms.;Experimental data on the premixed cases were analyzed to identify the combustion instability mechanism due to flame-vortex interactions, and the time lag and the characteristic frequency analyses were found to be useful. Based on the fact that the time lag only describes the phase relationship between parameters, the characteristic frequency analysis was introduced to account for the system gain as the other factor for the instability. The maximum gain frequency in the forced flame response measurement (fmax.gain ≈ 220 Hz) and the quarter-wave frequency of the mixing section (fmix ≈ 340 Hz) were found to be characteristic frequencies for the combustor configuration. As a result, two conditions were identified as the requirements for the observed instability due to the flame-vortex interactions: (i) the time lag should be satisfied: Rayleigh's criterion and Ctn ≈ 1, and (ii) the acoustic frequency of the combustor, facs, should be close to either fmax.gain or fmix. Spectrograms drawn from the measured combustor pressure signals for varying combustor length have confirmed that the proposed instability mechanism of the flame-vortex interaction is valid.;In the analysis of the combustion instability mechanism of equivalence ratio fluctuations, a new way of assessing the equivalence ratio fluctuation convection time in the combustor, tau4(t), was successfully used to explain the instability characteristics for varying fuel injection locations at a given operating condition. In self-excited combustion instability of lean premixed flames, the resultant effect of equivalence ratio fluctuations at the flame was highly dependent on the velocity oscillations at the dump plane. The comprehensive experimental results for premixed and partially premixed cases confirmed that the flame-vortex interactions interact with the equivalence ratio fluctuations, and both of them contribute to driving the instability characteristics in partially premixed conditions.