The Effect of Acoustic Boundary Conditions on the Oscillation Mode Transition of Combustion System

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Abstract The lean premixed combustion technology makes thermoacoustic oscillation possible to occur in gas turbine combustor. With the change of operating condition, the amplitude and frequency of thermoacoustic oscillation also change. The oscillation mode can even change suddenly. This mode transition is closely related to the acoustic boundary conditions of the system. This paper investigates the impact of the acoustic boundary conditions of the outlet of the combustion system on the oscillation modes. By varying the equivalence ratio, the system is made to oscillate in different modes. The acoustic reflection coefficient and impedance of the system outlet are measured through double-sound-transducer method, and numerical simulations are conducted to investigate their impact on the growth rates of the oscillation modes. The results indicate that under different equivalence ratios, the combustion system exhibits three distinct oscillation modes. As the equivalence ratio in the combustor increases, the low-frequency mode gradually disappears and even undergoes a sudden transition to a high-frequency mode. The phase of the outlet acoustic reflection coefficient and impedance of the three modes are significantly different, which can reach up to 180°. There are two different principles for the mode disappearance. One is due to the acoustic reflectivity exceeding the critical value, while the other is due to the gradual decrease of it. Numerical simulations of oscillation modes are conducted based on the acoustic impedance measured. The simulated modes with the highest growth rate agree well with the experimental results, with an error within 1.94%. This study provides experimental and numerical methods for accurately obtaining growth rates of oscillation modes. Accurate boundary acoustic impedance is crucial for predicting oscillations. The reflectivity and the phase of boundary acoustic impedance are the primary reasons for mode transitions.