We experimentally study thermoacoustic transitions in an annular combustor consisting of sixteen premixed and swirl-stabilized turbulent flames. We show the changes in the characteristics of the bifurcations leading to the state of longitudinal thermoacoustic instability (TAI) when equivalence ratio and bulk velocity are systematically varied. Depending upon the bulk velocities, we observe different states of combustor operation when the bulk velocity is fixed and the equivalence ratio is varied. These states include combustion noise, intermittency, low-amplitude thermoacoustic instability, mixed-mode oscillations (MMO), and high-amplitude thermoacoustic instability. We replicate these varied dynamics of the annular combustor in a phenomenological model containing higher-order nonlinearities. In addition, we examine the special case of mixed-mode oscillations that are encountered during the transition from low-amplitude TAI to high-amplitude TAI. We model the variation in the amplitude of oscillations between low and high values observed during mixed-mode oscillations (MMO) by coupling the acoustic velocity fluctuations with low-frequency periodic oscillations in the model. We also discuss the global and local flame dynamics observed during the state of mixed-mode oscillations (MMO). We find that during epochs of low-amplitude oscillations of mixed-mode oscillations (MMO), all the flames are partially synchronized, while during epochs of high-amplitude oscillations, all the flames are perfectly synchronized.


Intermittency, Secondary Bifurcation and Mixed Mode Oscillations in a Swirl-Stabilized Annular Combustor: Experiments and Modeling

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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