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

Abstract

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

Critical transitions and their early warning signals in thermoacoustic systems

“…Ananthkrishnan et al [34] has explained another route to large-amplitude oscillations as a primary Hopf bifurcation followed by a secondary fold bifurcation. Such a secondary bifurcation has been observed experimentally in a laminar system by Mukherjee et al [35] and in a turbulent system by Singh et al [36].…”

Abrupt transitions in turbulent thermoacoustic systems

“…The power of the combustor based on the fuel flow rate varies between 39 and 79 kW. Please refer to [16,17] for more information on the annular combustor setup.…”

“…Ananthkrishnan et al [13,14] hypothesized the possibility of a secondary bifurcation from an initially stable primary limit cycle to a large amplitude secondary limit cycle solution in thermoacoustic systems having higher-order nonlinearities. Secondary bifurcation was then experimentally confirmed in laminar [15] and, very recently, in turbulent [16][17][18] thermoacoustic systems.…”

“…The occurrence of both abrupt and continuous transitions in thermoacoustic systems makes apparent the significant challenge in their modeling. In addition, the observation of abrupt secondary transition [16][17][18] in highly turbulent thermoacoustic systems is not understood clearly. Specifically, the explanation of what makes a transition continuous and another abrupt has been found lacking in the literature.…”