Abrupt transitions to the state of thermoacoustic instability (TAI) in gas turbine
combustors are a significant challenge plaguing the development of next-generation
low-emission aircraft and power generation engines. In this paper, we present the
observation of abrupt transition in three disparate turbulent thermoacoustic systems: an
annular combustor, a swirl-stabilized combustor, and a preheated bluff-body stabilized
combustor. Using a low-order stochastic thermoacoustic model, we show that the
reported abrupt transitions occur when an initially stable, supercritical limit cycle
becomes unstable, leading to a secondary bifurcation to a large amplitude limit cycle
solution. The states of combustion noise and intermittency observed in these turbulent
combustors are well captured by the additive stochastic noise in the model. Through
amplitude reduction, we analyze the underlying potential functions affecting the
stability of the observed dynamical states. Finally, we make use of the Fokker-Planck
equation, educing the effect of stochastic fluctuations on subcritical and secondary
bifurcation. We conclude that a high enough intensity of stochastic fluctuations
which transforms a subcritical bifurcation into an intermittency-facilitated continuous
transition may have little effect on the abrupt nature of secondary bifurcation. Our
findings imply the high likelihood of abrupt transitions in turbulent combustors
possessing higher-order nonlinearities where turbulence intensities are disproportionate
to the large amplitude limit cycle solution.