Abrupt transitions in turbulent thermoacoustic systems

TL;DR

This paper investigates abrupt transitions to thermoacoustic instability in turbulent combustors, revealing that stochastic fluctuations and nonlinearities can induce sudden state changes, with implications for engine stability.

Contribution

The study introduces a low-order stochastic model to explain abrupt thermoacoustic transitions and analyzes the role of noise and bifurcations in these phenomena.

Findings

Abrupt transitions occur when stable limit cycles become unstable.

Stochastic noise captures combustion noise and intermittency.

High turbulence can lead to sudden state changes despite stochastic effects.

Abstract

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.