We experimentally investigated the noise-induced dynamics of a prototypical thermoacoustic system undergoing a subcritical Hopf bifurcation to limit cycle oscillations. The study was performed prior to the bistable regime. Analysis of the characteristics of pressure oscillations in the combustor and fluctuations in the heat release rate from the flame-the two physical entities involved in thermoacoustic coupling-at increasing levels of noise indicated precursors to the Hopf bifurcation. These precursors were further identified to be a result of coherence resonance.

The advent of annular combustors introduced a new facet of flame-acoustic interaction: flame coupling with standing and spinning azimuthal acoustic waves. Such coupling involves an acoustic field that is essentially transverse to the flame. Recent experiments on single burner test rigs have provided significant insight into flame interaction with transverse standing waves. However, experiments that focus on the spinning/rotating nature of azimuthal instabilities are still lacking. In this report, we demonstrate a methodology for studying spinning azimuthal instabilities on a single burner test rig. This methodology is based on analyzing flame response to a traveling acoustic wave generated in the combustor. We generate traveling acoustic waves in our transverse acoustic forcing test-rig by converting one end of the transverse extensions to a non-reflecting end. This is achieved through the implementation of the technique of impedance tuning. In the paper, we have discussed this implementation, followed by discussions on the effects of a traveling acoustic wave on a swirl-stabilized flame. The discussion is in the form of a comparison of flame oscillations for traveling wave and standing wave transverse forcing cases. Results show that the effect of transverse pressure oscillations dominates the flame response to traveling acoustic waves.

This article is a report of experiments conducted in order to investigate the role of noise on thermoacoustic systems. In contrast to most studies in this direction, in the present work, the role of noise in the subthreshold region, prior to the (subcritical) Hopf bifurcation and the associated saddle-node bifurcation, is considered. In this regime, a thermoacoustic system is stable and does not undergo transition to self-excited thermoacoustic oscillations. However, the system can feature dynamics, which arise due to the proximity of the system to the approaching Hopf bifurcation, in response to noise. Experiments were performed on a model thermoacoustic system featuring a laminar flat flame. Noise was introduced in a controlled manner, and the effect of increasing levels of noise intensity was studied. Results presented here show that noise addition induces coherent oscillations. The induced coherence is observed to depend on the noise amplitude and the proximity to the Hopf bifurcation. Furthermore, this noise-induced behavior is characterized by a well-defined “resonance-like” response of the system: An optimum level of coherence is induced for an intermediate level of noise. These results can be of importance in practical thermoacoustic systems (e.g., combustors), which are inherently noisy due to factors such as flow turbulence and combustion noise.

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