Noise-Induced Dynamics in the Subthreshold Region in Thermoacoustic Systems

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.