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. Although, in this regime, a thermoacoustic system is stable and does not undergo transition to self-excited thermoacoustic oscillations, the system can feature dynamics that 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 can be quite significant, and is dependent on the noise amplitude and 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. For practical thermoacoustic systems (e.g. combustors), which are inherently noisy due to factors such as flow turbulence and combustion noise, these results can have important implications.