Abstract :

The talk is based on recent investigations conducted in order to investigate the role of noise on thermoacoustic coupling—a phenomenon which affects combustors employed for rocket engines, furnaces, and gas turbine combustors. In contrast to most studies in this direction, this work looks at the role of noise in the subthreshold region, prior to the (subcritical) Hopf bifurcation and the associated saddle-node bifurcation. In this regime, a thermoacoustic system is stable and does not undergo transition to self-excited thermoacoustic oscillations. However, in response to noise, the system can feature dynamics which arise due to the proximity of the system to the approaching Hopf bifurcation. 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 shed light on a fundamental aspect of the dynamics of combustors which must be taken into account in the modeling and prediction of stability boundaries. Additionally, results from the numerical analysis of a representation model for thermoacoustic instability in combustors have been obtained. These, when compared to the experiments, provide further details of the phenomenon governing noise-induced response in combustors. Reference: Aditya Saurabh, Lipika Kabiraj, Richard Steinert, Christian Oliver Paschereit, Noise-Induced Dynamics in the Subthreshold Region in Thermoacoustic Systems, J. Eng. Gas Turbines Power 139(3),031508.