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Thursday, July 11 • 09:00 - 09:20
UNDERSTANDING THE ROUTE TO LOCK-IN PHENOMENON IN VORTEX SHEDDING COMBUSTORS

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Vortex-driven thermoacoustic instability is an adverse phenomenon observed in a variety of combustion chambers in which hydrodynamics is dominant. Vortices are carriers of heat source as it contains unmixed cold reactants and hot products. As a vortex disintegrates, the reactants are ignited by the products due to mixing, which leads to a sudden heat release. This mechanism creates an instantaneous acoustic field that perturbs pressure and velocity fields inside the chamber. The velocity fluctuations in turn cause variations in the shedding frequency and hence, the heat release rate. A positive coupling between heat release and pressure fluctuations lead to thermoacoustic instability. During instability (large amplitude oscillations), vortex shedding frequency locks into the acoustic frequency of the chamber. Since the acoustic frequency is almost unaltered, it can be replaced by an upstream excitation to study lock-in. In this paper, we perform a numerical investigation to study the route to lock-in across a range of upstream excitation frequencies. In this regard, we used an existing low-dimensional mathematical model for vortex shedding. It is transformed to a first-order implicit map equation that relates time instances of successive vortex shedding. The map is obtained for a sinusoidal velocity excitation, parameterized by its amplitude (A) and frequency (f). We observed that, when the excitation frequency is close to the natural vortex shedding frequency, the model encounters a quasiperiodic route to lock-in. Whereas, it endures a higher periodic orbit route when the excitation frequency is far away from the natural vortex shedding frequency. We investigate the first return map to identify regions in the A-f plane, where lock-in occurs through the above two routes.

Moderators
MH

Maria Heckl

Professor, Keele University

Authors

Thursday July 11, 2019 09:00 - 09:20 EDT
Outremont 5
  T03 Aero… aircrft noise & vibr., SS03 Combust noise & thermoac