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10-01-14 14:15 Age: 4 years

By:  Giulio Ghirardo (University of Cambridge)

Standing and spinning acoustic waves in annular combustors


Combustors are prone to thermoacoustic instabilities, resulting from the coupling between acoustic modes and flame response. In the case of axisymmetric annular combustors, the excited acoustic modes are often of azimuthal type, with two azimuthal modes becoming linearly unstable at the same time, sharing the same growth-rate and oscillation frequency. This makes the problem quite different from longitudinal instabilities, where often only one mode is unstable. Annular rigs often saturate nonlinearly to a spinning acoustic wave, rotating in the chamber in clockwise/anticlockwise direction, or a standing wave pattern, with fixed pressure and velocity nodes at certain azimuthal positions. I’ve been investigating why and how some combustors prefer standing to spinning or vice versa, or are happy to stay on both solutions. I will cover: 1) in a qualitative fashion, the effect of transverse forcing on the flames; 2) the effect of nonlinear saturation of the flame response with amplitude, as modelled by a flame describing function. On the way, you will be entertained with multi-stability, double-hopf bifurcations, averaging methods and triggering.


10-01-14 14:15 Age: 4 years

By:  Giulio Ghirardo (University of Cambridge)

Standing and spinning acoustic waves in annular combustors


Combustors are prone to thermoacoustic instabilities, resulting from the coupling between acoustic modes and flame response. In the case of axisymmetric annular combustors, the excited acoustic modes are often of azimuthal type, with two azimuthal modes becoming linearly unstable at the same time, sharing the same growth-rate and oscillation frequency. This makes the problem quite different from longitudinal instabilities, where often only one mode is unstable. Annular rigs often saturate nonlinearly to a spinning acoustic wave, rotating in the chamber in clockwise/anticlockwise direction, or a standing wave pattern, with fixed pressure and velocity nodes at certain azimuthal positions. I’ve been investigating why and how some combustors prefer standing to spinning or vice versa, or are happy to stay on both solutions. I will cover: 1) in a qualitative fashion, the effect of transverse forcing on the flames; 2) the effect of nonlinear saturation of the flame response with amplitude, as modelled by a flame describing function. On the way, you will be entertained with multi-stability, double-hopf bifurcations, averaging methods and triggering.


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