Abstract

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 oscillating frequency. This makes the problem quite different from longitudinal instabilities, where often only one mode is unstable. Annular combustors can 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 some combustors prefer standing to spinning or viceversa, or can converge to both solutions. Existing literature can explain stable spinning modes. I will present 2 different mechanisms that can explain stable standing modes as well: 1) the effect of transverse forcing on the flames, sweeping the flames back and forth in the azimuthal direction; 2) the effect of a weak flame response at small amplitudes, and strong response at large amplitudes.

The focus will be on unveiling the fundamental physical phenomena of the problem, and on providing new mathematical tools to study thermoacoustic problems. On the way, you will be entertained with multi-stability, double-hopf bifurcations (sub/supercritical), thermoacoustic triggering.