Spinning and azimuthally standing acoustic modes in annular combustors

Damaging combustion instabilities can occur in annular combustors used in low NO_x stationary gas turbines. Both spinning and azimuthally standing oscillations have been observed, without a real understanding of the causes of such mode selection. This paper proposes a new quantitative measure, called the spin-ratio, to describe the spinning tendency of an acoustic mode. The spin-ratio is based on an acoustic energy density formulation, in which the acoustic field in the annular combustor is described analytically as the super-position of helical waves travelling in the upstream or downstream direction, and with clockwise or counter-clockwise sense of rotation, respectively. The spin-ratio formulation is applied to study the impact of various parameters (axial mean flow, non-axisymmetric burner arrangement) on the selection of the thermoacoustic mode types in a generic annular combustor configuration. Results are analysed in terms of active control strategies.