Effect of equivalence ratio on the modal dynamics of azimuthal combustion instabilities

The present paper investigates the effect of equivalence ratio on the modal dynamics of self-excited azimuthal combustion instabilities in a laboratory-scale annular combustor. It is shown that operating at different equivalence ratios not only affects the mode of oscillation (whether the instabilities are mixed, or predominantly spinning or standing modes), but also the way in which mode switching between these states occurs. Using pressure time-series data obtained at multiple locations around the annulus the phenomenon of mode switching is investigated through the spin ratio, the orientation of the nodal lines and the envelope of the pressure oscillations. These three parameters all suggest that mode switching events occur almost periodically, and over azimuthal convective time scales. Moreover, analysis of the spin ratio and orientation of the nodal lines show that these parameters are correlated, and that their mean rates of change or trajectories also have a preferred direction. Therefore, these quantities were found to oscillate backwards and forwards in-phase with each other, as opposed to the mode simply rotating in one direction, providing new insight into the nature of modal dynamics of self-excited azimuthal modes.