“Rumble” is a self-excited combustion instability, usually occurring at the start-up of aero-engines with fuel-spray atomizers at sub-idle and idle conditions, and exhibiting low limit frequencies in the range of 50 Hz to 150 Hz. Entropy waves at the (nearly) choked combustor outlet are supposed to be the key feedback-mechanism for the observed self-excited pressure oscillations. The experimental study presented here aims to clarify the role of the entropy waves for the occurrence of rumble. A generic air-blast atomizer with a design being prone to self-excitation has been incorporated into a thermoacoustic combustor test rig with variable outlet conditions. The flame thermoacoustics were characterized by recording the OH*-chemiluminescence, the dynamic pressures, the dynamic temperatures, and by applying PIV. The measurements have shown the occurrence of periodic hot spots travelling with the mean flow with considerable dispersion. Measurements have been conducted with an open-ended resonance tube in order to eliminate the impact of entropy waves on the mechanism of self-excitation. The oscillation obtained, comparable in amplitude and frequency, proved that self-excitation primarily depends on convective time delays of the droplets in the primary zone and thus on the atomization characteristics of the nozzle.

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