Role of low-bandwidth open-loop control of combustion instability using a high-momentum air jet—mechanistic details

Low-frequency modulation of a high-momentum air jet is shown to be effective in controlling combustion oscillations in a swirl-stabilized spray combustor. The oscillations in the combustor studied are dominated by a longitudinal acoustic mode (235 Hz) with a self-excited low-frequency (13 Hz) bulk mode (of the upstream cavity) superimposed. In this paper, the mechanisms leading to control using external low-frequency modulation of a high-momentum air jet are studied. It is shown that low-frequency modulation periodically alters the phase relation between pressure and heat release, and for unstable conditions, this periodic alteration of the phase leads to a reduction in pressure oscillations.