Thermoacoustic instability is a plaguing problem in the combustion chambers of aircraft and rocket propulsions systems. Thermoacoustic instability manifests itself as high amplitude pressure oscillations in combustion systems and can even result in failure of the components.
We setup experiments to study the mechanism causing high frequency oscillations pertaining to transverse mode instability i.e. screech in a model afterburner. The model afterburner features liquid fuel injection and multiple bluff-body flame holders, and simulates the high
temperature inlet conditions of a typical afterburner.
We do not observe screech under quasi-static increase in flow Reynolds number of the afterburner; on the contrary, screech onsets when the Reynolds number is increased at a higher rate. Such a phenomenon is known as rate induced tipping or R-tipping. Rate dependent transition to screech happens through bursts of high amplitude pressure
oscillations known as intermittency. Promotion of intermittency occurs with increasing rate of change of Reynolds number.
We observe a combined longitudinal-transverse mode in the region upstream of the flame holders and can result in perturbation of fuel spray. We imaged the region downstream of flame holders at high frame rates and observed that the flame in the wakes of the flame
holders roll-up periodically in an asymmetric manner between the top and bottom flame holders. We show that the spatial distribution of phasor of unsteady heat release rate, consequently Rayleigh index is dictated by the convection time of the reacting mixture to
different regions in the test section.