Thermoacoustic instability is a plaguing problem in the combustion chambers of aircraft and rocket propulsion systems. Thermoacoustic instability manifests itself as high amplitude pressure oscillations in combustion systems and can even result in failure of the components. Traditionally, linear stability analysis was used to obtain the linear growth rates and identify the regions of thermoacoustic instability in the operating regimes of combustion systems. The observation of non-linear behaviour such as limit cycle oscillations, intermittency, triggering, sub-critical Hopf bifurcation, etc. required non-linear analysis of thermoacoustic instability. Recently, thermoacoustic instability is shown to exhibit behaviours of complex systems with tools from synchronization, complex networks, self- organization, etc.
We set up 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 obtained the operating conditions and geometric arrangements under which the model afterburner exhibited screeching combustion. Under quasi-static increase in flow Reynolds number of the afterburner, screech is not observed; 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 observed in the model afterburner suggests that the combustion system of a gas turbine engine should be subjected to different engine throttling rates to define the thermoacoustic stability map.