Criterion for vortex acoustic lock-on in combustors with backward facing step

Abstract

In thermoacoustic systems vortex separation and breakdown are an important source of heat release rate fluctuations. The coupling between the acoustic field and the energy released by vortex breakdown can cause instability in these systems under certain operating conditions. Non-linear simulations show that combustion instability occurs after vortex acoustic lock-on. The objective of this work is to determine the criterion for vortex acoustic lock-on and thus the criterion for combustion instability as well. The 0-to-peak amplitude of the velocity fluctuations in the system are used as a criterion to determine the transition of Helmholtz number from vortex shedding mode to acoustic mode. Helmholtz number and acoustic pressure fluctuations computed using the model are compared with experimental investigations. The phenomenological model shows a good agreement with the experiments and the criterion determined for vortex lock-on provides a better understanding of the phenomenon.