Acoustically Driven Combustion Instabilities in Dump Combustors

Abstract

Reacting flow experiments conducted in a dump combustor with an inlet-tocombustor length ratio of greater than 10, resulted in self-sustained, combustion driven oscillations at various longitudinal acoustic mode frequencies of the setup. Under certain operating conditions, vortex-driven combustion instabilities gave rise to pressure oscillations at the 15 th harmonic. Since a natural selection of such a high harmonic over the lower ones is very unlikely, plausible physical mechanisms explaining the selective amplification of the higher modes were investigated. Analytical results show that the observed instability mode could be naturally selected under properly timed vortex shedding and vortex bursting processes if those processes took place at specific locations with respect to the shape of the acoustic mode that was eventually amplified. To explain the physics of this unlikely mode selection process, three specific steps of frequency analysis are described. The first two steps have to do with the well-known duct acoustic resonance and the Rayleigh criterion. The last step is associated with the vortex shedding and bursting locations. It is shown that in order to sustain a vortex-driven feedback mechanism, vortex shedding has to occur at or near a velocity anti-node, while vortex bursting has to occur near a pressure anti-node. Equivalently, it is shown that for such vortex driven modes in a dump combustor, a velocity anti-node has to occur near the dump plane and a pressure anti-node has to occur near the exit nozzle. When applied to the observed experimental conditions, those frequency selection steps accurately analyzed the 15 th harmonic as the main mode of oscillations.