Investigation on mechanism of low frequency combustion instabilities in a dump combustor

Dump combustors exhibit undesirable combustion instabilities under certain operating conditions. Experimental and numerical investigations were implemented to study low-frequency combustion instabilities. The results show that the longitudinal mode is dominant in pressure oscillations of the dump combustor whose frequency is not completely same as acoustic frequency. Including multi-steps chemical mechanisms coupling with large eddy simulation (LES), numerical simulation methods for combustion instabilities analysis are established. Large eddy simulation of low-frequency combustion instabilities is carried out for experimental combustor. The detailed forming and evolution process of combustion instabilities are obtained. It is discovered that vortices shedding from dump plane, pulse of combustion area, local equivalent ratio and heat release fluctuations during vortices motion process in combustor are responsible for the low-frequency combustion instabilities. Pressure fluctuations result in flow velocity pulse. Furthermore, vortex shedding is formed. The motion of large scale in combustor will result in large altitude pulse of heat release, and will excite pressure oscillations. The low-frequency combustion instabilities appear to be controlled by the motion of both vortices and acoustic waves.