Large Eddy Simulation and Acoustic Analysis of Combustion Instability in an Annular Combustor with Low-Swirl Flames

Abstract Self-excited combustion instability in an annular combustor with low-swirl flames is studied with the combination of LES and acoustic solvers. Acoustic analysis with a Helmholtz solver provides an estimate of frequencies and modal structure in the annular combustor. LES gives detailed modal dynamics at specific instability modes. Combustion instabilities in the annular combustor including longitudinal, spinning and standing modes are successfully captured in single LES. Numerical results show the instability modes are not constant, they switch among these modes randomly and rapidly. The flow oscillates back and forth in phase with the largest pressure amplitude locating near the outlet of the injectors for the longitudinal mode. The azimuthal instability oscillates at the 1A2L mode of the annular system. In the spinning mode, the pressure anti-nodes move forwards while the modal structure keeps constant. For the standing mode, the locations of pressure anti-nodes are fixed in the annular combustor and the fluctuations at the pressure anti-nodes keep out of phase. The near-zero value of mean spin ratio indicates the dominant azimuthal mode is the standing mode. The azimuthal modes captured by LES are in good agreements with that predicted by Helmholtz solver in terms of frequency and modal structure. The maximum deviation of the predicted frequency is less than 5%. This adds values before putting the low-swirl injector into the actual annular combustor.