Effect of fuel-feeding excitation on the isothermal turbulent swirling flow structure in a lean gas turbine model combustor

Abstract

Fuel-feeding excitation (ff) is an important factor in lean combustors because it controls the combustion process and maintains a stable mixing process. In this work, the structure of turbulent swirling flow is analyzed via large eddy simulation (LES) for a lean partially premixed gas turbine model combustor. The fuel injection is artificially excited at various frequencies of 0 Hz, 15 Hz, 50 Hz, 100 Hz, and 200 Hz. The LES findings show that all fuel-feeding excitation cases have a bubble vortex breakdown as the predominant flow topology. The size of the corner recirculation zone (CRZ) is constant at ff values of 50 Hz, 100 Hz, and 200 Hz, stabilizing the swirling flow in the upstream direction. The fuel-feeding excitation increases the swirl intensity by a rate of 35% and the instantaneous counterrotating vortices spiral further, creating a modulation for the shear layers that increases the rolling-up of small vortices. A mushroom-shaped vortex is formed near the fuel jet, where its position is fixed when ff = 0 Hz and oscillates periodically toward the downstream direction when fuel-feeding excitation is applied. The mushroom-shaped vortex shrinks to a balloon-shaped vortex and merges with the fuel jet when ff = 200 Hz. For all fuel-feeding excitation cases, the precessing vortex core (PVC) considerably increases the velocity fluctuations in the axial directions by 20%. The fuel feeding excitation improves the production of turbulence near the shear layers and the fuel jet exit by rates of 30% and 20%, respectively. In addition, the fuel-feeding excitation enhances the vorticity generation by small structures with a characteristic size that highly depends on the PVC. According to the proper orthogonal decomposition (POD), the first mode has the most energy compared to the other modes due to the prevalence of the PVC and Kelvin-Helmholtz instability in the shear layer region.