Analysis of the local equivalence ratio under fuel-feeding excitations in a lean gas turbine model combustor: Large eddy simulation approach

The synchronization process between the fuel-feeding excitation (f_f) and the local equivalence ratio (ϕ_local) is a key issue that has not been well studied numerically under lean conditions. Therefore, in this study, a large eddy simulation (LES) with a wall-adapting local eddy-viscosity (WALE) subgrid model was utilized to analyze the interaction between various fuel-feeding excitations and the local equivalence ratio in a lean partially premixed gas turbine model combustor. The model combustor was simulated using five values of fuel-feeding excitation: 0 Hz, 15 Hz, 50 Hz, 100 Hz and 200 Hz. The results demonstrated that the complex interaction between the swirling flow and the fuel-feeding excitation affected the flow structure. These changes enhanced the excitation of toroidal counter-rotating vortices in the region between the fuel jet exit and shear layers, which improved the homogeneity of ϕ_local inside the inner recirculation zone (IRZ). Furthermore, the fuel-feeding excitation modulated the shear layer thickness by increasing the rolling-up of the instantaneous counter-rotating vortices, leading to spatial regularity for ϕ_local inside the IRZ. The coupling process between the fuel-feeding excitation and the precessing vortex core (PVC) significantly increased the fluctuations of ϕ_local around the fuel jet exit and the shear layer region. An analysis of the probability distribution showed a nonlinear relationship between f_f and ϕ_local, where ϕ_local was high when f_f = 50 Hz, enhancing the uniformity of ϕ_local inside the IRZ. However, ϕ_local was more spread radially when f_f = 0 Hz in the downstream direction. The merging of vortex shedding and the PVC increasingly excited the self-oscillation of ϕ_local, while the fuel-feeding excitation dominated the oscillation of the counter-rotating vortices by increasing the subharmonic vortices. Moreover, the fuel-feeding excitation controlled and organized the small vortical structures, leading to stabilization of ϕ_local when f_f = 50 Hz, 100 Hz, and 200 Hz.