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Abstract The utilization of centrally-staged lean-premixed swirl combustion is prevalent due to its capacity for stable flame maintenance and reduced emissions. The configuration of the pilot and main flames, whether they are stratified or merged, significantly influences flame behavior and combustion dynamics. This study explores the underlying mechanisms of merged and stratified flames in a newly designed dual-swirl model combustor. Utilizing a 10 kHz PIV/OH* synchronized measurement and a 20 kHz CH2O PLIF method, we delineate the flame and flow structures under varying Air Split Ratios (ASR) and Stratification Ratios (SR). The results indicate that ASR alters the heat release intensity by modifying the flame’s radial dimensions, whereas SR determines the spatial separation between the pilot and main flames. The merged flame displays a single-layer M-shaped structure, with the inner flame front perturbed by vigorous vortex shedding. In contrast, the stratified flame exhibits a dual-layer structure with an M-shaped pilot flame and a V-shaped main flame, where a precessing vortex core (PVC) emerges whose frequency correlates positively with the equivalence ratio of the pilot flame. The formation of these two modes is attributed to the stabilization of the outer branch of the pilot flame, governed by the interplay between flame propagation and flow convection of the pilot stage.
Li et al. (Mon,) studied this question.