Lean blowout mechanisms of centrally staged swirl premixed flames

The lean blowout (LBO) of premixed methane/air flames in a centrally staged swirl burner is investigated using simultaneous OH* chemiluminescence and stereoscopic particle image velocimetry at a repetition rate of 5 kHz. Spectral proper orthogonal decomposition is employed to identify the dominant frequencies and characterize the temporal evolution of the precessing vortex core (PVC). A spatial linear stability analysis is applied to the time-averaged flow fields to calculate the maximum growth rates of the PVC. Two types of flames with distinct shapes, namely, the trumpet-shaped outer-stage flame and the V-shaped inner-stage flame, exhibit fundamentally different LBO mechanisms. For the inner-stage-dominated flame, the approach to LBO is characterized by repeated extinction–reignition cycles, which are associated with strain-rate distribution and PVC evolution. A critically amplified local PVC mode is observed as the total equivalence ratio is reduced, which destabilizes the flow field. Intensified convective disturbances eventually disrupt the reignition process within a single PVC rotation cycle at the final blowout. Conversely, as the outer-stage-dominated flame approaches LBO, the PVC-related locally unstable region expands, and the associated flow disturbances spread throughout the flow domain. However, there is no direct PVC–flame interaction due to the absence of spatial overlap, and the abrupt blowout is accompanied by large-scale flame wrinkling and stretching. The qualitative and quantitative insights obtained from this study deepen our understanding of the LBO dynamics in centrally staged swirl flames, offering a useful reference for the design of advanced centrally staged combustors.