Analysis of transient blow-out dynamics in a swirl-stabilized combustor using large-eddy simulations

In the present work, the flame structure and dynamics of a swirl-stabilized methane/air flame near and at blow-out conditions are investigated using large-eddy simulations (LES). To this end, simulations at stable conditions and during a transient blow-out sequence are performed employing a flamelet/progress variable (FPV) model and a thickened flame (TFLES) approach with finite-rate chemistry. Good agreement is obtained for velocities, OH mass fraction profiles, and statistics between LES and measurements at conditions near blow-out. Lift-off height comparisons reveal that the TFLES model predicts a significantly less stable flame compared with the FPV model, which is consistent with the stability limits observed for the two models. Subsequently, transient blow-out simulations are performed and examined. This analysis is guided by employing early warning indicators from dynamical system analysis to identify critical transition points and precursors that trigger the onset of blow-out. It is found that the variance of the integrated heat release is a sensible quantity as an early warning signal in detecting blow-out. Detailed comparisons are then performed between the two models to examine the difference in terms of the blow-out mechanism. The comparison reveals that the different phases of the transient blow-out sequence are qualitatively similar for the two models. However, local extinction phenomena are more significant for the TFLES model, reducing the stabilizing feedback by hot combustion products and leading to a total blow-off time, which is three times shorter compared to FPV.