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This paper presents key effects of gliding arc discharges on the stability limits of externally perturbed methane/air swirl flames. The low-frequency flow disturbance mimicking the transient conditions in practical combustor was generated by a flow pulsation system with repetition of ∼5 Hz. The synchronized OH planar laser-induced fluorescence and particle imaging velocimetry (OH-PLIF/PIV) measurements were utilized to visualize the instantaneous flow and flame structures. We find that, with gliding arc discharges, the lean blowout limits in perturbed flames are dramatically extended by up to 60%, which is more pronounced than in non-perturbed flames. OH-PLIF/PIV results show that, the stabilization effect can be explained by the expansion of the inner recirculation zone, wherein the opening angle increases from 52° to 62° due to the plasma enhancement. Time-resolved OH-PLIF images illustrate that, in the presence of gliding arc discharges, the flame response to the flow disturbance consists of three stages, including enhancement, extinction, and re-ignition processes. Furthermore, proper orthogonal decomposition (POD) analysis of OH-PLIF images reveals a considerable increase of mean energy contents and suppression of turbulent fluctuations. It suggests that the enhancement of gliding arc discharges is not a simple superposition of the energy deposit, but a strong non-linear coupling between the plasma and the turbulent flame with closed-loop feedbacks. [ABSTRACT FROM AUTHOR]

This paper investigates the extinction characteristics of premixed stagnation flames (PSFs) with controlled heat losses and flow disturbances. The low-frequency air flow pulsations that imitate the operational transients in practical combustors were specially introduced. The tunable diode-laser absorption spectroscopy (TDLAS) measurement was applied to obtain the temperature profile and wall heat flux. It is found that, for steady flame with a fixed equivalence ratio, the extinction stretch rate dramatically increases as the wall heat flux decreases. The extinction criterion is summarized as a global Karlovitz number of 0.57 by establishing a relationship between the global and local stretch rates. Numerical simulations reveal that the local extinction Karlovitz number of steady PSFs is approximately 1.0 regardless of the conditions such as heat flux and equivalence ratio. Further experiments present that the air pulsations with a repetition of ∼5 Hz significantly deteriorate the flame stability. Particularly, for unsteady perturbed flames, the extinction stretch rate exhibits a nonlinear trend, yielding two regimes with discrepant sensitivities to wall heat flux. The unsteady simulation then highlights a local stretch rate overshoot in the presence of pulsation. It is caused by the time delay between the inlet velocity and flame front movement that eventually leads to poor flame stability. Moreover, in the high heat-flux regime, a smaller local stretch rate overshoot results in the weak dependence of extinction limits on heat fluxes. [ABSTRACT FROM AUTHOR]