Extinction of non-equidiffusive premixed flames with oscillating strain rates

Extinction of premixed flames under non-uniform, unsteady strain is a phenomenon commonly observed in turbulent combustors. To assess the role of inequity in thermal and mass diffusion, represented by a global Lewis number (Le) - defined as the ratio of the mixture's thermal diffusivity to the mass diffusivity of the deficient species, on such extinctions, we present a study of counterflow twin-flames with various Le under oscillating strain rates. Experimental results confirm that for low mean strain rates, the amplitude of strain rate oscillation required for extinction is so large that the flow temporarily alters its direction, leading to distortion of the counterflow flow-field, destabilization, and eventual extinction of the flame irrespective of Lewis number. However, for relatively large mean strain rates, extinction results from flame-response to the peak instantaneous strain rate. For Le>1 mixtures, the maximum strain rate that the flames can sustain is greater than the steady state extinction values, while Le≤1 flames extinguish at a maximum strain rate, approximately equal to the steady state strain condition. This distinctively disparate behaviors of extinctions depending on the Lewis number are analyzed and delineated using numerical simulations of unsteady flames.