Pulsating instability and extinction of stretched premixed flames

Based on literature analytical and computational results that Le > 1 premixed flames are susceptible to exhibit pulsating instability, and that such propensity is promoted with increasing positive stretch, experiments were conducted on counterflow twin flames of lean and rich mixtures of methane in 15% O₂/(O₂ + He) such that the mixture Lewis number uniformly exceeds unity. Experimental results on the flame luminosity and position showed that these Le > 1 flames do exhibit pulsating instability as the strain rate is increased beyond a critical value, that pulsation develops for sufficiently off-stoichiometric (φ ≤ 0.80 and φ ≥ 1.30) flames while steady burning and extinction are associated with near-stoichiometric flames, and that the amplitude of the oscillatory flames increases with increasing strain rate until the flame extinguishes. Computations with detailed chemistry and transport showed qualitative and quantitative agreement with the experimental results, including the oscillation frequency. The calculation further reveals that during the course of largeamplitude oscillation, when the instantaneous flame temperature falls below its value at the steady-state extinction limit, the flame cannot revive itself and extinction occurs.