Propagation and extinction of premixed H₂-O₂-N₂ edge-flames

The effects of equivalence ratio ( phi), global strain rate ( sigma) and mixture strength on the shapes and propagation rates ( U-edge) of premixed edge-flames in H-2-O-2-N-2 mixtures were investigated experimentally using a counterflow slot-jet apparatus. Results are presented for both single (premixed gas against cold inert gas) and twin (premixed gas against premixed gas) configurations. Both advancing ( U-edge > 0) and retreating edge-flames ( U-edge < 0) were characterized. Flame images show that for lean mixtures where H(2 )is the deficient species and thus the effective Lewis number ( Le(eff)) is less than unity, the leading edge-flame has a stronger burning intensity than the trailing strained premixed flame. For mixtures with low phi (thus low Le(eff)) at near-extinction conditions corresponding to sufficiently high or low sigma, transitions from continuous to broken structures were observed due to diffusive-thermal instabilities. Such transitions enable flames to survive under conditions where extinction would have occurred without transition. The combined effects of phi, adiabatic flame temperature and sigma were found to correlate well in terms of the effect of a non-dimensional strain rate (epsilon) on a scaled propagation rate (sic). (c) 2023 The Combustion Institute. Published by Elsevier Inc. All rights reserved.