Propagation and extinction of premixed edge-flames

Abstract

The propagation rates (Uedge) of edge-flames in premixed hydrocarbon-oxygen-inert mixtures were measured as a function of global strain rate (σ), mixture strength, and (by changing fuel and inert type) Lewis number (Le). Using a counterflow slot-jet burner with electrical heaters at both ends to anchor the flame edges, both advancing (positive Uedge) and retreating (negative Uedge) edge-flames were characterized. Results are presented for both twin (premixed gas against premixed gas) and single (premixed gas against cold inert gas) edge-flames in terms of the effects of a non-dimensional strain rate (ε) and non-dimensional heat loss (κ) on a scaled propagation rate. Uedge showed a strong dependence on Le and flames images show that high (low) Le lead to weaker (stronger) edge-flame burning intensity. Uedge for single edge-flames scaled with the square root of the unburned to burned gas density ratio in a manner similar to nonpremixed flames whereas for premixed flames Uedge scaled linearly with density ratio. Edge-flames exhibited two extinction limits corresponding to a high-σ strain induced limit and a low-σ heat loss induced limit; a simple description of the low-σ limits was proposed and found to correlate well with experiments in twin-premixed, single-premixed, and nonpremixed edge-flames over more than a two-decade range of κ. Results are in good qualitative and reasonable quantitative agreement with simple theories except that retreating twin edge-flames in high-Le mixtures are predicted theoretically but were not observed experimentally.