Propagation rates of nonpremixed edge flames

The propagation rates (U_edge) of nonpremixed ignition (advancing) fronts and extinction (retreating) edge flames were measured as a function of global strain rate (σ), jet spacing (d), mixture strength, stoichiometric mixture fraction (Z_st), and Lewis number (Le) using a counterflow slot-jet burner in which edge flames propagated along its long dimension. Electrical heaters at both ends of the slot "anchored" the flames, allowing conditions resulting in negative values of U_edge to be studied by triggering local extinction of anchored flames with an N₂ jet. Results are presented in terms of the effects of a dimensionless flame thickness (ε) related to σ and a dimensionless heat loss (κ) on a scaled U_edge. Propagation rates were markedly enhanced/retarded in mixtures with low/high Le. Propagation rates and extinction conditions were highly asymmetric with respect to Z_st = 0.5 despite the symmetry of flame location; mixtures with Z_st greater/less than 0.5 behaved like stronger/weaker mixtures, apparently due to the relative locations of the radical production zone and maximum temperature zone for varying Z_st. Two extinction limits were identified, corresponding to a high-σ strain-induced limit that was strongly dependent on Le but nearly independent of κ and a low-σ heat-loss-induced limit that was strongly dependent on κ but not Le. Most experimental findings were in good agreement with theoretical predictions; however, unlike predictions, "tailless" triple flames were not observed; instead standard triple flames and "short length" edge flames were found, and only for a very narrow range of experimental conditions. This is proposed to be due to the difference between the volumetric heat loss presumed in the models and the conductive transfer to the jet exits that dominates heat loss in the experiments.