Effects of mixture fraction on edge-flame propagation speeds

The propagation speeds (U-edge) of nonpremixed edge-flames were measured as a function of stoichio-metric mixture fraction (Z(st)) and global strain rate (sigma) for several fuel/oxidant/diluent combinations using a counterflow slot-jet burner. It was found that for fuel Lewis number (Le(f)) approximate to 1 and oxidant Lewis number (Le(o)) approximate to 1 with fixed sigma, U-edge increases monotonically with increasing Z(st). In contrast, for Le(f) > 1 and Le(o) similar to 1 with fixed sigma, U-edge exhibits a minimum, typically at Z(st) approximate to 0.3, except for dimethyl ether which showed U-edge montonically decreasing with increasing Z(st). These results indicate that Z st has both chemical and Lewis number effects on nonpremixed edge-flame speeds. For Le(f) approximate to Le(o) approximate to 1, chemical effects dominate over the whole range of Z(st) whereas for Le(f) > 1 and Le(o) approximate to 1, Lewis number effects become important at low Z(st). It is shown that all observed U-edge vs. Z(st) trends are consistent with computed values of extinction strain rate (sext) of these mixtures in a 1D counterflow, thus sext serves as a simple surrogate for predicting edge-flame behavior. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.