Extinction of partially premixed flames

Experimental and numerical studies on extinction of laminar partially premised flames are carried out. The studies are performed in the counterflow configuration. A premixed fuel-rich mixture of methane (CH4), oxygen (O-2), and nitrogen (N-2) is injected from one duct while a fuel-lean mixture of CH4, O-2, and N-2 is injected from the other duct. The levels of partial premixing are given by the equivalence ratios phi(r) of the fuel-rich mixture and phi(1) of the fuel-lean mixture. Here phi = Y-F/(nuY(O2)), where Y-F and Y-O2 are, respectively, the mass fractions of fuel and oxygen at the injection planes (exit of the ducts), and nu is the stoichiometric mass ratio of fuel to oxygen. Previous studies have established that the scalar dissipation rate at extinction depends on the stoichiometric mixture fraction, xi(st), and the adiabatic temperature, T-st. To clarify the influence of partial premixing on extinction, studies are carried out at fixed values of xi(st) and T-st. For phi(r)(-1) = 0, experiments show that the value of the strain rate at extinction, a(q), increases with increasing phi(1). For phi(1) = 0, experiments show that the value of the strain rate at extinction, a(q), decreases with increasing phi(r)(-1). Numerical calculations are carried out using a detailed chemical-kinetic mechanism and an one-step chemical-kinetic mechanism. The values of a(q) calculated using the detailed mechanism are found to agree well with experiments. The values of a(q) calculated using the one-step mechanism do not agree with experiments. In fact, calculations with the one-step mechanism show the values of a(q) to decrease with increasing phi(1), and increase with increasing phi(r)(-1). The differences in critical conditions of extinction obtained using the detailed mechanism and the one-step mechanism are attributed to differences in flame structure.