Rate-ratio asymptotic analysis of the structure and extinction of partially premixed flames

Rate-ratio asymptotic analysis is carried out to elucidate the structure and mechanisms of extinction of laminar, partially premixed methane flames. A reduced chemical-kinetic mechanism made up of four global steps is used. The counterflow configuration is employed. This configuration considers a flame established between a stream of premixed fuel-rich mixture of methane (CH4), oxygen (O-2), and nitrogen (N-2) and a stream of fuel-lean mixture of CH4, O-2, and N-2. The levels of premixing are given by the equivalence ratios phi(r) of the fuel-rich mixture and phi(1) of the fuel-lean mixture. The value of phi(r) depends on the mass fraction of oxygen, Y-O2,Y-r, in the fuel-rich mixture, and the value of phi(1) depends on the mass fraction of fuel, Y-F,Y-1 in the fuel-lean mixture. The mass fraction of the reactants at the boundaries are so chosen that the diffusive flux of reactants entering the reaction zone is the same for all values of phi(r) and phi(1). The analysis shows that the value of the scalar dissipation rate at extinction increases with increasing Y-F,Y-1 for Y-O2,Y-r = 0, while it decreases with increasing Y-O2,Y-r for Y-F,Y-1 = 0. The results of the analysis agree with numerical calculations carried out using a chemical-kinetic mechanism made up of elementary reactions. They also agree with experimental data. A key finding is that at fixed values of the scalar dissipation rate, the leakage of oxygen from the reaction zone decreases with increasing Y-F,Y-1. As a consequence the flame temperature increases. This makes the flame more resistant to strain. At fixed values of the scalar dissipation rate, the leakage of oxygen from the reaction zone increases with increasing values of Y-O2,Y-r. This makes the flame less resistant to strain. (C) 2006 Published by Elsevier Inc. on behalf of The Combustion Institute.