Structural Transition of a Nonpremixed Flame to a Chemistry Controlled Premixed Flame Regime

Two regimes, that is, the transport controlled and chemistry controlled regimes of a nonpremixed flame are analyzed using large activation energy asymptotics adopting counterflow as a model problem. Results show that for high fuel loading, a distinct transition of the flame characteristics occurs such that as the stretch increases, burning velocity in the diffusion regime increases up to the maximum burning velocity which corresponds to that of a stoichiometric premixture of the fuel and oxidizer from the freestream. After the transition, the flame remains as its maximum burning velocity showing the premixed flame characteristics until the flame is extinguished, i.e. in the premixed regime. While for low fuel loading, the flame retains its diffusion flame characteristics all along up to the flame extinction. These characteristics agree well with the existing experimental data. Explicit transition criterion is also identified from which the concept of apparent flame strength can be clarified.