Extinction mechanisms of near-limit premixed flames and extended limits of flammability

The response of near-flammability-limit, weakly burning, counterflow premixed flames as a function of stretch rate has been studied computationally with detailed chemistry and transport properties. The limit mechanisms and extinction phenomena are found to be strongly influenced by the combined effects of flame stretch, mixture non-equidiffusion, and radiative loss. For subunity Lewis number flames such as those of lean methane/air, the combined effects of mixture non-equidiffusion and positive stretch elevate the combustion intensity such that steady burning persists beyond the fundamental flammability limit defined for the one-dimensional planar flame. Furthermore, the flame response to stretch rate variations exhibits a dual-extinction, turning-point behavior in that flame extinction occurs not only for sufficiently large stretch rates and minimal radiative heat loss, but also for sufficiently small stretch rates and relatively substantial heat loss. Consequently, for a given mixture strength, steady combustion is possible only within a finite range of the stretch rate. This range steadily diminishes with decreasing mixture strength such that there exists a critical equivalence ratio, the extended flammability limit, beyond which steady burning for the stretch-enhanced flame also ceases to be possible. For lean propane/air flames, however, the mixture Lewis numbers are greater than unity such that the combined stretch and non-equidiffusion effects always diminish the burning intensity. Consequently, the transition from stretch-dominated extinction to loss-dominated extinction is monotonic in terms of the equivalence ratio, and the fundamental flammability limit is the proper flammability limit. The present results agree well with the recent experimental observations of near-limit, lean methane/air and propane/air flames, obtained under microgravity conditions needed to eliminate the influence of buoyancy, which could severely affect the response of these weakly burning flames. Implications of the present understanding on the experimental determination of flammability limits using the counterflow flame technique are also discussed.