A steady, one-dimensional pemixed flame propagating in a duct with varying crosssectional area with external heat recirculation, is analyzed using activation energy asymptotics. Heat recirculation is achieved by transferring heat through a tube wall within a given distance L. The external heat transfer results in globally external heat loss and excess enthalpy burning (which is globally adiabatic), respectively, to the system with increasing wall temperature. The influences of external heat recirculation on the flammability limit and extinction of a premixed flame are examined with three parameters, namely, the amount of external heat transfer, the flame stretch and the external heat transfer coefficient.
