An Analysis of Lift-Off in Laminar Diffusion Flames

Abstract

Abstract : A jet diffusion flame attached to a burner rim may lift off and become stabilized further downstream when the jet velocity is sufficiently increased. In turbulent jet diffusion flames, such liftoff has been described alternately as the result of a stabilized premixed flame base, and as the result of extinguished diffusion flamelets at the flame base. Laminar flames exhibit liftoff behavior as well, and possess a relatively simpler flame structure which may be studied to provide insight into the basic mechanism responsible for flame lift. In the present study, an asymptotic solution and a numerical solution of a reduced set of equations are used to study the lifted structure of a laminar diffusion flame associated with a fuel jet. The numerical model solves a temperature equation with a finite chemical reaction rate. The asymptotic analysis is based on a flame-sheet model in the limit of large activation energy and large Damkoehler number. While it is likely that premixing at the flame base affects the structure of a lifted flame, the current analysis suggests that local extinction of the flame at its base is a contributing mechanism to flame lift.