Author:
GHOSAL SANDIP,VERVISCH LUC
Abstract
In non-premixed turbulent combustion the reactive zone is localized at the
stoichiometric surfaces of the mixture and may be locally approximated by a diffusion flame.
Experiments and numerical simulations reveal a characteristic structure at the edge
of such a two-dimensional diffusion flame. This ‘triple flame’ or ‘edge flame’ consists
of a curved flame front followed by a trailing edge that constitutes the body of
the diffusion flame. Triple flames are also observed at the edge of a lifted laminar
diffusion flame near the exit of burners. The speed of propagation of the triple flame
determines such important properties as the rate of increase of the flame surface in
non-premixed combustion and the lift-off distance in lifted flames at burners. This
paper presents an approximate theory of triple flames based on an approximation
of the flame shape by a parabolic profile, for large activation energy and low but
finite heat release. The parabolic flame path approximation is a heuristic approximation
motivated by physical considerations and is independent of the large activation
energy and low heat release assumptions which are incorporated through asymptotic
expansions. Therefore, what is presented here is not a truly asymptotic theory of triple
flames, but an asymptotic solution of a model problem in which the flame shape is
assumed parabolic. Only the symmetrical flame is considered and Lewis numbers are
taken to be unity. The principal results are analytical formulas for the speed and
curvature of triple flames as a function of the upstream mixture fraction gradient in
the limit of infinitesimal heat release as well as small but finite heat release. For given
chemistry, the solution provides a complete description of the triple flame in terms of
the upstream mixture fraction gradient. The theory is validated by comparison with
numerical simulation of the primitive equations.
Publisher
Cambridge University Press (CUP)
Subject
Mechanical Engineering,Mechanics of Materials,Condensed Matter Physics
Cited by
79 articles.
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