FLOW DIVERGENCE EFFECTS IN STRAINED LAMINAR FLAMELETS FOR PREMIXED TURBULENT COMBUSTION

Modelling of turbulent premixed flames by means of the laminar flamele t approximation leads to a requirement for information about the behav iour of strained laminar flames. Such information is frequently obtain ed by extensive laminar flame calculations involving detailed treatmen t of chemical reaction and molecular transport processes. The strain i s imposed by means of a counterflow geometry in which streams of fresh reactants and hot, burnt gas meet causing transverse outflow of the c ombustion products. in the present analysis the effects of this outflo w are shown to require careful examination. A formulation is proposed which recovers the desired one-dimensional nature of an equivalent fla me sheet in terms of the flow field while retaining the effects of str aining on the flame structure. The definition of a reaction progress v ariable is clarified and a well-defined flame sheet location within th e laminar flame structure emerges naturally from the analysis. Numeric al results are obtained for a stoichiometric methane-air flame which d emonstrate the validity of the approach and shed new light on the inte rpretation of laminar flame data. The analysis is shown to extend natu rally to strained flames with curvature. Finally the implications of t hese results for turbulent flame modelling are briefly considered.