Flow Divergence Effects in Strained Laminar Flamelets for Premixed Turbulent Combustion

Abstract

Abstract–:Modelling of turbulent premixed flames by means of the laminar flamelet approximation leads to a requirement for information about the behaviour of strained laminar flames. Such information is frequently obtained by extensive laminar flame calculations involving detailed treatment of chemical reaction and molecular transport processes. The strain is imposed by means of a counterflow geometry in which streams of fresh reactants and hot, burnt gas meet causing transverse out flow of the combustion products. In the present analysis the effects of this outflow are shown to require careful examination. A formulation is proposed which recovers the desired one-dimensional nature of an equivalent flame sheet in terms of the flow field while retaining the effects of straining on the flame structure. The definition of a reaction progress variable is clarified and a well-defined flame sheet location within the laminar flame structure emerges naturally from the analysis. Numerical results are obtained for a stoichiometric methane-air flame which demonstrate the validity of the approach and shed new light on the interpretation of laminar flame data. The analysis is shown to extend naturally to strained flames with curvature. Finally the implications of these results for turbulent flame modelling are briefly considered.

keywords:

• Premixed
• Laminar Flamelets
• Thrbulent Combustion