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In the present paper we investigate flame spread in laminar mixing layers both experimentally and numerically. First, a burner has been designed and built such that stationary triple flames can be stabilised in a coflowing stream with well defined linear concentration gradients and well defined uniform flow velocity at the inlet to the combustion chamber. The burner itself as well as first experimental results obtained with it are presented. Second, a theoretical model is formulated for analysis of triple flames in a strained mixing layer generated by directing a fuel stream and an oxidizer stream towards each other. Here attention is focused on the stagnation region where by means of a similarity formulation the three-dimensional flow can be described by only two spatial coordinates. To solve the governing equations for the limiting case in which a thermal-diffusional model results, a numerical solution procedure based on self-adaptive mesh refinement is developed. For the thermal-diffusional model, the structure of the triple flame and its propagation velocity are obtained by solving numerically the governing similarity equations for a wide range of strain rates.

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A laminar flamelet model of pre-mixed turbulent combustion is described in which a characteristic length scale L̂ y controls the flamelet surface-to-volume ratio. An analysis, based on the Bray-Moss-Libby model of turbulent combustion, leads to the conclusion that L̂ y /l is proportional to the ratio of the laminar burning velocity to the turbulence velocity u' , where l is the integral length scale of the turbulence. A fractal flame model and an analysis of experimental time series data both support this conclusion. Several different theories for the turbulent burning velocity are shown to be equivalent to each other and to be generalizations of the classical theory of Kolmogorov, Petrovski & Piskonov. A method of characteristics analysis confirms the resulting expression. This expression, containing only one disposable constant which must be of order unity, is compared with a published correlation of a large amount of experimental data. This leads to an experimental determination of the ratio of effective to true laminar burning velocities, as a function of Karlovitz number, which shows satisfactory agreement with results of strained laminar flame calculations.

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