Effect of Stretch on Local Burning Velocity of Premixed Turbulent Flames

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The effect of positive stretch on the local flame properties of turbulent propagating flames in the flamelet regime was investigated experimentally for methane-, hydrogen- and propane-air mixtures with lean and rich having nearly the same laminar burning velocity (SL0=25cm/s). The ratio of the turbulence intensity u to SL0 was varied as 1.4 and 2. A 2D laser tomography technique was used to obtain the temporal local flame configuration and movement in a constant-volume vessel. Some of the key parameters of turbulent combustion quantitatively measured are the local flame displacement velocity SF, curvature and stretch of turbulent flames. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effect of positive stretch on burning velocity. It was found that the obtained SF was distributed over a wide range with flame stretch as well as curvature, even for low turbulence (u/SL0=1.4). There also existed a good relationship between SF and the turbulent burning velocity. A quantitative relationship between Ma based on laminar flames and the SF for positive stretch and curvature of turbulent flames could be observed only for mixtures with Lewis number Le greater about one.