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 (S L0 =25cm/s). The ratio of the turbulence intensity u' to S L0 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 S F , 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 S F was distributed over a wide range with flame stretch as well as curvature, even for low turbulence (u'/S L0 =1.4). There also existed a good relationship between S F and the turbulent burning velocity. A quantitative relationship between Ma based on laminar flames and the S F for positive stretch and curvature of turbulent flames could be observed only for mixtures with Lewis number Le greater about one.

This experimental study is performed to investigate directly the local flame properties of turbulent propagating flames at the same weak turbulence condition (u'/S L0 =1.4), in order to clarify basically the influence of the addition of hydrogen to lean and rich methane or propane mixtures on its local burning velocity. The mixtures having nearly the same laminar burning velocity with different rates of addition of hydrogen δ H are prepared. A two-dimensional sequential laser tomography technique is used to obtain the relationship between the flame shape and the flame displacement in a constant-volume vessel. Some of the key parameters of local flame properties quantitatively measured are the local flame displacement velocity S F , 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 trends of the mean values of measured S F with respect to δ H , the total equivalence ratio Φ and fuel types corresponded well its turbulent burning velocity. The trend of the obtained Ma could explain the S F of turbulent flames only qualitatively. The local burning velocity at the part of turbulent flames with positive stretch and curvature using this Ma, S Lt , attempted to be estimated quantitatively. As a result, a quantitative relationship between the estimated S Lt and the S F at positive stretch and curvature of turbulent flames could be observed only for mixtures with Le > 1 or Ma >0.

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