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Results of experimental data analysis on turbulent flame propagation in obstructed channels is presented. The data cover a wide range of mixtures: H 2 /air and H2/air/steam mixtures (from lean to rich) at normal and elevated initial temperatures (from 298 to 650K) and pressures (from 1 to 3 bars); and stoichiometric H 2 /02 mixtures diluted with N2, Ar, He, and C02 at normal initial conditions. The data set chosen covers, also, a wide range of scales exceeding two orders of magnitude. It is shown that basic flame parameters, such as mixture expansion ratio 0', Zeldovich number ß, and Lewis number Le, can be used to estimate a priori a potential for effective flame acceleration for a given mixture. The critical conditions for effective flame acceleration are suggested in the form f3 > l3*(Le, ß) on the basis of experimental correlations. Critical l3*-values are found tobe in the range from 3.5 to 4.0, for stable flames (ß(Le-I)> -2). For unstable flames (ß(Le-I)< -2), f3* is given by a function of Zeldovich number ß. The requirement of large enough mixture expansion ratio (J > l3*(Le, ß) can be used in practical applications as the necessary condition or criterion for possible development of fast combustion regimes. On this basis, limits for effective flame acceleration can be defined for hydrogen combustibles. Uncertainties in determination ofcritical f3*-values, which should be taken into account in practical applications, are discussed. 5. ZUSAMMENFASSUNG Bestimmung der Bedingungen für Flammenbeschleunigung in Wasserstoffhaitigen BrenngasenPractical applications and uncertainties 10Summary and conclusions 12References 13 NOMENCLATURE LatinA -elementary area of the flame surface;Csr -sound speed in reactants;Csp -sound speed in combustion products;d -size of orifice;Ea -effective activation energy;h -height of obstacles;L-characteristic geometrical size (e.g., obstacle spacing);Lr-integrallength scale of turbulence;Le -Lewis number;Le1-Lewis number defined by fuel diffusion coefficient in stoichiometric mixture;Leo -Lewis number defined by oxidizer diffusion coefficient in stoichiometric mixture;Mab -Markstein number defined relative to burned mixture; n1-reaction order for fuel;no -reaction order for oxidizer; R -gas constant;SL -laminar burning velocityrelative to unburned mixture;Tb -maximum flame temperature;Tu -initial mixture temperature;Un -normal burning velocity of stretched flames relative to burned mixture; UL -laminar burning velocity relative to burned mixture; a-ratio of densities of reactants and products ( expansion ratio);X-temperature diffusivity.

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