大規模ガス爆発における水素 / 空気混合気中の球状伝ぱ火炎の加速現象

抄録

In the present study, the acceleration phenomena of expanding spherical hydrogen/air flames during large-scale unconfined hydrogen explosions have been investigated experimentally. Large-scale experiments, in which the hydrogen/air mixture of a prescribed concentration was filled in a plastic tent of thin vinyl sheet of 1 or 27 m³ and ignited by an electric spark at the center, at atmospheric pressure have been conducted. The propagation behaviors of expanding spherical flames were recorded by using an infrared photography and a visible photography. Results demonstrate the flame wrinkling on the flame surface progressively developed owing to the flame instabilities and the flame front area increased and thereby the speed also accelerated. The critical flame radius r_c associated with the onset of the self-acceleration due to the diffusional-thermal and hydrodynamic instabilities was evaluated by the plotting the measured flame speed versus different flame stretch rate and the critical Peclet number associated with the onset of the self-acceleration, Pe_c = r_c/δ (δ: Flame thickness), increases with the mixture equivalence ratio. Such acceleration has been evaluated using fitting formula associated with the fractal theory. Results demonstrate that the onset of the flame instabilities, when the flame radius is small, thus occurs and thereby the flame propagates with self-acceleration for Pe > Pe_c. Experimental acceleration exponent α increased with the flame propagation and eventually the power law exponent reached the limited values associated with the fractal dimension. Such results illustrate that the self-acceleration (α > 1) and the self-similarity (α=constant) regimes during the flame propagation definitely existed.