Abstract: The existing researches mainly focus on the development of internal pressures in the confined space during hydrogen explosions, and the studies about the distribution law and evolution mechanism of external overpressure during vented hydrogen explosions are still not enough. Therefore, a commercial FLACS was used to establish the numerical model of the chamber, the vent size was increased to 3.14 m2 and the maximum hydrogen concentration was increased to 30%, then numerous simulations were conducted to study the formation of the external explosion, the distribution rule of the external overpressure and the hazard scope. The effect of factors on the external overpressures during vented hydrogen deflagrations was also analyzed. The results show that when the vent size becomes relatively large, the vented hydrogen deflagration will lead to the formation of the external combustible gas cloud with a larger volume. Then the cloud is ignited by the emerging flame to produce an external explosion, and the whole formation process of the external explosion can be effectively simulated by FLACS. When the vent size is large, the maximum overpressures at different positions exhibit monotonic behavior as hydrogen concentrations increase, but a non-monotonic behavior of the change rate of overpressure between different positions near the vent opening as a function of hydrogen concentration is found. For the different ignition locations, maximum overpressures near the vent opening are distributed in different shapes of wedge, but the distribution modes of the hazard scope are totally different, the external overpressure and hazard scope with the back ignition are larger than those with the center ignition.

Key words: fire protection; hydrogen; vented deflagration; external overpressure; distribution mode; hazard scope