Influence of In-Duct Blockage Ratio on Hydrogen–Air Explosion Behaviour Inside a Vented Duct and in the External Space

Vented hydrogen explosions in ducts can transfer flame and pressure loads to external spaces, yet how internal obstructions regulate this external hazard remains insufficiently understood. Here, a three-dimensional FLACS model was used to examine hydrogen–air explosions with in-duct blockage ratios of 0–66.8%. Results show that the internal peak overpressure and flame propagation speed increased with increasing blockage ratio. At a blockage ratio of 66.8%, the internal peak overpressure exceeded 1.75 bar, more than twice that of the unobstructed case. In contrast, the external peak overpressure decreased markedly, with a reduction of approximately 79% at 66.8% blockage. The simulated results are consistent with stronger velocity shear and a less compact released hydrogen-cloud front at high blockage, which coincided with a weaker external second explosion. The findings support comparative hazard assessment of vented narrow hydrogen-handling spaces under the tested fuel-rich condition.