Mechanistic and Statistical Characterization of DDT in Hydrogen–Air Explosions Through Cylindrical Obstacles at Atmospheric Conditions

Deflagration-to-detonation transition (DDT) in hydrogen – air mixtures poses critical safety challenges in hydrogen technologies. The influence of geometry and mixture reactivity on the probability of DDT remains difficult to quantify, as the process is governed by inherently stochastic, localized interactions. This study aims to systematically investigate the mechanisms and conditions leading to DDT by analyzing hydrogen – air flame acceleration in a 1 m open-ended channel obstructed by cylindrical elements (77% blockage ratio) under atmospheric conditions. A total of 200 premixed tests (ϕ=0.8–1.6; H 2 = 25–40 vol. %) were conducted with four pressure transducers and three high-speed cameras capturing overpressure histories and flame-front dynamics. Flame propagation follows a three-zone progression. While most flames accelerate through the obstacle field to supersonic speeds, detonation occurs in only a subset of experiments, highlighting that supersonic shock – flame coupling alone cannot trigger DDT and requires specific local interactions. Diagnostics reveal that DDT occurs within a narrow reactive window (ϕ=1.0–1.4), peaking at 48% probability at ϕ=1.1. Statistical analysis confirms that only downstream sensors reliably detect DDT events, while upstream sensors offer modest early indicators. High-speed imaging captures a multi-stage, stochastic DDT mechanism driven by localized hotspots, wall-driven secondary ignitions, and shock reflections that merge into a planar detonation front.