Asymmetric deflagration characteristics of hydrogen-blended natural gas under multiscale conditions: The influence of ignition location

Hydrogen-blended natural gas (HBNG) is receiving increasing attention as a practical low-carbon gaseous fuel for the transition of natural-gas-based energy systems, but its vented deflagration behavior in partially confined spaces remains insufficiently understood, especially when ignition occurs at different locations and system scale changes. To address this issue, vented deflagration experiments were conducted in three geometrically similar chambers with internal volumes of 2809 L, 281 L, and 28 L. Six representative ignition locations were considered: chamber center (L_C), top center (L_T), bottom center (L_D), back wall (L_B), right wall (L_R), and quarter-diagonal intersection (L_CC). The results showed that ignition location strongly affected the explosion response, but its role changed markedly with scale. As the scale decreased, the internal pressure response evolved from a clearly separated multi-peak structure to a compressed single-peak composite waveform. Under the tested conditions, the 281 L chamber generated the strongest near-field external shock, with a maximum free-field peak overpressure of 53.8 kPa at 0.5 m under L_R ignition. The 2809 L chamber exhibited the most pronounced multi-peak external pressure propagation and vent competition, whereas the 28 L chamber showed highly localized near-vent external effects. Moreover, the thermal hazard zone shifted from concentration on the front and top walls at the large scale to more distributed multi-wall exposure at the small scale. These findings clarify the scale-dependent explosion behavior of HBNG and provide useful guidance for vent design and thermal-load management in enclosures and equipment associated with hydrogen use and natural gas utilization.