Experimental and numerical studies on hydrogen leakage and dispersion in underground parking garages: Impact of leakage direction on safety considerations

•Upward leakage significantly increases the likelihood of hydrogen accumulation, resulting in higher concentrations of hydrogen near the roof of the leak location and its surrounding areas within the garage.•Hydrogen diffusion through the roof preferentially spreads from the leakage location to the four immediately adjacent areas.•The volume of hydrogen concentration surpassing the lower explosion limit of hydrogen (18.3 %) is particularly elevated in the case of vertical downward leakage, attributed to the accumulation of hydrogen beneath the vehicle.•Horizontal leakage emerges as the least severe among the three leakage directions, resulting in the smallest overrun volume. Hydrogen leakage in confined spaces poses significant safety risks in hydrogen energy applications, potentially leading to fires and explosions. This paper focuses on the accident scene of hydrogen leakage and dispersion in underground parking garages. A medium-scale model (1/10) of an underground parking garage is designed and built to study the characteristics of the dispersion of hydrogen leaked from hydrogen fuel cell vehicles (HFCVs) in underground garages using experimental and numerical simulation methods. This paper focused on analyzing the dispersion characteristics of hydrogen leaked from hydrogen fuel cell vehicles (HFCVs) within these environments, with particular attention given to the influence of leakage direction—upward, horizontal, and downward—on hydrogen concentration distribution over space and time. For instance, it was observed that under equivalent flow rates, upward leakage tends to result in higher hydrogen concentrations at the leakage site. Conversely, downward leakage promotes wider hydrogen diffusion around the source, particularly evident at higher flow rates. Horizontal leakage, comparatively, exhibits lower risk due to greater air volume absorption. Complementing the experimental data, numerical simulations revealed consistent patterns in hydrogen diffusion. Specifically, the simulations illustrated a cyclic accumulation-diffusion-accumulation process. Notably, vertical upward leakage demonstrated the largest volume of regions exceeding a 4 % hydrogen volume fraction, whereas vertical downward leakage resulted in the greatest volume of regions surpassing an 18.3 % hydrogen volume fraction. This disparity arises from hydrogen accumulation beneath the vehicle in cases of vertical downward leakage. Conversely, regions of high concentration induced by horizo