Dispersion of hydrogen in confined spaces

Abstract

Usage of hydrogen as fuel gives rise to possible accidental risks due to leakage and dispersion. The highest risk from hydrogen leak is the formation of a large volume of the hydrogen-air mixture, which could be ignited leading up to a severe explosion. Prevention and control of formation and ignition of hydrogen combustible cloud necessitate sufficient knowledge of mechanisms of the hydrogen leak, dispersion, and over-pressures generated during ignition and explosion. This study aims to investigate the momentum-controlled jet, the buoyancy-controlled wave and the parameters influence hydrogen concentration distribution in confined spaces. It demonstrates experimental results and analysis from helium and hydrogen dispersion and explosion in a channel. two groups of experiments were carried out. Firstly, a set of experiments for the release of helium and hydrogen jets in a 3 m long channel to record their concentrations in the cloud by concentration sensors at different horizontal and vertical positions. Secondly, a series of hydrogen explosionsin the channel to examine the overpressures built-up during the combustion process and the flame propagation characteristics. Two flow visualization techniques were applied by means of Backgroundoriented Schlieren and Shadowgraph along with high-speed video camera to image the mixing process around the release point, the helium-, hydrogen-air cloud shape at the middle of the channel and flame evolution after ignition of hydrogen-air mixture cloud. Moreover, results were used for comparison of helium and hydrogen concentration gradients. Experiments results show swift mixing occurs at higher flow rates, smaller nozzle sizes, and downward release direction. Higher concentration recorded in the channel with negative inclination. Results also confirmed that hydrogen/ helium behavior pattern in the channel accords with mutual intrusion theory about gravity currents. On the question of an explosion of the hydrogen-air mixture in a channel, this study found that for short release time of hydrogen, the highest overpressure was generated when the flame passed through the mixing zone around the jet. For a long time release, the highest overpressure generated after the ignition in the proximity of ignition source. This could be a realistic scenario when hydrogen accumulates after accidental leakage in a tunnel or channel and subsequent ignition of the hydrogen-air cloud.