Hydrogen Bubble Dispersion and Surface Bursting Behaviour

In many processes where hydrogen may be released from below a liquid surface, there has been concern regarding how such releases might ultimately disperse in an ullage space. Knowledge of the extent and persistence of any flammable volume formed is needed for hazardous area classification as well as for validation of explosion modelling or experiments. Following an initial release of hydrogen, the overall process can be subdivided into three stages (i) rise and possible break-up of a bubble in the liquid, (ii) formation and bursting of a thin gas-liquid-gas interface at the liquid surface and (iii) dispersion of the released gas. An apparatus based on a large glass sided water tank has been constructed which employs two synchronised high-speed imaging systems to record the behaviour of hydrogen bubble release and dispersion. A high-speed digital video system records the rising of the bubbles, and the formation and bursting of the gas-liquid-gas interface at the liquid surface. An additional schlieren system is used to visualise the hydrogen release as bubbles burst at the liquid surface. The bubble burst mechanism can clearly be described from the results obtained. Following the nucleation of a hole, surface tension causes the liquid film to peel back rapidly forming a ring/torus of liquid around the enlarging hole. This process lasts only a few milliseconds. Although some hydrogen can be seen to be expelled from the bubble much seems to remain in place as the film peels away. To assess the extent of the flammable plume following a bubble burst, the apparatus was modified to include an electric-arc igniter. In order to identify plumes coincident in space with the igniter, a schlieren system was built capable of recording simultaneously in two orthogonal directions. This confirmed that clouds undetected by the schlerien imaging could not be ignited with the electric arc igniter.