Experimental and Analytical Study of Hydrogen Jet Fire in a Vented Enclosure

An experimental and numerical study of hydrogen jet fire in a confined space was performed forhydrogen safety purposes within the European HyIndoor project (www.hyindoor.eu). An existence of two combustion regimes was numerically found and then experimentally confirmed. Depending on hydrogen mass flow rate, volume of the enclosure and vent area a well-ventilated or under-ventilated jet fire may occur. A chamber of 1x1x1 m3 with upper and lower vent positions, vent areas from 1 to 90 cm2 and different hydrogen mass flow rates from 0.027 to 1.087 g/s were used for numerical simulations and experimental validation. A lower axial position of a jet fire produced by immediate ignition of a hydrogen leak was established in the tests. A Background Oriented Schlieren (BOS) technique combined with high speed camera, pressure and temperature measurements were utilized in the tests to evaluate dynamics of the combustion process. In case of small hydrogen release rate and large vent area, a relatively stable well-ventilated regime leading to over-pressure not more than 0.8 mbar and a maximum internal temperature of 540 C was established. In case of very high hydrogen mass flow rate and relatively small vent sizes three different scenario of under-ventilated jet fire behaviour with self-extinction, re-ignition and external flame modes leading to very high overpressure of 10-100 mbar and maximum temperatures of 1000-1200 C were experimentally measured. Strong influence of steam condensation on under-ventilated jet fire behaviour results in reduced sub-atmospheric pressures inside the chamber and intensive air ingress into the chamber. It may result in re-ignition of the quenched flame and then again to the extinction.