Highlights of the Flame Acceleration in a Confined Nonuniform H2/O2/N2 Mixture

International audienceGaseous explosion models generally assume the gas mixture to be uniform. However, in a real explosion, the vapor cloud may not be homogeneous, and repartitioning of the reactivity inside the cloud can be subject to wide spatial variations. In this work, experimental tests were run to study the flame propagation and acceleration in nonuniform mixtures. Experiments were performed in a long vertical confined tube with a square cross section, composed of four equal sections. A gate valve separated the tube into two parts, and the composition of the gases was different on each side of the valve. The opening of the valve permitted the mixing of gases by molecular diffusion. For nonuniform mixtures, a mode of propagation identical to that seen in uniform mixtures was observed; however, a third phase of propagation was found, in which the flame velocity increased strongly. This increase occurred with higher hydrogen concentration in an upwardpropagating flame. A concentration gradient can appreciably modify the trajectory and acceleration of a flame. Here, however, the incidence of pressure effects remained modest, since the combustion was confined and the final pressure depended mainly on the quantity of reactants available

Fuites d’hydrogène sous pression enflammées et non enflammées : concentration, mesure de turbulence et effets de surpression

Safety studies for production and use of hydrogen reveal the importance of accurate prediction of the overpressure effects generated by delayed explosions of accidental high pressure hydrogen releases. Analysis of previous experimental work demonstrates the lack of measurements of turbulent intensities and lengthscales in the flammable envelope as well as the scarceness of accurate experimental data for explosion overpressures and flame speeds. Industry and INERIS join a collaborative project to study un-ignited and ignited high pressure releases of hydrogen. The purpose of this work is to map hydrogen flammable envelopes in terms of concentration, velocity and turbulence, and to characterize the flame behaviour and the associated overpressure.Les études de sécurité relatives aux sites de production d’hydrogène ou au système pile à combustible hydrogène montrent que dans la plupart des cas l’explosion et plus particulièrement l’explosion à la suite de l’inflammation retardée d’une fuite gazeuse sous pression donne les distances d’effet maximales. L’évaluation des conséquences de ce type d’explosion nécessite des modèles précis et validés. De nombreux travaux se sont intéressés à l’explosion d’un jet gazeux turbulent à la suite d’une inflammation retardée. Ils se focalisent principalement sur les mesures de concentration et la dépendance des effets de pression à la position du point d’inflammation. Cependant, aucun d’entre eux n’apporte de données expérimentales précises quant à la mesure de turbulence dans le nuage inflammable. En effet, l’intensité de turbulence et la taille des structures tourbillonnaires déterminent la vitesse de flamme et par conséquent les surpressions d’explosion. Pour combler ce manque, l’INERIS et deux industriels se sont associés dans le cadre d’un projet de recherche partenariale pour étudier expérimentalement les fuites d’hydrogène sous pression non enflammées et enflammées

Prédire les effets et réduire les conséquences des phénomènes accidentels

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Dynamics of vented hydrogen-air deflagrations

International audienceThe use of hydrogen as an energy carrier is a real perspective for Europe since a number of breakthroughs now enable to envision a deployment at the industrial scale. However some safety issues need to be further addressed but experimental data are still lacking especially about the explosion dynamics in realistic dimensions. A set of hydrogen-air vented explosions were thus performed in two medium scale chambers (1m3 and 10m3). Homogeneous mixtures were used (10% to 30% vol.). The explosion overpressure was measured inside the chamber and outside on the axis of the discharge from the vent. The incidence of the external explosion is clearly seen. All the results in this paper and the predictions from the standards differ greatly meaning that a significant effort is still required. It is the purpose of the French project DIMITRHY to help progressing

Mitigation of confined gas explosions using ventilation grilles and access doors

International audienceDeveloping new energies leads to installing energy production and storage systems (batteries, fuelcells, electrolysers, etc.) in containers. It is important to note that some systems have the potential torelease and accumulate flammable gases, which can create a risk of a confined explosion. To addressthis risk, a specific research program is focused on studying the explosion protection of containerisedapplications using safety vents. The program aims to optimise the explosion discharge surfaces basedon the specific scenario for the formation of an explosive atmosphere.In practice, the safety studies carried out on this type of equipment show that few applications havespecific vent panels to discharge the explosion overpressure outside to maintain the internal pressureat a level compatible with the mechanical strength of the container. However, in most cases, thesecontainers are modified to accommodate fireproof access doors and ventilation grilles to ensurenatural and forced air intake.This paper aims to present the experimental study of the ability of ventilation grilles and access doorsto act as explosion vent surfaces.Two types of equipment were tested:- Square ventilation grilles measuring 0.8 x 0.8 m2;- fireproof doors (2 m high and 0.83 m wide).All the equipment was tested at explosion overpressures of 100 and 200 mbar

Influence of the flammable cloud geometry on the gas explosion effects

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Gestion du risque H2

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Enquête sur l'accident de Buncefield : examen d'un nouveau mécanisme d'explosion

National audienceL'explosion a eu lieu le 11 décembre 2005 vers 6h30 sur le dépôt pétrolier de Buncefield. Le débordement par le toit d'un bac d'essence entraine le déversement de 180 tonnes d'essence qui parvient à s'échapper de la cuvette de rétention. Ce déversement s'accompagne de la formation d'un aérosol et d'une nappe de liquide s'évaporant sur une grande surface. L'atmosphère est très stable avec une vitesse de vent inférieure à 1 m/s. La surface au sol du nuage inflammable a été estimée à 120 000 m2, soit environ un rayon de 200 m autour du point de fuite. La hauteur estimée du nuage est de l'ordre de quelques mètres. Les témoignages, des éléments visuels (vidéo surveillance) et l'examen de la chronologie des événements suggèrent fortement que l'inflammation aurait eu lieu dans la pomperie incendie adjacente au réservoir " fuyard " au moment du démarrage des pompes. L'environnement dans lequel se développe l'explosion est relativement dégagé avec un faible taux d'encombrement comprenant les bacs d'hydrocarbures, deux rangées de taillis bordant une route, quelques bâtiments et un parking relativement vide le matin de l'explosion. Dans ce type de configuration, les experts s'attendaient à une explosion peu violente avec des effets de pression au mieux de l'ordre d'une centaine de millibars..

Propagation of a confined explosion to an external cloud

Since the pioneering work of Harrison and Eyre (1986), the existence of secondary or external explosion outside explosion vents is recognized and rather systematic. This explosion can be much more powerful (Proust, 2004, 2010) than the internal explosion particularly when the mixture is very reactive. But today, the understanding of the formation of the external cloud and its subsequent combustion remains largely outstanding. Very rapid burning was noticed and significant UVCE pressure effects. In some circumstances, a preexisting flammable cloud encompasses the vented vessel, like in Buncefield for instance. This paper presents experimental work and CFD simulations (OpenFoam) which investigate the aerodynamics of the flow and the flame propagation. The experimental device is composed of a 4 m3 chamber linked to an unconfined 54 m3 volume via a square vent. These two volumes are filled with a stiochiometric propane air mixture and ignited by a pyrotechnical match in the 4 m3 chamber. When the vent area is small enough, the vortex bubble formed by the gas ejection is disrupted and a jet is formed entraining a significant portion of the outside atmosphere. The explosion overpressure outside can be 10 time larger as compared to the fully unconfined case (no chamber)

Turbulent flame propagation in large unconfined H2/O2/N2 clouds

Turbulence is a key aspect in hydrogen explosions. Unfortunately, only limited experimental data is available and the current understanding of flame turbulence interactions is too limited to permit safe predictions. New experimental data are presented in which the flame trajectory and pressure history are interpreted for unconfined explosions of H2/O2/N2 clouds of 7 m3. The intensity of the turbulence is varied between 0 and 5 m/s and the integral scale of the turbulence is on the order of 10 cm which is at least an order of magnitude larger than lab scale