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High-pressure hydrogen storage is currently the mainstream way of hydrogen storage. However, the risk of spontaneous ignition and subsequent jet flame are serious obstacles to the wide application of hydrogen energy. To further reveal the interaction of shock waves, vortices and flame induced by the spontaneous ignition of high-pressure hydrogen release under the effect of release process and burst pressure, a numerical study is conducted by employing LES, RNG, EDC models and detailed hydrogen/air combustion mechanism. The qualitative and quantitative comparison of flow field and flame splitting process outside the tube between this numerical study and previous experimental results validate the numerical results. It is found that an initial vortex is formed at the edge of the jet. Then, several vortices are generated at the edge of the jet and ahead of the Mach disk. The number of vortices in the outer side of the barrel shock is larger when the opening time is shorter. After moving out of the tube, the flame front becomes hemisphere and the lateral edge has a hemispherical bulge which is induced by the initial vortex. The temperature is low and the hydrogen concentration is high in the high-speed region which is induced by the reflected shock wave, leading to the splitting of the flame into two parts: the first and the second half of the flame. However, the flame cannot be formed at the initial stage when the opening time increases to 150 μs and the flame area is larger than with smaller opening times. After splitting, under the affection of the vortices generated at the lateral side, the first half of the flame rotates backward and gradually merges with the second half of the flame, finally forming a complete hydrogen jet flame. [ABSTRACT FROM AUTHOR]

Once fires occur unexpectedly at the oil-hydrogen integrated refueling stations, there is a high likelihood that they will escalate into a coupled fire, resulting in unpredictable casualties and property losses. In this work, experiments on coupled fire induced by hydrogen jet flame and a heptane pool flame were conducted with different nozzle diameters and release pressures. It has been found that there is a critical diameter above which the hydrogen jet can be ignited instantly. This critical diameter is determined by the release pressure and the location of the pool flame. The heat release rate is greater than that of the single pool flame, and it reaches its highest point at 0.2 MPa. Compared to a single fire, the height of the flame decreases while the length of the flame increases, a prediction model is proposed for the dimensionless flame length and width, taking into account the ratio of the heat release rate of the hydrogen jet flame to the heat release rate of the pool flame. The temperature of the coupled fire reaches its highest point at 0.2 MPa and increases as the nozzle diameter. And it decreases with the increase in the distance when Y= 0 m. However, it increases with the increase of the distance when Y= 0.05 m due to the inclination of the flame. [ABSTRACT FROM AUTHOR]

• Shock wave is reflected twice inside the tubes with different angles. • Two shock reflection mechanisms for acute and non-acute tubes are proposed. • It requires a longer time and a further distance to initiate the spontaneous ignition inside the tubes with larger angle. • The flame induced by the spontaneous ignition could move upstream steadily inside the 60° angle tube. High-pressure hydrogen storage is one of the best choices of hydrogen storage at present and in the future. However, the spontaneous ignition of high-pressure hydrogen release blocks its wide application. This paper further studies the shock evolution, temperature accumulation and spontaneous ignition inside the tubes with different angles by employing LES, RNG and EDC models, 10-step like opening process of burst disk and detailed hydrogen/air combustion mechanism. Four tubes with different angles (60, 90, 120 and 150°) are employed based on the previous experimental result. It is found that the smaller the angle of the tube, the greater the maximum pressure and temperature of the first shock reflection zone. And the maximum pressure and temperature of the second shock reflection zone is the highest inside the 90° angle tube. This is due to two shock reflection mechanisms inside the tubes: in acute angle tube, only the reflected hemispherical shock hits the inner wall of the tube corner, however, for non-acute tubes, both the reflected hemispherical shock and reflected normal shock hit the inner wall of the tube corner. In addition, it requires a longer time and a further distance to initiate the spontaneous ignition inside the tubes with larger angle. And the flame induced by the spontaneous ignition could move upstream steadily inside the 60° angle tube and the flame length increases with the release time to a maximum value of 21.44 mm. [ABSTRACT FROM AUTHOR]

Hydrogen is regarded as one of the promising fuels in the next decades. However, the spontaneous ignition of high-pressure hydrogen blocks its safe utilization. In this investigation, a visualization investigation is carried out to study on the characteristic and mechanism of spontaneous ignition condition of high-pressure hydrogen during its sudden release into a tube. A rectangular tube with transparent observation window is employed. The high-speed images of spontaneous ignition conditions and light signals emitted from ignition are obtained. Four spontaneous ignition conditions are observed: (1) no ignition; (2) failed ignition with one ignition spot; (3) failed ignition with two ignition spots and (4) successful ignition. The critical burst pressure for four ignition condition is determined. The jet fire (successful ignition) could be generated in the cases with a shorter distance from ignition spot to leading shock wave and contact surface due to the stronger shock wave affection and sufficient flammable mixture maintaining the chemical reaction and flame propagation. The second ignition is the precondition forming the successful ignition. The scaled ignition delay time and distance could be predicted by using the derived dimensionless shock pressure. • Four ignition conditions are captured during the release of high-pressure hydrogen. • Critical burst pressures of these four ignition conditions are proposed. • Mechanism of failed and successful ignition of high-pressure hydrogen release is reveled. • The scaled ignition delay time and distance could be predicted using the dimensionless shock pressure P∗. [ABSTRACT FROM AUTHOR]

Cryogenic storage option is one of the important ways of storing hydrogen, but under-expanded ignited releases of cryogenic hydrogen could easily lead to jet fires, causing serious damage to people and properties. Barrier walls are important means to reduce the severity of cryogenic hydrogen jet flames, however, its effect on the cryogenic hydrogen jet flame and related temperature distribution are still unclear. In this work, the effect of the distance of barrier wall (0.20, 0.25, 0.30, 0.35 and 0.40 m) and release temperature (140, 180, 220, 260 and 300 K) on the hydrogen jet flame width and temperature distribution is investigated experimentally. It is found that the flame width in the horizontal and vertical direction and the temperature on the vertical barrier wall increase with the decrease of the distance to the nozzle and release temperature. The normalized flame width and inverse excess maximum temperature on the barrier wall increases linearly with the scaled distance considering nozzle diameter and release temperature. The temperature distribution is normalized as Gaussian distribution both in the horizontal and vertical direction, while the excess temperature is higher upward than that of downward. The results can provide a theoretical basis and data support for the blocking of jet fires formed by cryogenic hydrogen ignited releases. [ABSTRACT FROM AUTHOR]

• The maximum hydrogen concentration increases as the distance of the vertical barrier wall decreases. • The inverse maximum hydrogen concentration increases linearly with the scaled distance. • The hydrogen concentration in the horizontal and vertical direction increases as the distance of vertical barrier wall decreases. • The hydrogen concentration in the horizontal and vertical direction follow Gaussian and exponential distribution, respectively. As a recognized clean energy source with advantages of non-pollution, high energy density and less energy loss during the cooling process, cryogenic hydrogen is widely used worldwide. The barrier wall is an effective means of preventing the backward diffusion of jets. However, the effect of distance of vertical barrier wall to the release nozzle on the hydrogen concentration profile is still unknown. In this work, cryogenic hydrogen is released with different distances of the barrier wall (0.20, 0.25, 0.30, 0.35, 0.40 m) and temperatures of cryogenic hydrogen (140, 180, 220, 260, 300 K) experimentally. 10 hydrogen concentration sensors are placed in the horizontal and vertical directions on the barrier wall, respectively, to record the hydrogen concentration. It is found that the maximum hydrogen concentration increases as the distance of the vertical barrier wall decreases. Models of the maximum hydrogen concentration in volume and mass fraction are proposed. The hydrogen concentration increases significantly in the horizontal and vertical directions as the vertical barrier wall is placed closer to the release nozzle. The hydrogen concentration in the horizontal and vertical directions of the vertical barrier wall follows the Gaussian and exponential distribution, respectively. The prediction models of hydrogen concentration in the horizontal and vertical directions are proposed as well. The research results can provide a theoretical basis and data support for the design of safety codes for cryogenic hydrogen storage and the safety distance of barrier walls in hydrogen refueling stations. [ABSTRACT FROM AUTHOR]

Hydrogen is regard as one of the most promising primary energy vectors solving the problems of air pollution and energy shortage. However, the spontaneous ignition is a major hazard during the application of high-pressure hydrogen. In this paper, a numerical investigation is conducted to study the shock wave propagation, hydrogen/air mixing and spontaneous ignition induced by high-pressure hydrogen release into the tubes with square, pentagon and circular cross-sections (tube wall angles are 90, 108 and 180° respectively). Large Eddy Simulation, Eddy Dissipation Concept coupling with the detailed hydrogen/air reaction mechanism and 10-step like opening process of burst disk are employed. The numerical results of ignition conditions and positions agree well against the previous experimental results. In the square and pentagon tubes, the inward-reflected and outward-reflected shock wave could be generated simultaneously and interact with each other due to the radial propagation of leading shock wave. In the circular tube, however, the two types of reflected shock generate alternately and the interaction disappears. The higher viscous resistance at wall corner induces the formation of velocity shear layer on the tube wall, which promotes the mixing of hydrogen and air. The increase of the angle of wall corner reduces the flammable mixture with the chemical equilibrium equivalence ratio on the tube wall due to the low viscous resistance and short velocity shear layer. The ignition delay time/distance and combustion intensity decrease with the increase of the angle of wall corner. After the occurrence of spontaneous ignitions in the non-circular tubes, the flame propagates downstream and its length increases rapidly along the wall corner. In addition, larger combustion area could be observed with the decrease of the angle of wall corner. [ABSTRACT FROM AUTHOR]