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• 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]

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]