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The effect of an extension tube with a rectangular slit inlet on the opening status of the metal diaphragm, as well as spontaneous ignition of pressurized hydrogen are experimentally investigated. The results show that the opening ratio of the metal diaphragm exhibits a positive correlation with the increase of the slit area. It also shows an initial increase followed by a decrease as the failure pressure rises. Moreover, the shock overpressure and Mach number inside the tube with the slit entrance are significantly reduced compared to those within the circular inlet tube. The critical pressure for spontaneous ignition in the tube with narrow inlet is notably affected by the slit parameters, increasing progressively with decreasing slit area and slightly increasing with raising slit aspect ratio. These experimental results would serve as valuable references for further exploring the failure modes of hydrogen storage containers and mechanisms underlying the spontaneous ignition. • The opening ratio of partially ruptured burst disks is statistically measured. • The slit inlet attenuates the overpressure but does not modify its axial trend. • The abnormal rupture of diaphragm leads to the compression waves preceding shock. • The threshold exceeded opening ratio is not the main determinant of self-ignition. [ABSTRACT FROM AUTHOR]

Due to the high probability of hydrogen leaking from narrow cracks of the vessels, an experimental study is conducted to investigate the spontaneous ignition of high-pressure hydrogen released into a tube through inlets with different narrowness, including circular, square (length-width ratio χ = 1), and two slit shapes of χ = 2 and 3, with the same area. Pressure transducers and photoelectric sensors are used to detect shock wave dynamic variation and ignition occurrence. The results indicate that narrow inlets weaken the shock wave intensity and speed, and the weakening effect increases with narrowness. Moreover, spontaneous ignition is affected. The minimum burst pressure required for spontaneous ignition is higher in the cases with square and slit inlets. Most significantly, when χ is 3 , it is 2.5 times that of circular inlet. Meanwhile, for narrow inlets, the ignition flame intensity and speed are enhanced, and the inlet of χ = 2 has the greatest impact. • The influence of narrow releasing ports on self-ignition hydrogen is investigated. • Narrow ports weaken the shock wave intensity and speed. • It requires higher burst pressure for self-ignition under narrow ports. • The flame intensity is enhanced under narrow ports. • Different narrowness of the ports brings different effects. [ABSTRACT FROM AUTHOR]

Experiments on shock waves propagation, spontaneous ignition, and flame development during high-pressure hydrogen release through tubes with symmetrical obstacles (O 1-1) and asymmetrical obstacles (O 1-2) are conducted. The obstacle's side is triangular with a length of 4 mm, a height of 3.6 mm, and its width is 15 mm. In the experiments, a reflected shock wave generates and propagates both upstream and downstream when the leading shock wave encounters the obstacle. At the same burst pressure, the reflected shock wave intensity in tube O 1-1 is significantly greater than that in tube O 1-2. Moreover, the presence of obstacles in the tube can induce spontaneous ignition. The minimum burst pressures for spontaneous ignition for tubes O 1-1 and O 1-2 are 2.84 MPa and 3.28 MPa respectively, lower than that for the smooth tube. Furthermore, both the initial ignition position and ignition time are greatly advanced in obstruction tubes, mainly affected by obstacle positions and burst pressures. Finally, the flame separation process near the obstacle is observed. After passing the obstacle, the flames grow rapidly in radial and axial directions on the tube sidewalls. And at the same burst pressure, the flame convergence time in tube O 1-2 is usually longer than that in tube O 1-1. • The influence of obstacles arrangements on self-ignition hydrogen is investigated. • Reflected shock wave forms and propagates both upstream and downstream. • The presence of the obstacles can facilitate the occurrence of self-ignition. • Obstacles arrangements have great effects on ignition positions and ignition time. • The flame separation is observed inside both obstruction tubes. [ABSTRACT FROM AUTHOR]

The shock wave dynamics, spontaneous ignition and flame variation during high-pressure hydrogen release through tubes with different cross-section shapes are experimentally studied. Tubes with square, pentagon and circular cross-section shapes are considered in the experiments. The experimental results show that the cross-section shape of the tube has no great difference on the minimum burst pressure for spontaneous ignition in our tests. In the three tubes with length of 300 mm, spontaneous ignition may occur when overpressure of shock wave is 0.9 MPa. When the spontaneous ignition is induced in a non-circular cross-section tube, the possible turbulent flow in the corner of the tube increases can promote the mixing of hydrogen and air, thus producing more amount of the hydrogen/air mixture. As a result, both the peak light signal and flame duration detected in the non-circular cross-section tubes are more intense than those in the circular tube. The smaller angle of the corner leads to a more intensity flame inside tube. When the hydrogen flame propagates to the tube exit from the circular tube, the ball-like flame developed near tube exit is relatively weak. In addition, second flame separation outside the tube is observed for the cases of non-circular cross-section tubes. • The effect of cross-section shapes of tube has been experimentally investigated. • The use of burst disk with cross notching on surface exhibits good repeatability. • The ignition occurs when the overpressure of shock wave is about 0.9 MPa. • The cross-section shape shows a significant influence on the flame intensity. • Twice flame separation are observed for cases of non-circular cross-section tubes. [ABSTRACT FROM AUTHOR]

An experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube are measured by pressure transducers and light sensors. Results show that the pressure behind a shock wave first increases, and subsequently remains near constant value with an increase of the propagation distance. That is, a certain propagation distance is required to form a stable shock wave in the tube. In the front of the tube, the minimum value of pressure behind the shock wave (P shock) required for spontaneous ignition decreases with the increase in axial distance to the diaphragm. However, the minimum P shock remains nearly a constant value in the rear part of the tube. Moreover, the critical values of shock Mach number (M S) for spontaneous ignition decrease with the increase in tube length. And the ignition delay time decreases with the increase of the M S. As the ignition kernel grows in size to a flame, it propagates downstream along the tube with velocity greater than the theoretical flow velocity of the hydrogen-air contact surface. The flame propagation velocity relative to tube wall increases with M S. When the self-sustained flame exits from the tube, a rapid non-premixed turbulent combustion is observed in the chamber. The combustion-wave overpressure increases with the increase of the M S. • The influence of shock wave intensity on spontaneous ignition is investigated. • A certain propagation distance is required to form a stable shock wave in the tube. • The minimum P shock for self-ignition decreases with increasing the axial location. • The critical shock Mach number for self-ignition decreases with the tube length. • Flame propagation velocity relative to tube wall increases with shock Mach number. [ABSTRACT FROM AUTHOR]

Highlights • Similitude analysis used to derive correlation for hydrogen spontaneous ignition. • The correlation is validated against tests performed in this and other studies. • Burst disks of nickel, aluminum, copper, brass of 0.13–0.4 mm thicknesses are used. • Tubes of 5–20 mm diameter, 80–360 mm length at burst pressure 2–10 MPa are used. • The correlation can be used as a tool for hydrogen safety engineering. Abstract Hydrogen is an emerging energy carrier for green hydrogen and fuel cell vehicles. The compressed gaseous hydrogen is selected by car manufacturers as a preferred option for onboard storage. One possible accident scenario is release of high-pressure hydrogen to the atmosphere through piping and tubes. The spontaneous ignition of accidently released hydrogen can be accompanied by pressure and thermal effects on humans and facilities. However, despite several previous studies, the critical conditions for spontaneous ignition to take place are not yet defined for an arbitrary case. Most of earlier studies present only a qualitative interdependence between the critical burst pressure for ignition and the ignition influencing factors, i.e. tube length and diameter, burst disk opening time, etc. The quantitative correlation is not yet available to underpin inherently safer engineering design of hydrogen systems and infrastructure. This paper focuses on quantitative analysis of the phenomenon of spontaneous ignition during high-pressure hydrogen release through a tube filled with air into the atmosphere. Similitude analysis of the problem is carried out. It is found that the ratio of shock pressure inside the tube to the atmospheric pressure, P s /P a , is a function of the dimensionless parameter, which is the product of the initial storage to the atmospheric pressure ratio, P b /P a , and the ratio of the characteristic shock propagation time to the burst disk opening time, (D/u s)/t. Pressure transducers and light sensors are used in the experiments to record the shock propagation and ignition time and location. The correlation to quantitatively define the critical conditions for spontaneous ignition of released hydrogen inside a tube with air is derived for the first time using the similitude analysis. The correlation is calibrated against experiments carried out in this study and by other authors, where four identical triangular petals are formed upon rupture of inertial burst disk made of metal. [ABSTRACT FROM AUTHOR]

Abstract Hydrogen is expected to be an alternative energy carrier in the future. High-pressure hydrogen storage option is considered as the best choice. However, spontaneous ignition tends to occur if hydrogen is suddenly released from a high-pressure tank into a tube. In order to improve the safety of hydrogen application, an experimental investigation on effects of CO 2 additions (5%, 10% and 15% volume concentration) on the spontaneous ignition of high-pressure hydrogen during its sudden expansion inside the tube has been conducted. Pressure transducers are used to record the pressure variation and light sensors are employed to detect the possible spontaneous ignition. It is found that the shock wave overpressure and the mean shock wave speed are almost the same inside the tube for different CO 2 additions under the close burst pressures. For cases with more CO 2 additions, the ignition detected time is longer and the average speed of the flame, the maximum value of light signals and the detected duration time of spontaneous ignition are smaller. It is shown that minimum burst pressure required for spontaneous ignition increase 1.47 times for 15% CO 2 additions. The minimum burst pressure required for spontaneous ignition increases from 4.37 MPa (0% CO 2) up to 6.41 MPa (15% CO 2). With the increasing of CO 2 additions, it requires longer distance and longer time for hydrogen and oxygen to mix and thus longer ignition delay distance/time. The results showed that additions of CO 2 to air have a good suppressing effect on hydrogen spontaneous ignition. Highlights • Effect of CO 2 doping to air on the spontaneous ignition occurrence is investigated experimentally. • Minimum burst pressure required for spontaneous ignition increases with the increasing of CO 2 addition. • Minimum burst pressure required for spontaneous ignition increase 1.47 times for 15% CO2 additions. • It requires a longer distance and a longer time for spontaneous ignition to take place if more CO 2 is added. [ABSTRACT FROM AUTHOR]

Highlights • The effect of opening ratios of burst disk has been experimentally investigated. • The burst disk cannot fully open and forms a divergent nozzle. • The mean shock speed and shock overpressure inside/outside the tube reduce significantly. • The minimum initial pressure for spontaneous ignition increases significantly when χ = 2/3. • When χ ≤ 1/2, no ignition occurs even though initial pressure ratio is as high as 90. Abstract Hydrogen is regarded as an alternative energy carrier in the next decades and high-pressure hydrogen storage is treated as the best option. However, unexpected spontaneous ignition would occur during high-pressure hydrogen sudden release, which induces a severe safety issue. For improving the safety application of hydrogen, an experimental investigation has been conducted. Different diameter ring gaskets are employed to change the opening ratio χ. Pressure transducers and light sensors are used to record the pressure variation and possible light signals inside the tube, respectively. It is found that the burst disk is unable to fully open during high-pressure hydrogen release when χ < 1, resulting in forming a convergent nozzle. This structure leads to the speed reduction for supersonic flow. Consequently, the speed of shock and shock overpressure inside the tube reduce significantly. The spontaneous ignition cannot be initiated even though the initial pressure ratio is as high as 90 when χ ≤ 1/2. The minimum initial pressure ratio required for spontaneous ignition increases to 64.1 when χ = 2/3. The flame is dimmer for small opening ratio cases. The shock overpressure outside the tube is reduced significantly, which decreases the damage to the facilities and humans to a large extent. [ABSTRACT FROM AUTHOR]

• The influence of a sho (ϸ)-shaped tube on the spontaneous ignition is investigated. • There are complex shock wave interactions in the bend section. • Spontaneous ignition is more prone to occur in the bend section. • The flame in the bend section cannot spread to the straight section. • There are two jet fires with a long-time interval outside the sho-shaped tube. The geometry of the extension tube has a great influence on the shock wave propagation and spontaneous ignition of high-pressure hydrogen release in this paper, piezoelectric pressure transducers, photodiodes and a high-speed camera are used to study the shock wave propagation and spontaneous ignition of high-pressure hydrogen released to the downstream sho (ϸ)-shaped extension tube. Experimental results show that the intensity of the leading shock wave is weakened due to the presence of branches when it propagates in the straight section of the sho-shaped tube, and a strong reflected shock wave is generated in the bend section. Spontaneous ignition is more prone to occur in the bend section than in the straight section, with a large optical signal value and long flame duration. However, due to the complex structure of the extension tube, there are still cases of the hydrogen flame extinguishing inside the tube even under high burst pressure. The flame detection time after the leading shock wave at the measured positions decreases with increasing burst pressure, and the time in the bend section is greater than that of the straight section. Radial flame expansion, flame separation, downstream flame extinction and upstream flame development, primary jet fire and secondary jet fire are successively observed outside the tube. [ABSTRACT FROM AUTHOR]

Abstract This paper investigates the effects of hydrogen additions on spontaneous ignition of high-pressure hydrogen released into hydrogen-air mixture. Hydrogen and air are premixed with different volume concentrations (0%, 5%, 10%, 15% and 20% H 2) in the tube before high-pressure hydrogen is suddenly released. Pressure transducers are employed to detect the shock waves, estimate the mean shock wave speed and record the shock wave overpressure. Light sensors are used to determine the occurrence of high-pressure hydrogen spontaneous ignition in the tube. A high-speed camera is used to capture the flame propagation behavior outside the tube. It is found that only 5% hydrogen addition could decrease the minimum storage pressure required for spontaneous ignition from 4.37 MPa to 2.78 MPa significantly. When 10% or 15% hydrogen is added to the air, the minimum storage pressure decreases to 2.81 MPa and 1.85 MPa, respectively. When hydrogen addition increases to 20%, the spontaneous ignition even takes place at burst pressure as low as 1.79 MPa inside the straight tube. Highlights • Effect of hydrogen doping to air on the spontaneous ignition occurrence is investigated experimentally. • Minimum storage pressure required for spontaneous ignition decreases with the increasing of hydrogen addition. • 5%, 10% and 15% hydrogen decrease minimum storage pressure from 4.37 MPa to 2.78, 2.81 and 1.85 MPa, respectively. • Spontaneous ignition takes place at storage pressure as low as 1.79 MPa when 20% hydrogen is added. [ABSTRACT FROM AUTHOR]

Combined effects of tube dimensions and burst pressure on the spontaneous ignition caused by high-pressure hydrogen release into a semi-confined space are investigated experimentally. An important finding is that the influence of tube diameter on spontaneous ignition shows complex behavior. For tubes with different diameters, the minimum burst pressure for spontaneous ignition depends on not only the strength of the shock wave but also the mixing of hydrogen and air. A dimensionless parameter of tubes, L/D —which is defined as the ratio of the tube length to the tube diameter—is introduced to describe the effect of tube size. The results show that the possibility of spontaneous ignition increases with increasing L/D . Under appropriate conditions, spontaneous ignition leads to flame development. As the flame propagates into a semi-confined space, it first forms an envelope structure in front of the hydrogen jet. Since some amount of a partially premixed combustible mixture, created by the hydrogen jet, exists in the semi-confined space, the flame subsequently undergoes deflagration. The overpressure caused by deflagration is significantly greater than that caused by the leading shock wave. In addition, both the deflagration overpressure and the shock-wave overpressure increase with increasing tube diameter and initial release pressure. [ABSTRACT FROM AUTHOR]

An experimental investigation of spontaneous ignition and flame propagation at high-pressure hydrogen release via cylindrical tubes with varying cross-section is presented. Tubes with different transverse cross-sections are considered in the experiments: (1) local contraction, (2) local enlargement, (3) abrupt contraction, and (4) abrupt enlargement. The results show that the presence of the varying cross-section geometries can significantly promote the occurrence of spontaneous ignition. Compared to the tube with constant cross-section, the minimum pressure release needed for spontaneous ignition for the varying cross-sections tubes is considerably lower. Moreover, the initial ignition location is closer to the disk in the presence of varying cross-section geometries in comparison with straight channel. As the flame emerges from the outlet of the tube, the velocity of the flame front in the vicinity of the nozzle increases sharply. Then, a deflagration develops across the mixing zone of hydrogen/air mixture. The maximum deflagration overpressure increases linearly with the release pressure. Subsequently, a hydrogen jet flame is produced and evolves different shapes at different release stages. A fireball is formed after the jet flame spouts in the open air. Later, the fireball develops into a jet flame which shifts upward and continues to burn in the vertical direction. [ABSTRACT FROM AUTHOR]

An experimental study on flow characteristics and spontaneous ignition produced by pressurized hydrogen release through an Omega-shaped tube into atmosphere was conducted. This paper aims to study the influence of geometry of downstream tube on the shock waves and the spontaneous ignition of pressurized hydrogen release. The tube used in present experiments is an Omega-shaped tube. It is found that the pressure of the tank will undergo an increase when hydrogen is releasing in the Omega-shaped tube because of the shock wave reflection which is detected by the piezoelectric pressure transducers. The pressure inside the Omega-shaped tube is much higher compared with straight tubes. Additionally, the spontaneous ignition even occurs inside the tube when burst pressure is as low as 2.19 MPa, which means that the geometry of downstream tube does have an important influence on the spontaneous ignition of hydrogen release and using an Omega-shaped tube reduces the critical pressure required for spontaneous ignition. When burst pressure is below 4.09 MPa, the spontaneous ignition occurs inside the tube and flame is formed inside the exhaust chamber but no flame separation happens. [ABSTRACT FROM AUTHOR]

Spontaneous ignition and subsequent flame propagation of high-pressure hydrogen release via a tube into air are experimentally investigated using pressure records, flame detection and direct high-speed photographs. The study shows that as the burst pressure increases the likelihood of spontaneous ignition increases and the initial ignition is closer to the burst disk. With the increase of tube length, the possibility of spontaneous ignition increases, while the critical release pressure for spontaneous ignition decreases. It is also found that a strong shock wave generated to trigger the ignition and a long tube to promote the growth of the flame are two key factors for the transition from spontaneous ignition inside the tube to jet flame in the air. After the flame exits from the tube, a flame envelope is formed in the front of the hydrogen jet, which gradually splits into upstream and downstream combustion regions. The upstream flame region propagates forward. However, the downstream flame region moves back toward the tube exit. The flame is then stabilized at the tube exit and gradually grows. Noticeable deflagration events were observed to occur successively in the semi-enclosed space. The deflagration leads to a significant increase of pressure in the chamber. And the overpressure of the deflagration is higher than that of the leading shock wave. Both the overpressures of the leading shock wave and the deflagration increase with the release pressure. A stable jet flame is formed outside the tube subsequent to the deflagration. And different jet flame configurations are observed at different controlling mechanisms of flow. [ABSTRACT FROM AUTHOR]

Shock waves and spontaneous ignition produced by pressurized hydrogen release through a tube into atmosphere are studied experimentally. The results showed that the speed of the leading shock wave increases at the initial stage and then decreases when propagating to the downstream inside the tube. The schlieren images of the of the hydrogen jet in the vicinity of the tube exit clearly show various classical flow structures, i.e. Mach disk, barrel shock, reflected shock and shock triple point. Due to the sharp decline of the strength of the shock wave, spontaneous ignition might be difficult to occur or quenched outside the tube. Additionally, Spontaneous ignition is more likely to happen in the downstream tube with small diameter and the possibility of spontaneous ignition increases with increasing the tube length. An accidental explosion was observed when the hydrogen jet was emitted into a semi-closed exhaust chamber. It is found that the rupture process of the diaphragm has an important influence on the formation of shock wave and spontaneous ignition. [ABSTRACT FROM AUTHOR]

Spontaneous ignition induced by high-pressure hydrogen leakage has primarily been discussed qualitatively, focusing on factors such as release pressure, downstream tube size, and the rupture of the burst disk. However, quantitative investigations and prediction models remain limited and unclear. In this paper, we introduce the relative molecular mass M and delve into its influence on the critical pressure threshold for spontaneous ignition of high-pressure hydrogen leaks. Additionally, we introduce the mass flow rate Q , as a metric to quantify the impact of opening area and shape on the ignition process. By integrating six key factors, including tube diameter D , tube length L , atmospheric pressure P a , burst disk opening time t , gas mass flow rate Q , and gas relative molecular mass M , four dimensionless factors affecting the critical leakage pressure were determined by using similarity analysis. The correlation between the dimensionless numbers was verified by correlation analysis and calibrated to the experiments of this paper and other authors. Finally, a critical criterion for spontaneous ignition from leakage in high-pressure hydrogen tubes was established to provide a basis for safety evaluation and prevention techniques for high-pressure hydrogen. [Display omitted] • The effect of the relative molecular mass of gases in the high-pressure region on the spontaneous ignition of high-pressure hydrogen leakage is discussed. • The dimensionless parameter affecting the critical pressure for spontaneous ignition of high-pressure hydrogen leakage is investigated. • The critical pressure criterion for the spontaneous ignition of high-pressure hydrogen leakage is established. [ABSTRACT FROM AUTHOR]

• Shock wave is temporarily weakened when the shock wave travels through obstacles. • Effects of obstacles in the tube on hydrogen self-ignition is investigated. • The further obstacles away from diaphragm, the more probability of flame enhancement. • Presence of obstacles in tube would only temporarily promote combustion inside tube. The spontaneous ignition and subsequent hydrogen combustion has become a great challenge for the safe use of hydrogen energy at high-pressure. The spontaneous ignition mechanism of high-pressure hydrogen in the tube with obstacles has still not fully understood. In this study, synchronization measurement was performed through simultaneous pressure and flame acquisition, and experiments of pressurized hydrogen sudden release through tubes with obstacles were conducted. Spontaneous ignition and subsequent hydrogen combustion were reproduced through varying initial burst pressure in different obstructed tubes, the presence of obstacles exhibit great influence on shock wave propagation and flame evolution inside the tube. The temporary weaken of shock wave strength caused by obstacles is not conducive for spontaneous ignition onset. At the contrary, the complex dynamic flow induced by obstacles facilitates the occurrence of the spontaneous ignition. The effect of obstacles on the spontaneous ignition onset can be further analyzed by the minimum burst pressure for spontaneous ignition. And it shows the effects of obstacles on spontaneous ignition onset depends on its comprehensive influences on the formation of hydrogen/air mixture and increment of temperature. Furthermore, locations of obstacles also play an essential role on spontaneous ignition onset and hydrogen combustion intensity inside tube. The pressure and flame dynamics evolution show that when increasing the distance between burst disk and obstacles, the obstacles exhibit less influence on the spontaneous ignition onset but temporarily enhance combustion inside tube. [ABSTRACT FROM AUTHOR]

This paper studies the effects of triangle obstacles inside the tube on the ignition mechanism and subsequent flame development in a semi-confined space when high-pressure hydrogen sudden release. Smooth tube and obstructed tube with optical glasses are constructed where controlled high-pressure hydrogen are released. High-speed direct photography is used to image the flame evolution while pressure transducers are used to obtain pressure-time traces both inside the tube and exhaust chamber. These cases that hydrogen with various burst pressure jets into the smooth and obstructed tubes are tested. It is found that spontaneous ignition occurs in the boundary layer of the tube in the smooth tube, a complete fire spanning the cross-section is reached as the flame propagates within the mixing region. The appearance of obstacles is found to have a significant effect on the ignition mechanism and flame structure. A two-way dissemination reflected shock wave forms and three possible ignition region emerges surrounding the obstacles. The flame propagation restricted between the leading shock wave and hydrogen jet. The flame experiences a split that the front flame in the vicinity of the obstacle gradually dies out and the rear flame continues propagating within the mixing region. The flame evolution and pressure variation in the exhaust chamber show that obstacles inside the tube do not aggravate the disaster in the semi-confined chamber. [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]

• 10-step like opening process is used to mimic the opening process of burst disk. • Hemispherical shock wave is firstly produced and changes into a normal shock. • Spontaneous ignition firstly occurs at the tube boundary. • Numerical results successfully reproduce the experimental results. Hydrogen is regarded as the promising energy carrier in the 21st century and high-pressure storage is selected as the best option. However, spontaneous ignition would be induced if high-pressure hydrogen is suddenly released. Three cases were numerically conducted to gain an insight into the mechanism of the spontaneous ignition and to validate against our previous experimental results. Result show that a hemispherical shock wave is first produced. Then it is reflected as reflected shock wave after hitting the tube wall and interacts with other reflected shock waves to form the Mach disk, shock triple point and a barrel shock. The height of the Mach disk gradually decreases and finally it disappears. Meanwhile, the shape of hydrogen jet changes from a forward convex shape to a backward concave shape and hydrogen/air mixture layer is formed near the tube wall and the center of the tube. The temperature of the shock-affected region gradually increases and its area thickens. After maintenance of high temperature for a period of time, spontaneous ignition firstly occurs at the tube boundary. Present numerical results not only reproduce the ignition conditions but also the ignition positions indicating that the model is an effective tool for hydrogen safety engineering. [ABSTRACT FROM AUTHOR]