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The dynamic mechanisms of self-ignition and flame propagation during high-pressure hydrogen release through a rectangular tube were experimentally investigated using pressure records, flame detection and high-speed photographs. Experimental results show that the minimum burst pressure for self-ignition decreases with an increase in axial distance to the diaphragm and then remains at an almost constant value. The self-ignition onset at the same location of the tube exhibits a certain randomness even if the intensity of the shock wave produced in the tube is similar. Multiple ignitions were observed at the early stage of hydrogen release. They usually had difficulty to sustainably develop and were extinguished owing to oxygen deficiency. At a subsequent stage, the ignition kernel appears again and grows rapidly in the axial and radial directions, finally converging to a complete flame across the tube width. It was found that the radial growth rate of the flame was lower than the axial growth rate. [ABSTRACT FROM AUTHOR]

In this study, an experimental investigation is conducted to study the effects of CO addition (0%–7.5% vol.) on the self-ignition of pressurized hydrogen leakage. The result shows that more CO addition significantly decreases the likelihood of self-ignition inside the tube and the formation of self-sustained jet flame outside the tube. This can be mostly explained by the reduction of leading shock intensity inside the tube. Furthermore, CO addition effectively inhibits the flame propagation and development inside the tube. For pure hydrogen, the ignited flame quickly develops an intense flame spanning the tube width and eventually form a jet flame in ambient air. However, the H 2 –CO diffusion flame initiated approaching tube sidewall tends to propagate adjacent to the wall or be soon quenched with more CO addition. If the CO-weakened flame spouts to the tube exit, it may not survive the expansion at the tube exit and thus it is quenched. • The shock intensity is reduced in the presence of CO. • The critical pressure for self-ignition increases with CO addition concentration. • Shock intensity variation by CO addition is the main factor affecting self-ignition onset. • Small CO addition can effectively inhibit flame development inside the tube. • H 2 –CO flame spouting from the tube exit tends to be blown out. [ABSTRACT FROM AUTHOR]

• The minimum release pressure for self-ignition is significantly higher. • The decrease of binary mixture self-ignition enhances with fuel molecular weight. • Nitrogen addition effectively inhibits flame development inside the tube. • Nitrogen-weaken ignited flame spouting from tube exit tends to blow out. • When self-ignition occurs, a partially premixed flame evolves first. This paper reports on the nitrogen addition effects on the self-ignition of high-pressure hydrogen leakage. It reveals that the intensity of produced incident shock decreases with more nitrogen addition, unbeneficial for self-ignition occurrence. Besides, nitrogen addition in hydrogen significantly reduces self-ignition possibilities inside the tube and self-sustained jet flame formation outside the tube. There exists a certain critical threshold of shock overpressure of about 1 MPa for self-ignition occurrence at different nitrogen additions. In our experiments, the impurity gases (N 2 , CO, CH 4) reduce the self-ignition possibilities in the same order as it reduces the shock intensity, namely, the effect of N 2 is similar to that of CO, larger than that of the same volume of CH 4. It suggests that the decrease of binary mixture self-ignition can be mostly explained by the reduction of shock intensity, and the decrease effect enhances with fuel molecular weight. Furthermore, nitrogen addition inhibits the flame development and propagation inside the tube. Nitrogen-weaken ignited flame barely survives during the expansion outside the tube or is soon extinguished inside the tube with more nitrogen addition. Moreover, the flame has a rapid flame length growth rate of 1–2 mm/μs in the initial stage, but the expanding velocity becomes lower to 0–1 mm/μs after a period of spread. Accordingly, a mechanism of the self-ignition process for high-pressure hydrogen release is proposed for cases with 0%-7.5% nitrogen addition. [ABSTRACT FROM AUTHOR]

• The critical pressure for ignition increases with methane addition concentration. • Methane addition can effectively inhibit flame development in the tube. • Three mechanisms of self-ignition for hydrogen with methane addition is proposed. • Hydrogen-methane flame spouting from tube exit tends to blow out. • The hydrogen-methane mixture brings greater combustion overpressure in chamber. In this paper, experimental investigations are carried out to study the effects of methane addition (2.5%, 5% and 7.5% vol.) on self-ignition and subsequent flame propagation of pressurized hydrogen release via a tube. The visualization method utilizing the high-speed direct photography and the measurements of pressure and light sensors are applied in this study. The result shows that the existence of methane can reduce the shock wave overpressure and the temperature of shock-heated air. Therefore, it has great effects on the self-ignition possibility. The minimum burst pressure required for self-ignition increases from 2.89 MPa (0% CH 4) up to 6.05 MPa (7.5% CH 4). Besides, methane addition can extremely reduce flame intensity and inhibit flame development inside the tube. Three possible self-ignition mechanisms for the hydrogen with methane addition are discussed. Under the effects of methane addition, hydrogen flame spouting from tube exit tends to blow out. In all cases with 7.5% methane addition, no self-sustained jet flame is formed outside the tube for burst pressure up to 10.0 MPa. When self-sustained flame is formed in a confined space, a great rise in overpressure is generated due to the intense combustion. It is suggested that more methane addition can reduce the self-ignition probability and the formation of self-sustained flame. However, with the increasing of CH 4 additions, once the jet fire is formed in a confined space, it may cause greater overpressure and thus bring greater casualties and property losses. [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]

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 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]

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]

• 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]