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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 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]

Spontaneous ignition induced by high-pressure hydrogen release is one of the huge potential risks in the promotion of hydrogen energy. However, the understanding of the microscopic dynamic characteristics of spontaneous ignition, such as ignition initiation and flame development, remains unresolved. In this paper, the spontaneous ignition caused by high-pressure hydrogen release through a tube is investigated by two-dimensional numerical simulation at burst pressure ranging from 2.67 to 15 MPa. Especially, the thermal and species characteristics in hydrogen shock-induced ignition under different strengths of shock wave are discussed carefully. The results show that the stronger shock wave caused by higher burst pressure leads to larger heating area and higher heating temperature inside the tube, increasing the possibility of spontaneous ignition. The shortening effect of initial ignition time and initial ignition distance will decrease with the increase of the burst pressure. Ignition will be initiated when the temperature is raised to about 1350–1400 K under the heating effect of shock waves. It is also found that the ignition occurs under the lean-fuel condition firstly on the upper and lower walls of the tube. The flame branch after spontaneous ignition is observed in the mixing layer. Two ignition kernels show different characteristics during the process of combustion and flow. The evolution of HRR and mass fraction of key species (OH, H, HO 2) are also compared to identify the flame front. The mass fraction of H has the better trend with HRR. It is suggested that H radical is a more reasonable choice as the indicator of the flame front. • The spontaneous ignition of the high-pressure hydrogen is numerically investigated. • Ignition will be initiated when the temperature rises to 1350–1400 K. • Ignition occurs under the lean-fuel condition firstly. • Branch of flame is observed in the mixing layer after spontaneous ignition. • The evolution of HRR and key species is analyzed. [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]