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ACS Omega
ACS Omega
ao
acsodf
ACS Omega
2470-1343
American Chemical Society

10.1021/acsomega.4c03651
Article
Experimental Study on the Influence of Ignition Position on the Overpressure of Hydrogen Jet Flame
Xu Dayong †‡⊥
Lu Langqing †⊥
Wang Zhilei *†§
Zhang Lijing †
https://orcid.org/0000-0002-5309-7817
Pan Xuhai *†§∥
https://orcid.org/0000-0001-7018-2709
Jiang Juncheng †§
† College of Safety Science and Engineering, Nanjing Tech University, Nanjing 210009, China
‡ Key Laboratory of Urban Safety Risk Monitoring and Early Warning, Ministry of Emergency Management, Shenzhen Urban Public Safety and Technology Institute, Shenzhen 518038, China
§ Petrochemical Industry Engineering Laboratory of Hydrogen Safety Technology, Nanjing Tech University, Nanjing 211816, Jiangsu, China
∥ Nanjing Vocational University of Industry Technology, Nanjing 211106, China
* Email: wangzl@njtech.edu.cn.
* Email: xuhaipan@njtech.edu.cn.
24 08 2024
10 09 2024
9 36 3786937881
26 04 2024
19 08 2024
15 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

The accidental leakage of hydrogen poses a significant barrier to the widespread adoption and development of hydrogen energy due to the potential risks of fire, explosion, and jet fire hazards. Experimental investigations have been conducted on the process of jet fires formed by igniting hydrogen jet streams after accidental releases in scenarios such as high-pressure hydrogen gas storage tanks and hydrogen transmission pipelines. These experiments utilized a release pipe with a diameter of 10 mm and a length of 0.75 m, along with three pressure sensors, to study the influence of release pressure and ignition position on jet flame overpressure and flame propagation. Extensive tests at 1.5 MPa yielded a hydrogen flammability map containing two nonflammable zones and one flammable zone, along with a graph illustrating the relationship between overpressure and ignition points. Furthermore, experiments conducted at ignition positions of 0.05, 0.5 and 1.0 m under release pressures ranging from 6 to 10 MPa revealed that release pressure had no significant effect, while ignition position notably influenced the waveform and peak of the shockwave. Additionally, a peak shockwave reaching 30 kPa was observed at the downstream of the pipe outlet when ignited at 0.05 m, far exceeding the threshold of 24 kPa associated with fatalities. This research aims to provide valuable insights for safety design and protection distance considerations in scenarios involving hydrogen release and ignition.

Key Research and Development Program of Ningxia 10.13039/100016692 2022BEE02001 Shenzhen Science and Technology Program NA ZDSYS20210929115800001 Shenzhen Urban Public Safety and Technology Institute NA NA Key Laboratory of Urban Safety Risk Monitoring and Early Warning NA NA Natural Science Foundation of the Jiangsu Higher Education Institutions of China NA 22KJB620004 National Key Research and Development Program of intergovernmental international scientific and technological cooperation and innovation special project NA 2023YFE0199100 State Key Laboratory Cultivation Base for Gas Geology and Gas Control 10.13039/501100021379 WS2022A06 State Key Laboratory of Fire Science 10.13039/501100011294 HZ2023-KF11 National Natural Science Foundation of China 10.13039/501100001809 52304230 National Natural Science Foundation of China 10.13039/501100001809 52074158 document-id-old-9ao4c03651
document-id-new-14ao4c03651
ccc-price
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pmc1 Introduction

Hydrogen is considered an exemplary energy carrier due to its effectiveness and cleanliness. However, it has a low lower explosive limit and a minimum ignition energy of only 0.018 mJ,1 which entail risks in the storage, transport and use of hydrogen gas. Furthermore, despite having a mass energy density much greater than that of gasoline,2 hydrogen possesses a low bulk energy density at atmospheric pressure. This is why, due to cost and technical limitations, high-pressure hydrogen storage has become the prevalent method for hydrogen storage.3,4

In industrial settings, accidental hydrogen releases typically occur when the hydrogen storage tank ruptures due to external forces or when elevated temperatures lead to tank overpressurization. In such instances, hydrogen is released through the rupture or vent, resulting in a rapid and substantial leakage, often leading to the formation of under-expanded jets.5 If hydrogen is released via a pipeline, it carries the risk of initiating a jet flame or even an explosion upon encountering an external ignition source.6 These incidents present significant challenges to personnel and equipment in the vicinity,7 and can potentially trigger secondary disasters due to jet flames and explosion overpressure.8,9

Schefer10 utilized flame length and Froude number to predict the radiative heating of a hydrogen jet, establishing a foundational method for predicting flame characteristics. Sandia National Laboratories (SNL)11,12 conducted an extensive analysis of hydrogen jet velocity and concentration distribution, deriving the concentration distribution function along the jet’s axial direction. Guo13 and colleagues investigated the impact of axial ignition distance on the flame velocity of high-pressure hydrogen jets. The development of jet flames is influenced by hydrogen concentration and jet velocity in various ways. Lowesmith and Hankinson14 conducted large-scale experiments to measure hydrogen jet flame length and thermal radiation field. Additionally, Shirvill’s research15 explored hydrogen release pressures, reaching up to the current maximum of 40 MPa. These experts have comprehensively characterized hydrogen jet flame length, downstream pressure distribution, and radiation characteristics.

Numerical methods have also been widely used by many researchers to simulate hydrogen leakage scenarios. Houf et al.11 employed a low-pressure leakage integral model for theoretical calculations of hydrogen jet flow. Wang16 conducted numerical simulations to investigate the development of hydrogen jet flames caused by spontaneous ignition near the nozzle and in the midfield. Jiang17 analyzed the transition conditions from the near-field to the far-field of flames through experiments and Large Eddy Simulation (LES), showing that the initial vortex generation can lead to flame extinction. Similar results were demonstrated by Agnieszka18 using LES simulations, highlighting the critical role of annular vortices in determining the ultimate development of jet flames.

Furthermore, optical techniques are frequently employed to investigate hydrogen jet flames in unconfined environments, including background oriented schlieren (BOS), infrared (IR) and ultraviolet digital imaging, particle image velocimetry (PIV), planar laser rayleigh scattering.19 These optical techniques find wide application in visualizing hydrogen jets in unobstructed environments, allowing for effective measurement of various parameters of a hydrogen jet fire, such as flame structure, length, width, high-temperature region, and hydrogen concentration. In previous years, BOS was initially utilized for visualizing hydrogen jet flames, leading to the formulation of empirical equations outlining the position and size of the Mach disc.20 Veser21 employed PLV and BOS to further examine hydrogen jets, while Di Sarli22 applied PIV to study vortex phenomena in flames rich in hydrogen-methane, determining the nature of interactions between the flames and vortices. The advancement of a new era of BOS methods23 has also enabled accurate analysis of hydrogen concentration.

Grune24,25 employed a similar electrode ignition method to investigate the overpressure caused by unsteady hydrogen jet flows, but has yet to explore the scenario of large-scale release pressures. Additionally, most numerical simulation-based studies lack experimental validation, raising questions about the accuracy of their results. Therefore, this study experimentally investigates the ignition outcomes and resulting overpressure of hydrogen jet releases under a wide range of release pressures and encountering ignition sources at different positions near the nozzle. The analysis aims to understand the influence of flame overpressure, ignition position, and release pressure. Drawing on the findings summarized by LaChance et al.26 regarding the overpressure levels required to cause damage to people and structures, this research provides insights for hazard assessment, disaster prevention, and the design of safe distances for release outlets.

2 Experimental Setup

Figure 1 displays the experimental setup used in this paper. Hydrogen is supplied through one or more hydrogen cylinders with a volume of 40 L and a purity of 99.999%, up to 12.5 MPa, via the gas supply system. Nitrogen cylinders are used to purge nitrogen after each experiment.

Figure 1 Schematic diagram of experimental setup.

Nitrogen and hydrogen are directed into the storage tank through a pneumatic valve situated ahead of the discharge pipeline. The tank has a volume of 1 L. Another pneumatic valve is positioned between the discharge pipeline and the storage tank to regulate gas entry. The discharge pipeline measures 0.75 m in length and has a diameter of 10 mm. When discharged, high-pressure hydrogen has the potential to generate a shock wave, which may result in spontaneous combustion.27−29 However, in this experiment, the risk of a shock wave was mitigated by controlling the fully open time of the pneumatic valve to 110 ms. Grune’s experiments24 have demonstrated the effectiveness of this method in preventing spontaneous combustion of hydrogen inside the tube, ensuring that the hydrogen jet ignites only outside the tube due to the igniter.

The ignition device utilizes an arc igniter to generate a stable electric arc that ignites the hydrogen jet. Figure 2 illustrates the placement of the igniter, which is divided into a near-field ignition area and a far-field ignition area. The near-field ignition region comprises 29 ignition positions, distributed axially between 0.01 and 0.1 m and radially between 0 and 0.03 m. The far-field ignition region comprises 31 ignition positions, distributed axially between 0.5 and 3 m and radially between 0 and 0.35 m. The origin of the coordinates for the ignition area is the center of the outlet of the discharge pipe. The axial distance indicates the distance downstream from the nozzle, and the radial distance indicates the distance of the ignition point from the axis of the discharge pipe.

Figure 2 Layout diagram of ignition position.

To gather data, three pressure sensors (PCB–106B piezoelectric pressure sensors), named P01, P02, and P03, were positioned at the same elevation as the discharge tube, which was 1.1 m above the ground. These sensors were located at a radial distance of 0.5 m from the axis of the high-pressure hydrogen discharge tube and at axial distances of 1, 1.5 and 2 m, respectively. Additionally, the tank was equipped with a static pressure sensor (Kulite 375M-250BARSG) to facilitate real-time monitoring of the internal pressure, ensuring that hydrogen was discharged at the specified pressure. An oscilloscope (HIOKI, MR6000) with a maximum sampling rate of 200 M/S was employed for recording and outputting pressure data. Flame images were captured using a high-speed camera (SSZN, SH3-105) with a frame rate set to 10,000 FPS.

Throughout the experiment, the oscilloscope recorded the values of each static pressure sensor arranged on the tank whenever the pressure drop detected by the sensor surpassed the predefined threshold. To ensure the safety of the experimental personnel, the pneumatic valve was controlled remotely by personnel to execute the experiment.

3 Results and Discussion

3.1 Ignition Results

From the images of the flames captured after the hydrogen jet was ignited and the data recorded by the pressure sensors, three scenarios can occur: combustion, explosion, and nonignition. Figure 3 shows the pressure recording history of the igniter placed at three different positions at a discharge pressure of 1.5 MPa, which represents a typical condition for the three scenarios.

Figure 3 Three typical pressure records for 1.5 MPa release. (a)Ignition position: axial 1.0 m, radial 0.02 m. (b)Ignition position: axial 0.03 m, radial 0.02 m. (c)Ignition position: axial 0.02 m, radial 0.03 m.

Please note that the pressure data obtained in the experiments exhibit a notable disparity between the maximum overpressure difference and the duration of overpressure, owing to differing ignition outcomes. Consequently, to accurately depict pressure fluctuations in the graphs, the termination points of the maximum overpressure on the vertical coordinate of the graphs are not uniform. Additionally, it is essential to acknowledge that the time axis T = 0 is manually chosen based on the duration of the pressure fluctuation and may not precisely align with the moment when the pressure wave reaches the sensor.

Figure 3a–c depict three distinct scenarios following the ignition of a hydrogen jet: combustion, explosion, and no ignition. In Figure 3c, high-pressure hydrogen discharged through the tube fails to ignite upon encountering the electrode igniter, resulting in noncombustion. In such cases, pressure sensors P01, P02 and P03 register only minor pressure fluctuations induced by the gas flow, with peak overpressure reaching approximately 0.0125 kPa, deemed safe for downstream personnel and equipment. Conversely, Figure 3b illustrates an explosion at the pipe outlet, where P01, P02 and P03 detect a swiftly rising pressure wave sequentially. The pressure trend remains consistent, with P01 consistently exhibiting higher peak pressure than P02 and P03 sensors. Over time, pressure gradually diminishes, followed by damped pulsation, with the pressure reaching its peak in approximately 0.5 ms. This indicates that the pressure wave has not yet coalesced into a shock wave. The pressure rise rate remains constant downstream, suggesting that the pressure wave attains maximum strength before reaching the P01 sensor position. Additionally, a certain negative pressure state ensues after the positive pressure of the shock wave. However, our study focuses solely on the positive pressure peak of the initial pressure wave. Figure 3a portrays a combustion scenario characterized by a notably slower pressure rise rate compared to an explosion, albeit with a significantly prolonged duration of overpressure. Pressure waves arise and compound due to the expansion stemming from the hydrogen combustion process. Weak combustion fails to sustain shock and combustion coupling, with the combustion front propagating at subsonic speeds.30 However, the pressure curve derived from our experiment exists in an intermediary state between combustion and explosion. The differentiation between combustion and explosion hinges on the time taken for the overpressure to rise to its peak, the characteristics of the pressure wave, and the observations during the experiment. The underlying factors for these outcomes primarily include hydrogen concentration at the ignition point, jet velocity, and other pertinent variables, as elaborated upon in the subsequent section.

3.2 Effect of Different Ignition Positions at 1.5 MPa Release Pressure

In this paper, the ignition region is divided into two parts: near-field and far-field, and next, the near-field ignition is first analyzed at a discharge pressure of 1.5 MPa.

Figure 4 illustrates the pressure history at the ignition point, which yields the maximum overpressure, along with adjacent ignition points in the radial direction within 0.05 m axially from the nozzle. The maximum explosion overpressure of 2.09 kPa occurs at an axial distance of 0.03 m and radial distance of 0.02 m. In Figure 4a,c, the maximum explosion overpressure generated by the radial ignition positions is depicted in the pressure history. Figure 4a displays the pressure curve of the explosion at a radial distance of 0.03 m and an axial distance of 0.01 m from the ignition point, resulting in a minimal overpressure of only 0.07 kPa. Conversely, Figure 4c illustrates points located further from the axis, beyond the hydrogen jet’s reach, where no ignition occurs. The experimental data indicates that hydrogen ignition and the resulting downstream overpressure postignition are sensitive to the ignition location within 0.05 m from the nozzle axis. This sensitivity arises due to the significant gradient of variation in the hydrogen concentration distribution near the nozzle. The hydrogen jet emerging from the orifice is under-expanded and exhibits a conical structure. In the near-field, the jet’s radial width is narrow, potentially leading to explosions or nonignition occurring at 0.01 m intervals from the ignition position.

Figure 4 1.5 MPa release pressure at the ignition position that generates the maximum overpressure within 0.05 m axially of the orifice and the pressure record of the ignition position on both sides of the radial direction. (a) Ignition position: axial 0.03 m, radial 0.01 m. (b) Ignition position: axial 0.03 m, radial 0.02 m. (c) Ignition position: axial 0.03 m, radial 0.03 m.

Figure 5 depicts the pressure history of the ignition point that produces the maximum overpressure and the pressure history of the ignition points on either side of it in the radial direction at 0.5 m axial ignition points. The maximum overpressure was generated at the 0.5 m axial and 0.1 m radial ignition points. The maximum overpressure for ignition at 0.5 m axial location has increased to 4.39 kPa, which is higher than the maximum overpressure observed at the 0.05 m axial location. In addition, explosions occurred at ignition points on both sides of the radial, but at a much slower rate of pressure rise. The maximum overpressure observed at the axial 0.05 m ignition point location was 0.83 kPa, which was lower compared to the axial 0.15 m ignition point location where the maximum overpressure observed was 1.58 kPa. Based on the hydrogen density function provided by SNL Laboratories,31 the hydrogen concentration and jet velocity decrease hyperbolically as the distance from the ignition position to the nozzle increases. Although all ignition points are located in the combustible region of the hydrogen jet, those at the hydrogen jet–air shear layer have better hydrogen–oxygen mixing and burn more vigorously. Additionally, the vortex generated at the boundary layer where the jet sucks in air increases the level of turbulence and gas flow rate, promoting faster flame propagation of the ignited hydrogen.

Figure 5 1.5 MPa release pressure at the ignition position that generates the maximum overpressure within 0.5 m axially of the orifice and the pressure record of the ignition position on . (a)Ignition position: axial 0.5 m, radial 0.05 m. (b) Ignition position: axial 0.5 m, radial 0.1 m. (c) Ignition position: axial 0.5 m, radial 0.15 m.

As the ignition position moves downstream, the radial location of the ignition point that generates the maximum overpressure remains unchanged at 0.1 m radially, but the trend of the pressure change received by the sensors changes. In Figure 6b, both P01 and P02 experience a sharp increase in pressure almost simultaneously, resulting in peak pressures of approximately 2.25 kPa. The time gaps between the pressures detected by the three sensors depicted in Figure 6a,c are also reduced because the sensor position is fixed in the experiment and the ignition point is much closer to the sensor position. The pressure curves reveal that the pressure waves are more intricate compared to the near-field ignition, with various peaks and longer overpressure durations. Figure 6a illustrates the overpressure line, which rapidly rises to the first peak, drops to negative pressure, and then quickly ascends again, resulting in two peaks. Figure 6c is comparable to Figure 6a, but the initial pressure rise is succeeded by a slower pressure drop after the peak.

Figure 6 1.5 MPa release pressure at the ignition position that generates the maximum overpressure within 1.0 m axially of the orifice and the pressure record of the ignition position on both sides of the radial direction.

At a release pressure of 1.5 MPa, no explosion occurred at the ignition position beyond an axial distance of 1.5, as shown in Figure 7. In this case, ignition only resulted in nonignition and flashover. On one hand, the ignition core propagates downward with the jet flow, resulting in weak effects of flame and pressure waves on the upstream sensors. On the other hand, due to the influence of the large release port (D = 10 mm) and the fixed tank capacity, the hydrogen released in the pipe quickly forms a buoyancy-dominated flow. The hydrogen fuel at the ignition point is diluted, and the high turbulence temperature dissipation of the jet flow under the influence of buoyancy results in insufficient support for strong, stable jet combustion.32 Therefore, the pressure history exhibits high-frequency, low-amplitude oscillations.

Figure 7 Typical nonignition conditions.

3.3 Relationship between Ignition Position and Peak Overpressure

In order to summarize the influence of different ignition positions on the generation of peak overpressure during hydrogen jet combustion and explosion at 1.5 MPa, Figure 8 illustrates the relationship between ignition position and overpressure. Only the maximum overpressure is recorded during explosion or combustion.

Figure 8 Relationship between the overpressure value at a release pressure of 1.5 MPa and the ignition position downstream of the orifice near-field.

All data points in Figure 8 with maximum overpressure values exceeding 1 kPa are marked with the specific overpressure values. It can be observed from the graph that all ignitions at the radial distance of 0.01 m from the jet axis resulted in relatively low overpressure, all below 1 kPa. However, as the ignition points moved downstream, those further away from the axis began to exhibit higher overpressure. The highest overpressure of 4.37 kPa was observed at the axial distance of 0.03 m and radial distance of 0.02 m. Generally, ignition at the jet boundary resulted in higher overpressure compared to ignition within the jet. The distribution of peak overpressure values is closely associated with the position of the jet–air shear layer. Within the near-field range, ignition along the jet axis typically does not lead to detonation due to a lack of mixing and high flow velocity. The influence of the igniter on the flow field may have caused ignition at the axial position of 0.01 m. Moreover, overpressures exceeding 1 kPa only occurred at the jet–air shear layer and were highly sensitive to ignition position changes. The ability of the initial flame to develop within the shear layer is crucial for the stability and overall combustion intensity of hydrogen jet flames. Specifically, as the ignition point moved radially further away by 0.01 m, the ignition outcome changed from producing the highest explosion overpressure at that axial position to no ignition occurring.

3.4 The combustion limit region

The conventional flammability limits of static fuel/air mixtures are insufficient to determine the flammable boundary of a jet flow. To further ascertain the influence of ignition position on whether a hydrogen jet flow can develop into a jet flame, conservative flammable region diagrams for both near-field and far-field ignition scenarios are depicted in the Figure 9. It can be observed from the diagram that the area where the hydrogen jet flow cannot be ignited is divided into two parts. In the near-field region, a small portion near the axis inside the jet flow is also nonflammable. This is because the oxygen content is low along the axis of the nozzle, and except for the igniter placed closely to the nozzle, hydrogen does not ignite under other conditions.

Figure 9 Downstream ignition results under 1.5 MPa release pressure. (a) Near-field ignition results diagram. (b) Far-field ignition results diagram. (c) Overall area ignition results diagram.

As the ignition position moves downstream into the far-field, the influence of the flow field structure weakens, the degree of expansion of the jet flow decreases, and the radial range of the flammable area expands. It can be observed that the axial limit position where hydrogen jet flow can be ignited increases from 0.15 to 0.3 m, and beyond 3 m, the hydrogen jet flow cannot be ignited. Furthermore, as the ignition position approaches 2.5 m, ignition near the jet axis becomes impossible. Equation 1 represents the function of hydrogen jet concentration with distance.1

In the equation, A is an empirical constant; C0 is the initial hydrogen concentration; d0 is the original nozzle diameter; x0 is the displacement of the virtual origin; ρa and ρH2 are the densities of ambient air and hydrogen in the reservoir, respectively. This implies that ideally, the concentration decreases hyperbolically with distance, and at 3 m, it has already fallen below the flammable range. Compared to ignition experiments by others, although the nozzle diameter and release pressure vary, the overall shape of the ignition probability diagram that can cause downstream ignition is similar to studies by others.33

3.5 Influence of Axial Ignition Position on Overpressure and Flame Propagation Process

To explore the variation in overpressure patterns at the same ignition point under different release pressures, Figure 10 presents the overpressure development history of ignition at axial positions of 0.05, 0.5 and 1.0 m under release pressures of 6, 7, 8 and 9 MPa. As the release pressure changes, the ignition position causing the maximum overpressure also changes accordingly, and hydrogen ignition occurs in all cases depicted in the figure. At a release pressure of 6 MPa, the maximum overpressure occurs at the ignition point of 1.0 m along the axial line, reaching nearly 8 kPa, while ignition at 0.05 and 0.5 m only results in relatively weak overpressure. Under release pressures of 7, 8 and 9 MPa, ignition at 0.5 m becomes the lowest point of overpressure, with the maximum positive overpressure peaks all below 2 kPa, lower than those at 0.05 and 1.0 m ignition points. For all four release pressures, the overpressure curves at the 1.0 m axial ignition point exhibit more complex waveforms. At the 1.0 m ignition point, the resulting overpressure is relatively low, and the jet velocity diminishes. The influence of combustion and flow instabilities becomes more significant. Specifically, at this point, there are two distinct overpressure peaks observed during the deflagration process. The instability caused by multiple deflagration centers within the jet leads to the overall maximum overpressure not necessarily occurring at the second peak. Additionally, the duration of the overpressure is prolonged. As the ignition point moves further downstream, both the jet velocity and flame speed decrease, the hydrogen concentration reduces, and turbulence intensifies, making the leading edge of the jet flame more unstable. This results in the generation of multidimensional flame pressure waves, leading to more complex pressure waveforms.

Figure 10 Pressure history diagram for ignition at three positions under 6, 7, 8 and 9 MPa release pressures. (a) 6 MPa release pressure. (b) 7 MPa release pressure. (c) 8 MPa release pressure. (d) 9 MPa release pressure.

Utilizing a high-speed camera set at a frame rate of 1000 frames per second, the flame propagation at the nozzle outlet was captured (Figure 11). From the imagery, the spread of the jet flame can be delineated into four discernible stages. Time zero is defined as the moment when the flame becomes visible. At 45 ms, the flame is observed to propagate both upstream and downstream from the ignition point, with a notably faster propagation rate upstream, marking the first stage of flame development. In the second stage, the front of the flame forms a fireball characterized by intense internal combustion, resulting in significant disparities in physical parameters at the flame boundary, thereby inducing instability effects and the emergence of large-scale vortices. Subsequently, in the third stage, these vortices further develop, pushing the fireball away from the primary flame body, leading to flame separation. The fourth stage signifies the establishment of a stable self-sustaining flame. Additionally, the flame intensity is weaker upstream of the ignition point and stronger downstream, indicating a facilitating effect of the continued open state of the ignition device on combustion.

Figure 11 Flame propagation process diagram (release pressure 1.5 MPa, ignition position axis 1.0 m).

The flame propagation process elucidates the intricacies observed in the pressure waveform at the axial position of 1.0 m in Figure 6. Figure 12 provides a comparative analysis of the flame propagation process at ignition positions of 0.5 and 1.0 m, with a time interval of 30 ms between the images. Transitioning from the 0.5 to the 1.0 m ignition position induces changes in flame brightness and morphology. At the same time as the formation of the fireball and vortices (at 110 ms), the flame image from the ignition at 1.0 m reveals the formation of larger-scale vortices at the flame jet boundary, propelling the flame radially further outward. Conversely, flames ignited at 1.0 m exhibit more pronounced axial asymmetry during the development stage, attributed to the destabilizing effects, resulting in a more complex flame pressure waveform. Additionally, as a reflection of combustion intensity, the flame brightness at 1.0 m ignition is weaker compared to that at 0.5 m ignition, with characteristics indicative of buoyancy influence evident during the stable flame stage (at 200 ms).

Figure 12 Comparison of the flame propagation process at different ignition positions under 1.5 MPa release pressure.

It is noteworthy that rare instances of extremely high explosion overpressure were observed in multiple experiments. In Figure 10c, under an 8 MPa release pressure, ignition at the nozzle at 0.05 m resulted in an overpressure of up to 30 kPa at P01. Similar occurrences were observed at the same ignition point under a 5 MPa release pressure. According to references on the damage caused by physical explosions under different levels of overpressure, this magnitude of overpressure can result in significant damage to steel structures,34,35 surpassing the threshold for causing fatalities,36 as detailed in Tables 1 and 2. This implies that not only is the downstream of the jet affected by extreme explosion overpressure, but also a certain radial distance on both sides of the axis is within the range affected by the hydrogen jet explosion.

Table 1 Pressure vs. Expected Damage Relationship

overpressure (kPa)	expected damages	
1.0–1.5	window glass cracks	
3.5–7.6	minor damage in some buildings	
7.6–12.4	metal panels deformed	
12.4–20.0	concrete walls damage	
>35.0	wooden construction buildings demolition	
27.5–48.0	major damage on steel construction objects	
40.0–60.0	heavy damage on reinforced concrete buildings	
70.0–80.0	probable demolition of most buildings	

Table 2 Threshold Values for the Five Severity Levels

 	damage area	
peak air overpressure and secondary fragmentation (people in the structures): distance were the positive peak overpressure reaches	highly lethal	lethal boundary	irreversible injury	reversible injury	domino effect	
 	55 kPa	24 kPa	16 kPa	8 kPa	2.75 kPa	

The exact reason for the occurrence of violent explosions at release pressures of 5 and 8 MPa remains uncertain. One possible explanation is the transient behavior of the gas flow during the valve opening process. The valve opening is not instantaneous; the time taken for the valve to fully open is set to 100 ms. This gradual opening process allows for better mixing of hydrogen and air inside the pipe, forming high-concentration flammable gas clouds. Upon ignition at the outlet, the igniter initially ignites a small amount of premixed gas at the front edge of the jet, leading to deflagration of the premixed gas and subsequently igniting the trailing hydrogen jet. The disturbance caused by the initial ignition promotes turbulence in the jet–air mixture, resulting in a stronger explosion shockwave.

However, this phenomenon is not observed when the ignition point is at the 0.5 m position, as the concentration distribution of the jet follows an inverse proportional law. The reason for the occurrence of such high explosion overpressure warrants further investigation. The appearance of this high explosion overpressure is alarming and provides valuable insights for setting protection distances for high-pressure hydrogen release pipes or torch system outlets. The consequences of accidental hydrogen release and ignition through pipelines may result in significant overpressure hazards near the pipe outlet.

3.6 Influence of Release Pressure and Ignition Position on Overpressure Peaks

To investigate the effect of different hydrogen release pressures on explosion overpressure, experiments were conducted with a fixed ignition position while varying the release pressure. Figure 13 shows the peak overpressure received by sensor P01 for release pressures ranging from 5 to 10 MPa, with the electrode igniter located at axial distances of 0.05, 0.5, 1 and 1.5 m. From the graph, it can be observed that the maximum overpressure generated by jet ignition is highly sensitive to the ignition position. At the axial ignition position of 0.05 m, significant variations in overpressure are observed, with the highest overpressure reaching 30 kPa at release pressures of 5 and 8 MPa, while at other release pressures, the maximum overpressure is distributed at 4 kPa and below. For ignition at 0.05 m axial distance, the pattern of overpressure remains consistently below 3 kPa. However, ignition at 0.5 m and downstream positions shows a trend of increasing overpressure followed by a decrease for each release pressure. At the axial ignition position of 0.5 m, except for the case of 10 MPa release pressure where the maximum overpressure exceeds 2 kPa, the maximum overpressure is below 2 kPa for other release pressures. As the ignition point shifts to 1.0 m, the maximum overpressure increases, but there is no clear correlation between the maximum overpressure and release pressure; the maximum overpressure ranges from 7.7 kPa for 6 MPa release pressure to 2.7 kPa for 5 MPa release pressure. When the ignition point is at 1.5 m, hydrogen remains unignited in all cases. In conclusion, there is no apparent pattern between release pressure and peak overpressure; except for two cases of extremely high overpressure, the ignition position has a more significant impact on overpressure. Ignition at the axial position of 0.5 m results in the lowest overpressure for most release pressures, and ignition at the axial position of 1.5 m results in no ignition for all release pressures.

Figure 13 Diagram of the effect of release pressure and ignition position on maximum overpressure.

4 Conclusions

In this study, three pressure sensors were placed downstream of the hydrogen release nozzle at radial distances of 0.5 m and axial distances of 1, 1.5 and 2 m. The experiment investigated the ignition results and overpressure peaks of horizontal hydrogen jets at different downstream positions under various release pressures, coupled with observations of flame propagation using a high-speed camera. The main conclusions are as follows:1 Under a release pressure of 1.5 MPa, ignition near the boundary layer of the jet results in higher downstream overpressure, while ignition within the jet’s interior leads to lower overpressure. The region where the hydrogen jet cannot be ignited is divided into two parts.

2 At different release pressures, the overpressure from ignition at the axial position of 0.5 m is lower than at 0.05 and 1.0 m. As the ignition position moves downstream, flame instability increases, resulting in more complex pressure waveforms.

3 Regarding different release pressures and ignition positions, the correlation between the maximum overpressure and release pressure is weak, with the maximum overpressure being more sensitive to the ignition position. In some individual cases at the nozzle, peak overpressure values of up to 30 kPa were observed.

Author Contributions

⊥ D.X. and L.L. to this article are equal.

The authors declare no competing financial interest.

Acknowledgments

The authors gratefully acknowledge the support provided by the National Natural Science Foundation of China (nos 52074158 and 52304230), Ningxia Autonomous Region Key Research and Development Program (no. 2022BEE02001), National Key Research and Development Program of intergovernmental international scientific and technological cooperation and innovation special project (no. 2023YFE0199100), Shenzhen Science and Technology Program (ZDSYS20210929115800001), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (no.22KJB620004), State Key Laboratory of Fire Science Open Project (No.HZ2023-KF11), State Key Laboratory Cultivation Base for Gas Geology and Gas Control (No.WS2022A06), Shenzhen Urban Public Safety and Technology Institute, and Key Laboratory of Urban Safety Risk Monitoring and Early Warning, Ministry of Emergency Management.
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