A pressure-ratio equivalent method for ultra-high pressure hydrogen spontaneous ignition experiment

Increasing hydrogen storage pressure brings high economic benefits and high risks. Pressurized hydrogen leakage spontaneous ignition experiment is an important means to reveal the mechanism of hydrogen leakage spontaneous ignition and improve the safety of hydrogen storage equipment. However, due to the extremely high cost and danger of ultra-high pressure, there is a serious lack of experimental data. In this paper, a pressure-ratio equivalent (PRE) method of experiments is proposed based on the theory of the shock tube problem. By keeping the hydrogen-air pressure ratio constant while reducing the absolute pressure of air and hydrogen, the difficulty of the experiment is greatly reduced. The effectiveness of the PRE method is evaluated theoretically and experimentally. The results show the PRE method retains the ignition characteristics of hydrogen leakage spontaneous ignition largely when the air pressure is within 0.05–0.1 MPa. It is found the pressure ratio of hydrogen to air dominates the leakage spontaneous ignition process. In the experiments of different air pressures, the shock Mach numbers are close to theoretical values. In addition, leakage spontaneous ignition of hydrogen mixed with 30% (vol.) CO is found in experiments using the PRE method, with pressure ratios of up to 250. This indicates that when the storage pressure is high enough, there is also a risk of spontaneous ignition of syngas from high-pressure leakage. The PRE method can widely broaden the pressure scope of experimental research on leakage spontaneous ignition, and it provides a new idea for obtaining the experimental data of gas high-pressure leakage spontaneous ignition.     

Experimental study of spontaneous ignition induced by sudden hydrogen release through tubes with different shaped cross-sections

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.     

Numerical simulation of the effect of multiple obstacles inside the tube on the spontaneous ignition of high-pressure hydrogen release

Self-ignition may occur during hydrogen storage and transportation if high-pressure hydrogen is suddenly released into the downstream pipelines, and the presence of obstacles inside the pipeline may affect the ignition mechanism of high-pressure hydrogen. In this work, the effects of multiple obstacles inside the tube on the shock wave propagation and self-ignition during high-pressure hydrogen release are investigated by numerical simulation. The RNG k-ε turbulence model, EDC combustion model, and 19-step detailed hydrogen combustion mechanism are employed. After verifying the reliability of the model with experimental data, the self-ignition process of high-pressure hydrogen release into tubes with obstacles with different locations, spacings, shapes, and blockage ratios is numerically investigated. The results show that obstacles with different locations, spacings, shapes and blockage ratios will generate reflected shock waves with different sizes and propagation trends. The closer the location of obstacles to the burst disk, the smaller the spacing, and the larger the blockage ratio will cause the greater the pressure of the reflected shock wave it produces. Compared with the tubes with rectangular-shaped, semi-circular-shaped and triangular-shaped obstacles, self-ignition is preferred to occur in tube with triangular-shaped obstacles.     

Experimental investigation of shock wave propagation,spontaneous ignition,and flame development of high-pressure hydrogen release through tubes with different obstacles arrangements

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.     

Experimental investigation on effects of CO2 additions on spontaneous ignition of high-pressure hydrogen during its sudden release into a tube

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 CO₂ 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 CO₂ additions under the close burst pressures. For cases with more CO₂ 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% CO₂ additions. The minimum burst pressure required for spontaneous ignition increases from 4.37 MPa (0% CO₂) up to 6.41 MPa (15% CO₂). With the increasing of CO₂ 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 CO₂ to air have a good suppressing effect on hydrogen spontaneous ignition.     

Spontaneous ignition of high-pressure hydrogen during its sudden release into hydrogen/air mixtures

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% H₂) 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.     

Effects of initial diaphragm shape on spontaneous ignition of high-pressure hydrogen in a two-dimensional duct

A two-dimensional (2-D) simulation of spontaneous ignition of high-pressure hydrogen in a length of duct is conducted to explore ignition mechanisms. The present study adopts a 2-D rectangular duct and focuses on effects of the initial diaphragm shape on spontaneous ignition. The Navier–Stokes equations with a detailed chemical kinetics mechanism are solved in a manner of direct numerical simulation. The detailed mechanisms of spontaneous ignitions are discussed for each initial diaphragm shape. For a straight diaphragm, ignition only occurs near the wall owing to the adiabatic wall condition, while three ignition events are identified for a greatly deformed diaphragm: ignition due to reflection of leading shock wave at the wall, hydrogen penetration into shock-heated air near the wall, and deep penetration of hydrogen into shock-heated air behind the leading shock wave.     

Numerical study of spontaneous ignition of pressurized hydrogen release into air

Numerical simulations have been carried out for spontaneous ignition in the sudden release of pressurized hydrogen into air. A mixture-averaged multi-component approach was used for accurate calculation of molecular transport. Spontaneous ignition and combustion chemistry were accounted for using a 21-step kinetic scheme. To reduce false numerical diffusion, extremely fine meshes were used along with the arbitrary Lagrangian–Eulerian (ALE) method in which convective terms are solved separately from the other terms.     

Experimental investigation of flame and pressure dynamics after spontaneous ignition in tube geometry

Spontaneous ignition processes due to high pressure hydrogen releases into air are known phenomena. The sudden expansion of pressurized hydrogen into a pipe, filled with ambient air, can lead to a spontaneous ignition with a jet fire. This paper presents results of an experimental investigation of the visible flame propagation and pressure measurements in 4 mm extension tubes of up to 1 m length attached to a bulk vessel by a rupture disc. Transparent glass tubes for visual observation and shock wave pressure sensors are used in this study. The effect of the extension tube length on the development of a stable jet fire after a spontaneous ignition is discussed.     

Mechanism of spontaneous ignition of high-pressure hydrogen during its release through a tube with local contraction: A numerical study

The tendency of spontaneous ignition of high-pressure hydrogen during its sudden release into a tube is one of the main threats to the safe application of hydrogen energy. A series of investigations have shown that the tube structure is a key factor affecting the spontaneous ignition of high-pressure hydrogen. In this paper, a numerical study is conducted to reveal the mechanism of spontaneous ignition of high-pressure hydrogen inside the tube with local contraction. Large Eddy Simulation, Renormalization Group, Eddy Dissipation Concept, 37-step detailed hydrogen combustion mechanism and 10-step like opening process of burst disk are employed. Three cases with burst pressures of 3.10, 4.90, and 8.45 MPa are simulated to compare against the pervious experimental study. The spontaneous conditions and positions agree well with the experimental results. The numerical results indicate that shock wave reflection takes place at the upstream vertical wall of contraction part. The interacted-shock-affected region is generated at the tube center because of the subsequent shock wave interaction. The forward reflected shock wave couples with normal shock wave and increases the pressure of leading shock wave. The sudden contraction of tube blocks the propagation of hydrogen jet and decreases the speed from supersonic flow to subsonic flow. More flammable mixture is generated inside the contraction part, as a results, the length of the flame is increased. Two mechanisms are proposed finally.     

Numerical simulation on the spontaneous ignition of high-pressure hydrogen release through a tube at different burst pressures

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, HO₂) 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.     

Experiment study on the pressure and flame characteristics induced by high-pressure hydrogen spontaneous ignition

A series of experiments were conducted to study the pressure and combustion characteristics of the high-pressure hydrogen during the occurrence of spontaneous ignition and the conversion from spontaneous ignition to a jet fire and explosion. Different initial conditions including release pressure (4–10 MPa), tube diameter (10/15 mm), and tube length (0.3/0.7/1.2/1.7/2.2/3 m) were tested. The variation of the pressure and flame signal inside and outside of the tube and the development of the jet flame were recorded. The experimental results revealed that the minimum ignition pressure required for self-ignition of hydrogen at different tube diameters decreased first and then increased with the extension of tubes. The minimum ignition pressure for tubes diameters of 10 mm and 15 mm is no more than 4 MPa and the length of the tubes is L = 1.7 m. The minimum release pressure required for spontaneous ignition of a tube D = 15 mm is always lower than that of a tube D = 10 mm at the same tube length. When the spontaneous ignition occurred, it did not absolutely trigger the jet fire. The transition from spontaneous ignition to a jet fire must go through the specific stages.     

The effect of tube internal geometry on the propensity to spontaneous ignition in pressurized hydrogen release

Spontaneous ignition of compressed hydrogen release through a length of tube with different internal geometries is numerically investigated using our previously developed model. Four types of internal geometries are considered: local contraction, local enlargement, abrupt contraction and abrupt enlargement. The presence of internal geometries was found to significantly increase the propensity to spontaneous ignition. Shock reflections from the surfaces of the internal geometries and the subsequent shock interactions further increase the temperature of the combustible mixture at the contact region. The presence of the internal geometry stimulates turbulence enhanced mixing between the shock-heated air and the escaping hydrogen, resulting in the formation of more flammable mixture. It was also found that forward-facing vertical planes are more likely to cause spontaneous ignition by producing the highest heating to the flammable mixture than backward-facing vertical planes.     

Physics of spontaneous ignition of high-pressure hydrogen release and transition to jet fire

The aim of this study is to gain an insight into the physical phenomena underlying the spontaneous ignition of hydrogen following a sudden release from high-pressure storage and transition to sustained jet fire. The modelling and large-eddy simulation (LES) of the spontaneous ignition dynamics in a tube with a non-inertial rupture disk separating the high-pressure hydrogen storage and the atmosphere is described. Numerical experiments confirmed that due to the stagnation conditions a chemical reaction first commences in the tube boundary layer, and subsequently propagates throughout the tube cross-section. The dynamics of flame formation outside the tube, simulated by the LES model, has reproduced the combustion patterns, including vortex induced “flame separation”, which have been experimentally observed by high-speed photography. It is concluded that the LES model can be applied for hydrogen safety engineering, e.g. for the development of innovative pressure relief devices.     

Numerical study on spontaneous ignition of pressurized hydrogen release through a length of tube

The issue of spontaneous ignition of highly pressurized hydrogen release is of important safety concern, e.g. in the assessment of risk and design of safety measures. This paper reports on recent numerical investigation of this phenomenon through releases via a length of tube. This mimics a potential accidental scenario involving release through instrument line. The implicit large eddy simulation (ILES) approach was used with the 5th-order weighted essentially non-oscillatory (WENO) scheme. A mixture-averaged multi-component approach was used for accurate calculation of molecular transport. The thin flame was resolved with fine grid resolution and the autoignition and combustion chemistry were accounted for using a 21-step kinetic scheme.The numerical study revealed that the finite rupture process of the initial pressure boundary plays an important role in the spontaneous ignition. The rupture process induces significant turbulent mixing at the contact region via shock reflections and interactions. The predicted leading shock velocity inside the tube increases during the early stages of the release and then stabilizes at a nearly constant value which is higher than that predicted by one-dimensional analysis. The air behind the leading shock is shock-heated and mixes with the released hydrogen in the contact region. Ignition is firstly initiated inside the tube and then a partially premixed flame is developed. Significant amount of shock-heated air and well developed partially premixed flames are two major factors providing potential energy to overcome the strong under-expansion and flow divergence following spouting from the tube.Parametric studies were also conducted to investigate the effect of rupture time, release pressure, tube length and diameter on the likelihood of spontaneous ignition. It was found that a slower rupture time and a lower release pressure will lead to increases in ignition delay time and hence reduces the likelihood of spontaneous ignition. If the tube length is smaller than a certain value, even though ignition could take place inside the tube, the flame is unlikely to be sufficiently strong to overcome under-expansion and flow divergence after spouting from the tube and hence is likely to be quenched.     

Experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube

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.     

Mechanism of high-pressure hydrogen auto-ignition when spouting into air

High-pressure hydrogen leak is one of the top safety issues presently. This study elucidates numerically the physics and mechanism of high-pressure hydrogen jet ignition when the hydrogen suddenly spouts into the air through a tube. The direct numerical simulation based on the compressible fluid dynamics was carried out. When high-pressure hydrogen is passing through the tube filled by atmospheric air, a strong shock wave is formed and heats up hydrogen behind the shock wave by compression effect. The leading shock wave is expanded widely after the tube exit, auto-ignitions of hydrogen occur. When the tube becomes longer, the tendency of auto-ignition is increased. Other type of auto-ignitions is also predicted. An explosion is also occurred in the tube under a certain condition. Vortices are generated behind the shock wave in a long tube. There is a possibility of an auto-ignition induced by vortices.     

Experimental study on spontaneous ignition and flame propagation of high-pressure hydrogen release through tubes

An experimental study was conducted to research the mechanism of spontaneous ignition induced by high-pressure hydrogen release through tubes with a diameter of 10 mm and varying lengths from 0.3 to 3 m. The pressure and light signals inside the tube were collected. The propagation of shock wave inside and outside the tube was also systematically investigated. The development process of the jet flame in the atmosphere was completely recorded, and the multiple Mach disks at the tube exit were observed by using a high-speed camera. The results show that the minimum release pressure, at which the jet flame is formed, is found to be 3.87 MPa with the tube length of 1.7 m. When the tube length was longer than 1.7 m, the critical pressure for forming jet flame increased rapidly. The velocity attenuation of the shock wave is mainly affected by the burst pressure but not sensitive to the tube length, and the flame propagates in the tube at a slower velocity than the shock wave. The compression of the hydrogen-air mixture by the Mach disk causes it to burn more violently after passing through the Mach disk. It is confirmed that the flame at the tube exit is lifted in the atmosphere, then a jet flame initiates behind the second Mach disk.     

Effects of turbulence on nonpremixed ignition of hydrogen in heated counterflow

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet. In contrast to pseudo-turbulent flows, where turbulence was generated by only a perforated plate on the fuel side, resulting in little effect on ignition in a hydrogen system, fully turbulent flows with perforated plates on both sides of the flow were found to produce noticeable effects. The difference was attributed to the fact that in fully turbulent flows, a significantly larger range of turbulent eddies extend to smaller scales than in pseudo-turbulent flows. At atmospheric pressure, the lowest turbulence intensity studied had ignition temperatures notably lower than laminar ones, while further increases in turbulence intensity resulted in rising ignition temperatures. As a result, optimal conditions for nonpremixed hydrogen ignition exist in weakly turbulent flows where the ignition temperature is lower than can be obtained in other laminar or turbulent flows at the same pressure. Similar trends were seen for all fuel concentrations and at all pressures in the second ignition limit (below 3-4 atm). At higher pressures, turbulent flows caused the ignition temperatures to continue to follow the second limit resulting in ignition temperatures higher than the laminar values. The extension of the second limit ends at the highest pressures (7 to 8 atm) where evidence of third limit behavior appears. Three mechanisms were noted to explain the experimental results. First, turbulent eddies similar in size to the ignition kernel can promote discrete mixing of otherwise isolated pockets of gas. Second, this mixing can promote HO₂ chain branching pathways, which can account for the enhanced ignition noted in the second limit where reaction is governed by crossover temperature chemistry. Third, turbulence limits the excursion times available for reaction, inordinately affecting the slower HO₂ reactions. This is responsible for the increasing ignition temperature with turbulence intensity and pressure.     

Spontaneous ignition of high-pressure hydrogen and boundary layer characteristics in tubes

Hydrogen is efficient and environmentally friendly, but the danger of self-ignition resulted from the leakage of high-pressure hydrogen cannot be ignored. In this work, the self-ignition of high-pressure hydrogen released through different conditions was studied. 700-mm-length tubes with different diameters were adopted in our experiments. It summarized the characteristics of shock waves' attenuation and evolution process of hydrogen jets in tubes. In addition, effects of the boundary layer on the leading shock waves, the contact surface, and expansion waves were discussed. Results indicate that minimum pressure when self-ignition occurs for 15-mm-diameter tube is similar to 10-mm-diameter. And they have closely velocity of shock waves. Simulations show that the greater the release pressure, the more ignition products of hydrogen. Higher release pressure and smaller diameter can create a thicker boundary layer in micro shock tubes, and the boundary layer can lead to a change in the velocity of shock waves’ structures.