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.     

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.     

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.     

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.     

Visualization of spontaneous ignition under controlled burst pressure

A high-pressure hydrogen jet released into the air has the possibility of igniting in a tube without any ignition source. The mechanism of this phenomenon, called spontaneous ignition, is considered to be that hydrogen diffuses into the hot air caused by the shock wave from diaphragm rupture and the hydrogen-oxidizer mixed region is formed enough to start chemical reaction. Recently, flow visualization studies on the spontaneous ignition process have been conducted to understand its detailed mechanism, but such ignition has not yet been well clarified. In this study, the spontaneous ignition phenomenon was observed in a rectangular tube. The results confirm the presence of a flame at the wall of the tube when the shock wave pressure reaches 1.2–1.5 MPa in more than 9 MPa burst pressure and that ignition occurs near the wall, followed by multiple ignitions as the shock wave propagates, with the ignitions eventually combining to form a flame.     

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.     

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.     

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.     

基于扩散点火理论的高压氢气泄漏自燃研究

运用扩散点火理论对高压氢气泄漏到下游管道内的自燃点火情况进行了分析。利用激波管流动理论讨论了氢气射流前端激波加热区域的参数变化情况,分析了前沿激波强度、均匀区压力和温度与初始压力的关系,给出了高压氢气泄漏到下游管道后,预测前沿激波强度、均匀区压力和温度的数学方程,建立了判断高压氢气泄漏到下游管道内是否发生自燃点火的函数表达式。提出了理论点火临界压力的概念,计算发现氢气的理论点火临界压力明显低于其他几种常见的气体燃料。讨论了影响泄漏自燃发生的可能因素,结果可为预防高压储氢泄漏自燃提供科学依据。     

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.     

Large eddy simulation of laser ignition and compressible reacting flow in a rocket-like configuration

The control of ignition in a rocket engine is a critical problem for combustion chamber design. Delayed ignition may lead to high-amplitude pressure fluctuations that can damage the burner (strong ignition), whereas early ignition may fail. This paper describes a numerical study of a strong ignition sequence observed in a laboratory-scale single-injector rocket chamber ignited by a laser and fueled with gaseous oxygen and hydrogen. OH-emission images, Schlieren pictures, and pressure measurements make it possible to follow the flame propagation experimentally. The present large eddy simulation (LES) approach includes shock treatment, a six species-seven reaction chemical scheme for H₂-O₂, and a model for the energy deposition by a laser. Flame/turbulence interaction is modeled with the thickened flame concept. LES is used to compute both the filling phase (during which the gaseous hydrogen and oxygen mix) and the ignition phase. The flame location and structure, as well as the temporal evolution of the chamber pressure obtained numerically, are in good agreement with the experiment. The use of complex chemistry in the computation also allows the comparison of LES data with experimental OH-images and shows that the sensitivity of the CCD camera used to record the spontaneous emission of the OH^∗ radical is not high enough to properly locate the flame front in rich regions. The combined experimental and numerical results lead to a more detailed analysis of the ignition processes and its coupling with flow rate oscillations in the H₂ and O₂ feeding lines.     

Effects of congestion and confining walls on turbulent deflagrations in a hydrogen storage facility-part 2: Numerical study

Safety studies for hydrogen retail stations involve identification of possible accidental scenarios, modelling of consequences and measures to mitigate associated hazards with it. Accidental release of hydrogen during its handling and storage can lead to formation of ignitable mixture in a very short time. Ignition of such a mixture can lead to generation of overpressure affecting structure and people. Understanding of the possible overpressures generated is critical in designing the system safe from explosion hazards. In the present study, the worst-case scenario where high-pressure hydrogen storage cylinders are enveloped by a premixed hydrogen-air cloud is numerically simulated. The computational domain mimics the setup for premixed hydrogen cloud in a mock hydrogen cylinder storage congestion environment experimentally studied by Shirvill et al. [1]. Large Eddy Simulations (LES) are performed using OpenFOAM CFD toolbox solver. The Flame Surface Wrinkling Model in LES context is used for modelling deflagrations [2]. Numerical simulation results are compared against experiments. Simulations are able to predict experimental flame arrival and overpressure reasonably well. The effects of ignition location, congestion and confining walls on the turbulent deflagrations in particular on explosion overpressure are discussed. It was concluded that explosion overpressure increases with increase in confinement.     

Numerical study on unsteady characteristics of high-pressure hydrogen jet ejected from a pinhole

The aim of this study was to delineate the unsteady fluid dynamics of the high-pressure hydrogen jet to clarify the relationship between the forced ignition position and the flame development characteristics in a high-pressure hydrogen jet leaking from a pinhole. The Navier–Stokes equation for a compressible multi-component gas was used to simulate a high-pressure (82 MPa stagnation pressure) unsteady hydrogen jet ejected into the atmosphere through a pinhole (diameter = 0.2 mm). The results indicated that the flapping jet at the base of the jet formed a cloud of highly concentrated hydrogen that flowed downstream. A correlation was observed between the spatio-temporal distribution of hydrogen concentration and velocity was observed. The unsteady high-pressure hydrogen jet obtained by simulation will be used in subsequent studies focusing on flame development under forced ignition.     

Influence of tube cross-section geometry on high-pressure hydrogen-flow-induced self-ignition

Self-ignition within a cylindrical tube that discharges high-pressure hydrogen results in flame formation. Because rectangular tubes are used to visualize fuel-flow dynamics, the influence of the tube cross-section on the self-ignition characteristics is investigated. As experimental investigation of the mechanisms underlying the self-ignition phenomenon in cylindrical tubes is difficult, three-dimensional numerical simulation is employed. Following the bursting of diaphragm by high-pressure hydrogen with a storage pressure of 9.0 MPa and a temperature of 300 K, initial self-ignition occurs at the center of the rectangular tube sidewall. This is because of the mixing of air and hydrogen induced by the bow-shock reflection-generated jet flow and resulting adiabatic shock compression-induced temperature increase. This temperature rise induces a secondary self-ignition at the tube corners. In the cylindrical tube, a solitary ring-shaped self-ignition occurs near the sidewall. The flame evolutions in rectangular and cylindrical tubes reveal similar flame-spreading trends, which indicates similar bow-shock reflection-induced self-ignition mechanisms.     

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.     

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 ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

Effects of the geometry of downstream pipes with different angles on the shock ignition of high-pressure hydrogen during its sudden expansion

To investigate the effects of the geometry of downstream pipes on the shock ignition and the formation of the shock waves during high-pressure hydrogen sudden expansion, a series of bench-mark experiments were designed and high-pressure hydrogen were released into five types of pipes with different angles (60, 90, 120, 150 and 180°). It was found that the geometry of downstream pipes had a significant influence on the shock ignition of hydrogen. The incident shock wave would be reflected at the corner of the pipes with angles of 60, 90, 120 and 150°. The intensity of the reflected shock wave is higher if the angle is smaller. In addition, the average velocity of the leading incident shock wave would decrease when it passed the corner of the pipe. Using a pipe with smaller angle significantly increases the likelihood of shock ignition and lowers the minimal required burst pressure for shock ignition. The overpressure of the incident shock waves inside the exhaust chamber (for the cases with the angles of 60, 90, 120 and 150°) decreases sharply. There are three flame propagation behaviors inside the exhaust chamber: flame quenching, flame separation and no flame separation. The results of this study have implications concerning designs for storage safety of hydrogen energy and may help get better understanding of shock ignition mechanism of high pressure hydrogen and effect of pipeline geometry on ignition.     

An experimental study of the effect of 2.5% methane addition on self-ignition and flame propagation during high-pressure hydrogen release through a tube

This paper demonstrates experimental investigation on the self-ignition and subsequent flame propagation of high-pressure hydrogen-methane mixture release via a tube. The proportion of methane added to hydrogen is 2.5% (vol.). A transparent rectangular tube (d = 15 mm, L = 400 mm) is used in the experiments. It is shown that the minimum burst pressure required for self-ignition increases 1.57 times for only 2.5% methane addition from 2.89 MPa (pure hydrogen) up to 4.68 MPa (2.5% CH₄ addition). This is mainly caused by the following reasons: on the one hand, methane addition can result in the decease of shock intensity inside the tube, thereby lowering the temperature of the combustible mixture; on the other hand, the hydrogen-methane mixture has the higher minimum ignition energy than that of pure hydrogen. Besides, 2.5% methane addition can increase the initial ignition time, weaken the flame intensity and reduce the flame propagation velocity relative to tube wall inside the tube. Moreover, for cases with 2.5% methane addition, the complete flame throughout the tube is formed closer to the back end of the tube. When the self-sustained flame exits from the tube, the maximum overpressure in a confined space increases with 2.5% methane addition.