Experimental and numerical study on spontaneous ignition of hydrogen and hydrogen-methane jets in air

This paper is an investigation of the spontaneous ignition process of high-pressure hydrogen and hydrogen-methane mixtures injected into air. The experiments were conducted in a closed channel filled with air where the hydrogen or hydrogen–methane mixture depressurised through different tubes (diameters d = 6, 10, and 14 mm, and lengths L = 10, 25, 40, 50, 75 and 100 mm). The methane addition to the mixture was 5% and 10% vol. The results showed that only 5% methane addition may increase even 2.67 times the pressure at which the mixture may ignite in comparison to the pressure of the pure hydrogen flow. The 10% of methane addition did not provide an ignition for burst pressures up to 15.0 MPa in the geometrical configuration with the longest tube (100 mm). Additionally, the simulations of the experimental configuration with pure hydrogen were performed with the use of KIVA numerical code with full kinetic reaction mechanism.     

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.     

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.     

Effects of CO addition on shock wave propagation,self-ignition,and flame development of high-pressure hydrogen release into air

In this study, an experimental investigation is conducted to study the effects of CO addition (0%–7.5% vol.) on the self-ignition of pressurized hydrogen leakage. The result shows that more CO addition significantly decreases the likelihood of self-ignition inside the tube and the formation of self-sustained jet flame outside the tube. This can be mostly explained by the reduction of leading shock intensity inside the tube. Furthermore, CO addition effectively inhibits the flame propagation and development inside the tube. For pure hydrogen, the ignited flame quickly develops an intense flame spanning the tube width and eventually form a jet flame in ambient air. However, the H₂–CO diffusion flame initiated approaching tube sidewall tends to propagate adjacent to the wall or be soon quenched with more CO addition. If the CO-weakened flame spouts to the tube exit, it may not survive the expansion at the tube exit and thus it is quenched.     

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.     

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 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.     

Experimental study on explosion characteristics of syngas with different ignition positions and hydrogen fraction

The influence of different ignition positions and hydrogen volume fractions on the explosion characteristics of syngas is studied in a rectangular half-open tube. Three ignition positions were set at the axis of the tube, which are 0 mm, 600 mm and 1100 mm away from the closed end, respectively. A range of hydrogen volume fraction (φ) from 10% to 90% were concerned. Experimental results show that different ignition positions and hydrogen volume fraction have important influence on flame propagation structure. When ignited at 600 mm from the closed end on the tube axis, distorted tulip flame forms when flame propagates to the closed end. The formations of the tulip flame and the distorted tulip flame are accompanied by a change in the direction of the flame front propagation. The flame propagation structure and pressure are largely affected by the ignition position and the hydrogen volume fraction. At the same ignition position, flame propagation speed increases with the growing of hydrogen volume fraction. And the pressure oscillates more severe as the ignition location is closer to the open end. And pressure oscillations bring two different forms. The first form is that the pressure has a periodic oscillation. The amplitude of the pressure oscillation gradually increases. It takes several cycles from the start of the oscillation to the peak. For the second form, the pressure reaches the peak of the oscillation in the first cycle of the start to the oscillation.     

Research progress on the self-ignition of high-pressure hydrogen discharge: A review

As one of the most promising fossil energy substitutes, hydrogen energy is receiving increasing attention, and it has been greatly developed in recent years. However, hydrogen safety issues limit the large-scale application of hydrogen energy. Since 1922, the issue of self-ignition of high-pressure hydrogen discharge has gradually become the focus of attention of scholars in the field of hydrogen energy. Particularly fruitful research results have been obtained in the past 20 years, showing that the minimum discharge pressure of hydrogen self-ignition is approximately 2 MPa. In particular, the discharge tube shape and bursting disc rupture have a significant effect on the characteristics of hydrogen self-ignition. Moreover, the study of the hydrogen self-ignition mechanism under special working conditions has been extended by shock-induced ignition theory. Initial conditions mainly affect the critical pressure of hydrogen self-ignition by changing the formation, development and propagation of shock waves. Finally, the deficiencies and future research trends in research methods, self-ignition characteristics, and dynamic mechanisms are analysed.     

Improving burning speed by using hydrogen enrichment and turbulent jet ignition system in a rotary engine

Low flame speed restrains engine efficiency and increases HC emissions in rotary engines. Hydrogen addition and turbulent jet ignition have a great potential in increasing engine performance as they increase fuel burning speed. In this study, the classical R13b-Renesis Wankel engine and a modified one with a turbulent jet ignition configuration are numerically investigated by using hydrogen as a supplement. Eccentric motion of the rotor was generated by using User Defined Function in ANSYS-Fluent software. Pure methane and methane blended with 3% and 6% hydrogen energy fractions were used as fuels in the calculations. Combustion was modeled by using reduced mechanism of hydrogen-methane combustion having 22 species and 104 reactions. The Wankel engine was simulated at 2000 rpm speed and partial load conditions. At first, classical engine configuration having two spark plugs was simulated with pure methane. Then, hydrogen blended methane simulations were conducted to investigate the benefits of the hydrogen addition. Similar procedure was applied for the turbulent jet ignition application. The results show that both approaches are effective on increasing the burning speed of the fuel. It is revealed that hydrogen addition increases the indicated mean effective pressure (IMEP) by 1.8% and 5.2% for 3% and 6% hydrogen fraction cases respectively in the classical engine. Turbulent jet ignition with pure methane increases IMEP by 4.7% compared to the classical engine. Hydrogen addition only in pre-chamber is effective as much as 6% hydrogen fraction of classical engine. As the burning speed is increased by the application of these methods, CO and HC emissions are reduced and NO emission is increased. It is concluded that benefits of hydrogen addition and turbulent jet ignition applications can be optimized for both reducing harmful emissions and increasing engine performance.     

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 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.     

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.     

Suppression effects of ultrafine water mist on hydrogen/methane mixture explosion in an obstructed chamber

The mitigation effects of ultrafine water mist on hydrogen/methane mixture explosions with hydrogen fraction (ϕ) of the range from 0% to 60% were experimentally studied in a vented chamber with obstacles. The spraying time, droplets size of water mist and the volume ratio of hydrogen were varied in the tests, and the key parameters that reflect the explosion characteristics such as the flame propagation imagines, flame propagation velocity, and explosion overpressure were obtained. The results show that the ultrafine water mist presents a significant mitigation effect on hydrogen/methane mixture explosions. The flame propagation structures are similar under the condition of without and with ultrafine water mist while the flame temperature is declined by the physical and chemical inhibition by ultrafine water mist. In addition, the mitigation effect increases with the increase of water mist flux. As a result, the maximum flame speed and overpressure of ϕ = 30% hydrogen/methane mixture explosion are declined by 33.3% and 58.4% under the condition of spraying for 2 min with 15 μm ultrafine water mist, respectively. Besides, the mitigation effects of ultrafine water mist on ϕ = 30% hydrogen/methane mixture explosion descends evidently with the increase of the droplets size of the range from 6 μm to 25 μm, which due to the easier evaporation and the greater total droplets surface area of the smaller water mist. However, the explosion mitigation effect of ultrafine water mist on the hydrogen/methane mixture actually descends with the increase hydrogen fraction.     

Investigating the explosion hazard of hydrogen produced by activated aluminum in a modified Hartmann tube

Metallic powders exposed to water are sources of hydrogen gas that may result in an explosion hazard in the process industries. In this paper, hydrogen production and flame propagation in a modified Hartmann tube were investigated using activated aluminum powder as fuel. A self-sustained reaction of activated aluminum with water was observed at cool water and room temperatures for all treatments. One gram of Al mixed with 5 wt% NaOH or CaO resulted in a rapid rate of hydrogen production and an almost 100% yield of hydrogen generation within 30 min. The flame structures and propagation velocity (FPV) of released hydrogen at different ignition delay times were determined using electric spark ignition. Flame structures of hydrogen were mainly dependent on hydrogen concentration and ignition delay time, likely due to different mechanisms of hydrogen generation and flame propagation. As expected, FPVs of hydrogen in the Hartmann tube increased with ignition delay time. However, the FPV of upward flame propagation was much larger than that of downward flame propagation due to the effect of spreading acceleration at the explosion vent. Once ignited, the FPV of upward flame propagation reached 31.3–162.5 m/s, a value far larger than the 7.5–30 m/s for downward flame propagation. Hydrogen explosion caused by the accumulation of wet metal dust can be far more dangerous than an ordinary hydrogen explosion.     

Experimental study on pressure dynamics and self-ignition of pressurized hydrogen flowing into the L-shaped tubes

To investigate the effects of varying right-angle corner locations inside the L-shaped tube on self-ignition induced by high-pressure hydrogen release, a series of experiments were carried out on L-shaped tubes with different right-angle corner locations and a straight tube was adopted for comparison. It is found that compared with the straight tube, the propagation of shock wave in the L-shaped tubes becomes more complicated due to the existence of reflected shock wave. The pressure profiles detected by pressure transducers before the right-angle corner will undergo secondary rapid rise. The varying right-angle corner locations inside the L-shaped tube have a significant influence on self-ignition of hydrogen. The closer the right-angle corner is to the burst disk, the lower the critical pressure that causes hydrogen self-ignition is. Three possible mechanisms of self-ignition inside the L-shaped tubes are discussed. Nevertheless, different right-angle corner locations have no obvious effects on development process of out-tube jet flame, only the velocity of flame tip and the length and width of jet flame have slight differences.     

Pressure dynamics,self-ignition,and flame propagation of hydrogen jet discharged under high pressure

The self-ignition of hydrogen released from a high-pressure tank using extension tubes (2200 mm) with different diameters was studied. The processes of flame transition at a nozzle and jet flame development were characterized using a high-speed camera. The results indicated that the intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube than that in a 15-mm-diameter tube. The pressure in a 15-mm-diameter tube was weaker than that in the 10-mm-diameter tube at the initial stage; however, it became higher in the later stage. Spontaneous ignition was more likely to happen in a 15-mm-diameter tube. The formation of a stabilized flame at the tube exit and Mach disk were observed during the transition of the flame to a jet fire. The stabilized flame showed a triangular shape because of the influence of a Prandtl–Meyer flow when a hydrogen jet entered a suddenly expanding environment. The formation and separation of a spherical flame were recorded during jet flame development. Large vortexes were formed in front of the flame because of the Kelvin–Helmholtz instability, which resulted in the separation of the spherical flame. The vortexes stopped rotating until the separated flame disappeared.     

Assessment of elliptic flame front propagation characteristics of hydrogen in an optically accessible spark ignition engine

The absence of carbon content of hydrogen fuel makes it an attractive candidates for future energy carriers. Hydrogen or dual fuelled engines are a practical alternative to pure hydrocarbon fuelling modes. However, fine tuning of current engines is necessary. In this study premixed hydrogen flame propagation is investigated in a single-cylinder, spark-ignited, four-stroke optically accessible spark ignition test engine using high-speed imaging. Ellipses were fitted on the flame contours during the analysis to obtain flame speeds and flame centre motion. The test conditions covered a range of engine speeds from 1000 rpm to 2000 rpm with 100 rpm increments using a lean mixture, ? = 0.67. The fine temporal resolution allowed the time, at which spark governed kernel formation becomes a function of engine parameters to be determined. The few data that have been published in the literature regarding hydrogen flame speeds were compared with the finding of this study.     

Self-ignition and flame propagation of high-pressure hydrogen jet during sudden discharge from a pipe

Hydrogen is expected to serve as a clean energy carrier. However, since there are serious ignition hazards associated with its use, it is necessary to collect data on safety in a range of possible accident scenarios so as to assess hazards and develop mitigation measures. When high-pressure hydrogen is suddenly released into the air, a shock wave is produced, which compresses the air and mixes it with hydrogen at the contact surface. This leads to an increase in the temperature of the hydrogen–air mixture, thereby increasing the possibility of ignition. We investigated the phenomena of ignition and flame propagation during the release of high-pressure hydrogen. When a hydrogen jet flame is produced by self-ignition, the flame is held at the pipe outlet and a hydrogen jet flame is produced. From the experiment using the measurement pipe, the presence of a flame in the pipe is confirmed; further, when the burst pressure increased, the flame may be detected at a position near the diaphragm. At the pipe outlet, the flame is not lifted and self-ignition is initiated at the outer edge of the jet.     

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.