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In the study of self-ignition during pressurized hydrogen release, the morphology of the rupture disk alters the flow and mixing processes of the gas jet, as well as the development of shock waves, thereby affecting the occurrence of self-ignition. This study employs the LES method based on the RNG-SGS model, EDC model, and NUIMech1.1 H 2 combustion mechanism to investigate the flow mixing, shock wave development, and self-ignition phenomena under three different the rupture disk morphologies. The research findings indicate that as the growth trend of the rupture disk rupture ratio increases, the propagation speed of shock waves within the tube and the compression speed of the air and hydrogen-air mixing region accelerate, resulting in an earlier occurrence of self-ignition. A more complex rupture morphology leads to higher mixing rates between hydrogen and air and increased complexity of shock waves. Self-ignition initially occurs in the fuel-lean region of the boundary layer, followed by ignition at the center of the tube. The characteristic number of rupture disk is proposed to describe its impact on the self-ignition time. The competitive relationship between the two elementary reaction pathways of oxygen influences the development of the flame. • A CFD model for hydrogen release and self-ignition is established and validated. • The development process of multidimensional shock waves is described in detail. • The process of self-ignition and entire flame development is fully captured. • The reaction mechanism and HRR of self-ignition are analyzed. • The impact of rapture disk morphology on flow and self-ignition is fully obtained. [ABSTRACT FROM AUTHOR]

The ignition of lean hydrogen-air mixtures behind reflected shock waves at the temperature range of 902–1630 K and the pressure of 1 atm is studied experimentally and numerically. For the first time, reliable experimental data is obtained for temperatures below 1000 K, directly in the zone of change in the dominance of chain branching and chain termination reactions, which influence the formation of the ignition delay value. Expressions are formulated for ignition delay in two temperature intervals below and above 970–980 K, and the corresponding activation energy values are determined. Good agreement between the experimentally observed and calculated ignition delay values is demonstrated. The local sensitivity analysis allowed us to reduce the kinetic mechanism. • The new data on ignition of lean H 2 -air mixtures at the T of 902–1630 K and the P of 1 atm is presented. • Correlation expressions for ignition delay in two temperature intervals are formulated. • Good agreement between the experimentally observed and calculated ignition delay values is demonstrated. • The reactions determining the kinetic mechanisms of ignition delay formation of lean H 2 -air mixtures are identified. • Reduced kinetic mechanism is presented. [ABSTRACT FROM AUTHOR]

Induction and reaction times of hydrogen–air mixtures (ϕ = 0.5–2) have been measured behind reflected shock waves at temperatures of 1000–1600 K, pressures of 0.1, 0.3, 0.6 MPa in the domain of the extended second explosion limit. The measurements were performed in the shock tube with a completely transparent test section of 0.5 m long, which provides pressure, ion current, OH and high-speed chemiluminescence observations. The experimental induction time plots demonstrate a clear increasing of the global activation energy from high- to low temperature post-shock conditions. This trend is strongly pronounced at higher post-shock pressures. For a high-temperature range of T > 1200 K, induction time measurements show an activation energy for the global reaction rate of hydrogen oxidation of 64–83 kJ/mole. Detected reaction times exhibit a big scatter and a weak temperature dependence. The minimum reaction time value was nearly 2 µs. Obtained induction time data were compared with calculations carried out in accordance with the known kinetic mechanisms. For current and former shock-tube experiments within a pressure range of 0.1–2 MPa, critical temperatures required for strong (1000–1100 K), transient and weak auto-ignition modes behind reflected shock waves were identified by means of the pressure and ion-probe measurements in stoichiometric hydrogen-air mixture. The transfer from the strong volumetric self-ignition near the reflecting wall to the hot spot ignition (transient) was established and visualized below <1200 K with a post-shock temperature decreasing. [ABSTRACT FROM AUTHOR]

Hydrogen and ammonia have attracted increasing attention as carbon-free fuels. Ammonia is considered to be an effective energy storage and hydrogen storage medium. However, a small amount of unremoved NH3 is still present in the product during the decomposition of ammonia to produce hydrogen. Therefore, it is very essential to investigate the self-ignition of hydrogen-ammonia mixtures in order to accommodate the various scenarios of hydrogen energy applications. In this paper, the effect of NH3 addition on the self-ignition of high-pressure hydrogen release is numerically investigated. The RNG k-ε turbulence model, EDC combustion model, and 213-step detailed NH3/H2 combustion mechanism are used. CHEMKIN-Pro programs for zero-dimensional homogeneous and constant volume adiabatic reactor models are used for sensitivity analysis and ignition delay time of the chemical reaction mechanism. The results showed that the minimum burst pressure required for self-ignition increased significantly after the addition of ammonia. The maximum temperature and shock wave intensity inside the tube decreases with increasing ammonia concentration. The ignition delay time and H, HO2, and OH radicals reduce with increasing ammonia concentration. H and HO2 radicals are suggested as indicators for tracking the second and third flame branches, respectively. [ABSTRACT FROM AUTHOR]

The geometrical diversion of combustible gas molecules notably affects their self-ignition in high-pressure tubes. This study investigates the propagation of shock waves and flame induced by a sudden pressure release of hydrogen in various Y-shaped tubes. The impact of the initial burst pressure, downstream tube length, and bifurcation angle (60°, 120°) were analyzed by recording the pressure signals, photoelectric curves, and jet fire evolution. The results showed that the gas diversion caused by the Y-shaped structure significantly reduces the maximum overpressure of the hydrogen jet, the intensity, and the velocity of the leading shock wave. Also, it was indicated that increasing the Y-shaped tube's length downstream of the bifurcation point could enhance the strength of leading and reflected shock waves after the Y-bifurcation, making self-ignition more likely to happen. Applying a 60°-bifurcation angle could create stronger leading and reflected shock waves during the propagation downstream of the bifurcation point, while a 120°-bifurcation angle could make a more substantial reflected shock wave propagating upstream. It leads to the re-ignition downstream of the bifurcation in the 60°-bifurcation tube after the flame quenching and stronger far-field jet fire outside the tube. • The impact of geometrical diversion of pressurized hydrogen was investigated. • The influence of tube length and bifurcation angle were analyzed. • The flame quenching and re-ignition were explored. • The evolution of double far-field jet fire was recorded. [ABSTRACT FROM AUTHOR]

The accidental release of high-pressure hydrogen into a tube can induce self-ignition, and special tube configurations can complicate their characteristics. Hence, this paper studies the shock waves and self-ignition in tubes with T-shaped structure. Results show that the shock wave intensity decreases after passing through the T-shaped region, and its intensity is lower in the branch pipeline. The flow field is extremely complex after shock waves enter the T-shaped region, accompanied by different shockwave structures (e.g. barrel shock and Mach disk), multiple reflections, and shock-shock interaction. In addition, the location of the bifurcation point affects not only the downstream velocity evolution but also the time of upstream pressure to reach the peak pressure. When the bifurcation point is the closest to the rupture disc, spontaneous ignition is least likely to take place. The flame intensity decreases after passing via a T-shaped zone. Besides, if the T-shaped zone is far from rupture disc, even extinguishment can occur. [ABSTRACT FROM AUTHOR]

The self-ignition delays of concentrated syngas mixtures are determined by the method of reflected shock waves in the temperature range T0 = 750–1150 K at pressures P0 ≈ 1 atm. The experiments are carried out using a shock tube setup equipped with a calibrated pressure sensor and spectral-optical measuring instruments. The time profiles, the pressure, radiation intensity of the chemiluminescent reactions O + H → OH+ hν (λ = (308 ± 2) nm) and CO + O → CO2 + hν (λ = (410 ± 3 nm)), and the thermal radiation reaction CO2 (λ = (4.35 ± 0.07 µm)) are recorded. The ignition of the mixture manifests itself as a sharp simultaneous increase in all signals; thus, the delay can be determined based on any of them. Specifically, the delay is determined from the pressure oscillograms as the time between the arrival of the reflected shock wave at the pressure sensor and the sharp pressure jump caused by the ignition of the mixture. Kinetic modeling is carried out using the NUIG1.1 mechanism and our own mechanism, which is a modification of the Keromnes mechanism. Sensitivity analysis shows that the branching reactions, H + O2 = O + OH (1), and chain termination, H + O2 (+M) = HO2 (+M) (2), play the most significant role in syngas ignition. It is established that with an increase in the proportion of CO in the composition of the syngas, the delay increases for two reasons: the contribution of the chain termination reaction CO + O (+M) = CO2 (+M) grows as does the rate constant of the main chain termination reaction (2), since CO is more efficient as a third body (M) than H2. The results of kinetic simulations are in good agreement with the experimental data, with the relative discrepancy being within 30%. [ABSTRACT FROM AUTHOR]

Accidental leakage of pressurized hydrogen into the pipeline is often accompanied by the non-premixed self-ignition flame, whilst the complex structure of pipeline affects the flame propagation. In this study, the propagation of self-ignition hydrogen flame in different three-way tubes is studied and the flame evolution near the two exits is compared. Effects of the tube length and the location of bifurcation points are investigated. Results show that the bifurcation structure has the attenuation effect on the intensity of flame and shock wave, and they attenuate more sharply in the branch pipeline. But the flame intensity can increase again downstream the bifurcation point in the trunk pipeline, and this increase is not continuous and decrease again as the tube length increases. In addition, the flame evolution can be different near two exits of the tube. As the bifurcation point moves downstream, the number of Mach disk and the time required to form the stable flame will increase near the exit of branch pipeline. Despite different flame evolution, the flame morphology and propagation are all controlled by the shock wave structures before the formation of the vortex, but the flame changes dramatically and grows rapidly to form the stable lift-off flame after the formation of the vortex. [ABSTRACT FROM AUTHOR]

Spontaneous ignition resulting from the accidental release of high-pressure hydrogen is an important safety issue, and the self-ignition flame can eventually induce a jet flame. However, the links between the self-ignition flame inside a tube and an external jet flame are unclear. Hence, this paper presents a study on how the self-ignition flame transforms into the jet flame in the near-field region of the nozzle. Effects of release pressure and tube length are investigated. Changes in release conditions can lead to changes in the flow characteristics of the self-combustible jet at the nozzle. Results show that the difference in the flow parameters is manifested in three aspects, which directly contribute to the diversity of transition forms. The expansion processes and shock structure govern the flame transition. The expansion process consists of two typical stages, which lead to two different flame morphologies. Besides, the presence of discontinuous surfaces in the shock wave structure can cause the self-ignition flame to extinguish or re-ignition in some transition processes, resulting in the flame appearing in different zones during different transitions. Finally, five forms of flame transition are proposed and their formation reasons are analyzed. Dominant factors and links between different transitions are eventually identified. [ABSTRACT FROM AUTHOR]

The dynamic mechanisms of self-ignition and flame propagation during high-pressure hydrogen release through a rectangular tube were experimentally investigated using pressure records, flame detection and high-speed photographs. Experimental results show that the minimum burst pressure for self-ignition decreases with an increase in axial distance to the diaphragm and then remains at an almost constant value. The self-ignition onset at the same location of the tube exhibits a certain randomness even if the intensity of the shock wave produced in the tube is similar. Multiple ignitions were observed at the early stage of hydrogen release. They usually had difficulty to sustainably develop and were extinguished owing to oxygen deficiency. At a subsequent stage, the ignition kernel appears again and grows rapidly in the axial and radial directions, finally converging to a complete flame across the tube width. It was found that the radial growth rate of the flame was lower than the axial growth rate. [ABSTRACT FROM AUTHOR]

In this study, an experimental investigation is conducted to study the effects of CO addition (0%–7.5% vol.) on the self-ignition of pressurized hydrogen leakage. The result shows that more CO addition significantly decreases the likelihood of self-ignition inside the tube and the formation of self-sustained jet flame outside the tube. This can be mostly explained by the reduction of leading shock intensity inside the tube. Furthermore, CO addition effectively inhibits the flame propagation and development inside the tube. For pure hydrogen, the ignited flame quickly develops an intense flame spanning the tube width and eventually form a jet flame in ambient air. However, the H 2 –CO diffusion flame initiated approaching tube sidewall tends to propagate adjacent to the wall or be soon quenched with more CO addition. If the CO-weakened flame spouts to the tube exit, it may not survive the expansion at the tube exit and thus it is quenched. • The shock intensity is reduced in the presence of CO. • The critical pressure for self-ignition increases with CO addition concentration. • Shock intensity variation by CO addition is the main factor affecting self-ignition onset. • Small CO addition can effectively inhibit flame development inside the tube. • H 2 –CO flame spouting from the tube exit tends to be blown out. [ABSTRACT FROM AUTHOR]

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. • Summarized the research results on the self-ignition of hydrogen discharge. • Introduced the deficiencies in the study of self-ignition of hydrogen discharge. • Discussed the urgency of multi-element gas self-ignition research. • Pointed out three fields that need further research. [ABSTRACT FROM AUTHOR]

Acetylene is an effective additive to various rocket fuels. Acetam, an equilibrium highly concentrated solution of acetylene in liquid ammonia, is a promising fuel for use in liquid−propellant jet engines. To gain insights into the processes occurring in such systems and provide means for predicting their characteristics, the self-ignition of model acetylene−oxygen−argon mixtures is experimentally and theoretically studied. The experiments are performed behind reflected shock waves in a temperature range from 1100 to 1870 K at a total gas concentration of [M] ≈ 1.0 × 10−5 mol/cm3, with chemiluminescence signals from electronically excited C 2 * (λ ≅ 516 nm), CH* (429 nm), CO 2 *(363 nm), and OH* (308 nm) recorded. Numerical simulations within the framework of various detailed kinetic mechanisms closely reproduce the measured temperature dependences of the ignition delay time for the tested mixtures, but in some cases fail to satisfactorily describe the time profiles of the emission signals from C 2 *, CH*, CO 2 *, and OH*. The performed numerical simulations make it possible to explain the observed characteristic features of the emission profiles of electronically excited C 2 *, CH*, CO 2 *, and OH* during the self-ignition of C 2 H 2 –O 2 −Ar mixtures. Sensitivity analysis demonstrates that the existence and characteristics of the preignition stage in the CO 2 * and OH* emission signals are largely determined by the kinetic behavior of intermediate species, such as the ketene molecule and CH 2 radical. Based on the results obtained, a new detailed kinetic mechanism has been proposed for describing processes in the combustion products of acetylene-containing propellants, in particular chemiluminescent glow, which are of considerable interest for the development of new technologies for astronautics and aeronautics. • Results of shock-tube acetylene autoignition are presented. • A detailed kinetic mechanisms capable of describing acetylene ignition delay is developed. • The problems of simulation of chemiluminescence are discussed. • The importance of the kinetics of intermediate products are demonstrated. [ABSTRACT FROM AUTHOR]

The most influential factor for self-ignition of high-pressure hydrogen is known to be the strength of the shock. Thus, the self-ignition can be suppressed by weakening the shock strength, which is possible by reducing the area where the hydrogen is ejected in this study. To confirm the possibility of this method, experiments were done by controlling the burst pressure of up to 302 bar and the ratio of the opening area. The experimental results showed that the minimum burst pressure of self-ignition is increased exponentially as the opening area is reduced. This confirmed that reducing the opening area under the same burst pressure conditions has an effect on the suppression of self-ignition. However, it was also found that the minimum shock speed that causes self-ignition gradually decreases as the opening area becomes smaller, which results from an increasing in mixing. The CFD simulation results showed that the volume of the flammable region in the tube was increased and the hydrogen-air mixing efficiency also increased when the opening area became smaller. The results suggest that reduction of the opening area can suppress a self-ignition by weakening the shock strength, but it should be noted that an increase in mixing effect also occurs. Image 1 • The self-ignition of hydrogen can be effectively suppressed by reducing the opening area. • The burst pressure for self-ignition increases exponentially with the reduction of opening area. • The small opening area ratio results in weaker shock strength, but stronger air/hydrogen mixing. • Strong mixing was observed to increase the possibility of self-ignition even with weak shock wave. [ABSTRACT FROM AUTHOR]

The self-ignition of a stoichiometric acetone−oxygen mixture diluted with argon was studied both experimentally and numerically. The experiments were performed behind reflected shock waves over a temperature range from 1280 to 1810 K at a total gas concentration of [ M ] 50 ≈ 10−5 mol/cm3. The process was monitored by recording the signals of absorption by methyl radicals (λ = 216.5 nm) and emission from electronically excited OH* radicals (λ = 308 nm) and CO 2 * molecules (λ = 370 nm). Numerical simulations within the framework of various detailed kinetic mechanisms were performed to reproduce our own and published experimental data on the pyrolysis and self-ignition of acetone behind reflected shock waves, including the concentration profiles of acetone and CH 3 in the ground state and electronically excited OH* and CO 2 *, as well as the temperature dependence of the self-ignition delay time. It has been established that various detailed kinetic mechanisms describe the kinetic characteristics of the pyrolysis of acetone in shock waves with varying degrees of accuracy. At the same time, all of them predicted the measured ignition delays within a factor of two. A sensitivity analysis showed that the reactions determining the pyrolysis of acetone produce only a slight effect on the branched-chain ignition process, so that the relevant rate constants only slightly affect the ignition delay time. • Shock tube results of acetone autoignition are presented. • Detailed kinetic mechanisms are capable to describe acetone ignition delay. • The problems of simulation of acetone pyrolysis were revealed. • Kinetics of methyl radicals reactions should be refined. [ABSTRACT FROM AUTHOR]

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. • The minimum pressure when self-ignition occurs is similar in 700-mm-length tubes with different diameters. • Higher release pressure and smaller diameter tubes result in thicker boundary layer. • Boundary layer of micro shock tube affects shock-wave propagation and hydrogen self-ignition. [ABSTRACT FROM AUTHOR]

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. • The effect of varying right-angle corner locations on self-ignition is investigated. • The relationship between the reflected shock wave and burst pressure in L-shaped tube is discussed. • Self-ignition is easy to occur in L-shaped tube of smaller distance from corner to diaphragm. • Three possible mechanisms of self-ignition in L-shaped tube are proposed. [ABSTRACT FROM AUTHOR]

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% CH 4 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. • The effect of 2.5% methane addition to hydrogen is experimentally investigated. • The shock wave intensity is reduced in the presence of the methane. • The addition of methane can decrease the possibility of hydrogen self-ignition. • Initial ignition occurs in the tube wall and then develops into a complete flame. • The hydrogen-methane mixture brings greater combustion overpressure in chamber. [ABSTRACT FROM AUTHOR]

The time evolution of the chemiluminescence emission signals of CH*, OH*, C 2 *, and CO 2 * during the self-ignition of a number of the simple hydrocarbons is studied. The experiments are performed behind reflected shock waves over a temperature range of 1100–1900 K at a pressure of ∼1 bar. The effects of fuel-to-oxidizer equivalence ratio and the structure of the hydrocarbon molecule on the time profiles of the signals for each of the emitters are examined. A detailed kinetic mechanism for describing the emission signals from CH*, OH*, C 2 *, and CO 2 * recorded during the self-ignition of hydrocarbons is developed. To make the simulations more rigorous and reliable, the NASA thermodynamic polynomials for C 2 *, and CO 2 * were calculated based on the respective rotational and vibrational parameters given in the literature. Numerical simulations satisfactorily reproduce the measured time profiles of the signals from the studied emitters. • Chemiluminescence emission of CH*, OH*, C2*, and CO2* is studied. • Detailed kinetic mechanism for describing the emission signals was developed. • Simulation results well reproduce the measured intensity of emission. • Full width at half maximum of the emission signals are presented. [ABSTRACT FROM AUTHOR]

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. • The intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube. • Spontaneous ignition is more likely to happen in a 15-mm-diameter tube. • The stabilized flame at the tube exit is affected by the Prandtl-Meyer flow. • The formed large vortexes have a relationship with the separation of spherical flames. [ABSTRACT FROM AUTHOR]

• The strength of shock wave is different before and after obstacles. • Obstacles decrease the minimum burst pressure and self-ignition occurs around obstacles. • Mechanism of self-ignition is different before and after obstacles. • Obstacles before the initial ignition location significantly accelerates initial self-ignition. The safe application of hydrogen has attracted increased attention. In particular, fire and explosion disasters may be induced by self-ignition during high-pressure hydrogen leakage. This study experimentally investigates the effect of obstacles on initial self-ignition when high-pressure hydrogen is suddenly released. High-pressure hydrogen release experiments are performed at burst pressures of 2–8 MPa. The hydrogen flame inside the tube is captured by a high-speed video camera and light sensors, and shock wave variations are measured by pressure transducers. The results show that the presence of obstacles inside the tube significantly affects shock wave flow. It is found in this study that obstacles decrease the minimum burst pressure needed for self-ignition and self-ignition is prone to occur in the vicinity of obstacles. The influence of obstacles on the initial ignition process are different at different self-ignition locations. On the one hand, the reflected shock wave originating from obstacles is the main reason for the promotion of self-ignition upstream of obstacles. On the other hand, the formation of a hydrogen/air mixture under the effect of turbulent flow and multi-dimensional shock waves leads to self-ignition downstream of obstacles. Meanwhile, the presence of obstacles before the initial ignition location (L in) shortens the distance between the ignition location and the burst disk and significantly accelerates the occurrence of initial self-ignition. Obstacles participate in the self-ignition process earlier when they are placed closer to the burst disk. However, obstacles after the initial ignition location (L in) show no effect on the initial ignition process. The results can guide the safe use of high-pressure hydrogen. [ABSTRACT FROM AUTHOR]

The influence of addition of methane on the self-ignition of a hydrogen jet was studied experimentally for the pulse discharge into an open channel filled with air. During the pulse discharge from a high pressure chamber a shock wave is formed, which heats the surrounding air and a contact surface. The self-ignition of the jet occurred at the contact surface of the jet with air. A mixture of hydrogen with methane was preliminary prepared in a vessel of 40 L. The initial pressure of the mixture was varied from 3 to 15 MPa. During the discharge of the binary mixture ignition delays were measured relative to the moment of a breaking of the diaphragm. Ignition delays were determined by a photomultiplier tube. The delays were measured for different initial pressures and the methane addition. A molar concentration of the methane addition varied from 5% to 18%. Analysis of the effect of impurities on the thermodynamic parameters of the pulsed jets was carried out. [ABSTRACT FROM AUTHOR]

In the present work, self-ignition of high-pressure hydrogen that is released to air through a ruptured diaphragm is studied experimentally under various test conditions. Diaphragms with several thicknesses and scores are ruptured by high-pressure hydrogen, and the formation of a shock wave in the tube is experimentally observed. The behavior of the ruptured diaphragm and the subsequent self-ignition is photographed using a high-speed camera, while the self-emission intensity from a flame is measured using a photomultiplier. The experimental results show that the diaphragm rupturing condition is one of the main factors affecting the self-ignition phenomena. In addition, the self-ignition causes a ring-shaped flame that travels downstream, along a boundary between the hydrogen jet and the shock-heated air. [ABSTRACT FROM AUTHOR]

[Display omitted] • Examined self-ignition in high-pressure hydrogen jet owing to diaphragm rupture. • Visualization of flame growth behavior in a tube and out of a tube. • The longer a tube, the more flames develop in the tube. • The flame trapped in the vortex ring is the ignition source of the continuously released hydrogen jet. • The longer the tube length, the lower the burst pressure, and the jet flame is observed on the outside of the tube. This study investigates the behavior of self-ignited flames caused by high-pressure hydrogen release in seamless acrylic tubes. Such flames develop in and around vortex rings generated by the release of a preceding shock wave. The combination of hydrogen, which mixes with air as it flows through the vortex rings, and a sufficiently developed self-ignited flame yields a jet flame. The experiments performed in this study investigate flame retention outside circular acrylic tubes by varying their length and the burst pressure. As observed, for tube lengths below 1000 mm, the burst pressure decreases with increase in the tube length, and a jet flame is observed outside the tube. For tubes exceeding 1000 mm in length, jet flames are observed at burst pressure exceeding 4.8 MPa. Compared to extant research, a low critical pressure is observed in this study owing to high flame temperature and low thermal conductivity of the acrylic tube. [ABSTRACT FROM AUTHOR]

Abstract: The self-ignition of low alkylbenzenes at engine-relevant conditions has been studied with kinetic modeling. A previously developed chemical kinetic model for gasoline surrogate fuels [J.C.G. Andrae, R.A. Head, Combust Flame 156 (2009) 842–51] was extended with chemistry for ethylbenzene and m-xylene resulting in an overall model consisting of 150 species and 759 reactions. In model validation, comparisons were made between model predictions and experimental data of ignition delay times measured behind reflected shock waves, laminar burning velocities collected at elevated temperature and pressure and species profiles in a high-pressure single pulse shock tube. Generally good agreement was found and the model is sensitive to changes in mixture strength, pressure and temperature. Shock tube ignition delay modeling results for ethylbenzene and m-xylene also compare well to the ones for toluene. The rate controlling step for the ignition of ethylbenzene in the current mechanism is the reaction with ethylphenyl radical and oxygen. Ignition delay time for m-xylene was found to be very sensitive to reactions involving hydrogen atom abstraction from fuel by hydroxyl and oxygen and to branching reactions where methylbenzyl reacts with oxygen and hydroperoxide. The validated mechanism was used to study fuel chemistry effects when blending ethylbenzene with the paraffinic fuels iso-octane and n-heptane. A sensitivity- and flow path analysis showed that a higher consumption of hydroperoxide by ethylphenyl than expected from the contribution of neat ethylbenzene in a fuel mixture with iso-octane inhibits both iso-octane and ethylbenzene ignition. This can explain the observed increase in ignition delay time and octane number for fuel mixtures compared to neat fuels. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

• The shock wave and self-ignition of hydrogen in the arc-shaped tube are studied. • Reflected shock waves generated in the bend will spread both upstream and downstream. • Changing the position of the bend can promote or inhibit self-ignition. • Two possible processes of self-ignition are summarized. The self-ignition is a potential risk during the use of hydrogen, and arc-shaped bends exist in industrial hydrogen pipelines. So the experimental and numerical study were conducted to investigate the shock wave and self-ignition induced by high-pressure hydrogen release via three 90° arc-shaped tubes. The pressure profiles, photoelectric signals and jet flame were recorded. It was found that reflected shock waves generated in the bend would spread both upstream and downstream. And the reflected shock wave propagating downstream can enhance the intensity of the leading shock wave. But the velocity of leading shock wave in the bend decreased faster than that in the straight part. In addition, based on numerous experiments, two processes of self-ignition in the 90° arc-shaped tube were summarized. One was only affected by the strong leading shock wave, the other was affected by the combination of reflected shock wave and leading shock wave. Moreover, the distance from the bend to the burst disc had an effect on the self-ignition. When the distance from the bend to the burst disc was the shortest, it was least likely to cause self-ignition. Besides, the phenomena of flame quenching cannot occur in the bend of arc-shaped tube. The development characteristics of the external flame was studied, including the flame morphology near the Mach disk and the variation in the size of the jet flame. [ABSTRACT FROM AUTHOR]

• The critical pressure for ignition increases with methane addition concentration. • Methane addition can effectively inhibit flame development in the tube. • Three mechanisms of self-ignition for hydrogen with methane addition is proposed. • Hydrogen-methane flame spouting from tube exit tends to blow out. • The hydrogen-methane mixture brings greater combustion overpressure in chamber. In this paper, experimental investigations are carried out to study the effects of methane addition (2.5%, 5% and 7.5% vol.) on self-ignition and subsequent flame propagation of pressurized hydrogen release via a tube. The visualization method utilizing the high-speed direct photography and the measurements of pressure and light sensors are applied in this study. The result shows that the existence of methane can reduce the shock wave overpressure and the temperature of shock-heated air. Therefore, it has great effects on the self-ignition possibility. The minimum burst pressure required for self-ignition increases from 2.89 MPa (0% CH 4) up to 6.05 MPa (7.5% CH 4). Besides, methane addition can extremely reduce flame intensity and inhibit flame development inside the tube. Three possible self-ignition mechanisms for the hydrogen with methane addition are discussed. Under the effects of methane addition, hydrogen flame spouting from tube exit tends to blow out. In all cases with 7.5% methane addition, no self-sustained jet flame is formed outside the tube for burst pressure up to 10.0 MPa. When self-sustained flame is formed in a confined space, a great rise in overpressure is generated due to the intense combustion. It is suggested that more methane addition can reduce the self-ignition probability and the formation of self-sustained flame. However, with the increasing of CH 4 additions, once the jet fire is formed in a confined space, it may cause greater overpressure and thus bring greater casualties and property losses. [ABSTRACT FROM AUTHOR]