Ignition limits of rapidly expanding diffusion layers: Application to unsteady hydrogen jets

The present paper addresses the ignition problem of a one-dimensional unsteady diffusion layer of fuel and oxidizer, undergoing volumetric expansion. The problem is applied to shock induced diffusion-ignition of pressurized fuel jets that are released into an oxidizing atmosphere. Upon the sudden release of a pressurized gaseous fuel into the ambient atmosphere through a hole, a strong shock wave forms, driven by rapid expansion of the forming jet. The model follows the thin diffusion layer at the head of the jet in Lagrangian coordinates, with its rate of expansion dictated by the local pressure evolution of the surrounding gasdynamic flow. Following the analysis of Radulescu and Law, the latter can be calculated a priori before the ignition event. Hence, the expansion rate is prescribed as a source term in our calculations of the diffusion layer. The calculations, which are performed for hydrogen and air with realistic thermo-chemical data and transport properties of the chemical species, revealed the transient events leading to ignition in this unsteady diffusion layer. Furthermore, the calculations showed that when the rate of expansion was sufficiently strong, which may occur for releases through sufficiently small holes, ignition can be prevented. This illustrates the important role that gasdynamic expansion plays on ignition phenomena. The results of the present model are found to be in very good agreement with previous numerical and experimental results of transient jet release ignition.     

Experimental study of ignited unsteady hydrogen jets into air

In order to simulate an accidental hydrogen release from the low pressure pipe system of a hydrogen vehicle a systematic study on the nature of transient hydrogen jets into air and their combustion behaviour was performed at the FZK hydrogen test site HYKA. Horizontal unsteady hydrogen jets with an amount of hydrogen up to 60 STP dm³ and initial pressures of 5 and 16 bar have been investigated. The hydrogen jets were ignited with different ignition times and positions. The experiments provide new experimental data on pressure loads and heat releases resulting from the deflagration of hydrogen-air clouds formed by unsteady turbulent hydrogen jets released into a free environment. It is shown that the maximum pressure loads occur for ignition in a narrow position and time window. The possible hazard potential arising from an ignited free transient hydrogen jet is described.     

Experimental study on spark ignition of non-premixed hydrogen jets

The leaks of pressurized hydrogen can be ignited if an ignition source is within a certain distance from the source of the leaks, and jet fires or explosions may take place. In this paper, a high speed camera was used to investigate the ignition kernel development, ignition probability and flame propagation along the axis of hydrogen jets, which leaked from a 3-mm-internal-diameter nozzle and were ignited by an electric spark. Experimental results indicate that for successful ignition events, the ignition delay time increases with an increase of the distance between the nozzle and the electrode. Ignitable zone of the hydrogen jets is underestimated if using the predicted hydrogen concentration along the jets centerline. The average rate of downstream flame decreases but that of the upstream flame increases with the electrode going far from the nozzle.     

Review on hydrogen safety issues: Incident statistics,hydrogen diffusion,and detonation process

The development and application of hydrogen energy in power generation, automobiles, and energy storage industries are expected to effectively solve the problems of energy waste and pollution. However, because of the inherent characteristics of hydrogen, it is difficult to maintain high safety during production, transportation, storage, and utilization. Therefore, to ensure the safe and reliable utilization of hydrogen, its characteristics relevant to leakage and diffusion, ignition, and explosion must be analyzed. Through an analysis of literature, in combination with our practical survey analysis, this paper reviews the key issues concerning hydrogen safety, including hydrogen incident investigation, hydrogen leakage and diffusion, hydrogen ignition, and explosion.     

The effect of ignition delay time on the explosion behavior in non-uniform hydrogen-air mixtures

The purpose of this study is to examine the explosion characteristics of non-uniform hydrogen-air mixtures with turbulent mixing. In the experiment, hydrogen is first filled into a 20 L spherical chamber to a desired initial pressure, then air is introduced into the same chamber through a fast response solenoid valve, by adjusting the ignition delay time (t_d), i.e., the time period between the end of air injection and the action of ignition, the turbulent mixing strengthen (or called uniformity of hydrogen-air mixture) is then changed. The experimental results show that the explosions are overall enhanced as t_d decreases, which indicates that turbulence plays a leading role in enhancing the explosion behaviors. In addition, it is found that the effect of turbulence on p_max is more prominent in end-wall ignition than that in center ignition. This is because the heat loss per unit time is higher in end-wall ignition due to the flame front continuously contacts with inner wall of the chamber throughout the explosion process, although the explosion duration time t_e for both ignition cases is reduced when turbulence is introduced, heat loss reduction for end-wall ignition is generally larger than that in center ignition. Lately, a systematical analysis of the turbulent effect associated with various equivalence ratios on the explosion characteristics is conducted in end-wall ignition. Those experimental results illustrate that the turbulence-enhancing influence is more noticeable when hydrogen-air mixtures move toward the lower explosion limit. However, no significant influence of turbulence on explosion process can be found as combustible mixtures tend to the fuel-rich side. This is mainly because that when hydrogen-air mixtures tend to fuel-rich side, τ_e reduction caused by the presence of turbulence is relatively weak as compared with that under quiescent condition, resulting in heat loss during explosion process changes slightly, hence there is no significant impact on explosion parameters.     

Effect of hydrogen and syngas addition on the ignition of iso-octane/air mixtures

There is worldwide interest in using renewable fuels within the existing infrastructure. Hydrogen and syngas have shown significant potential as renewable fuels, which can be produced from a variety of biomass sources, and used in various transportation and power generation systems, especially as blends with hydrocarbon fuels. In the present study, a reduced mechanism containing 38 species and 74 reactions is developed to examine the ignition behavior of iso-octane/H₂ and iso-octane/syngas blends at engine relevant conditions. The mechanism is extensively validated using the shock tube and RCM ignition data, as well as three detailed mechanisms, for iso-octane/air, H₂/air and syngas/air mixtures. Simulations are performed to characterize the effects of H₂ and syngas on the ignition of iso-octane/air mixtures using the closed homogenous reactor model in CHEMKIN software. The effect of H₂ (or syngas) is found to be small for blends containing less than 50% H₂ (or syngas) by volume. However, for H₂ mole fractions above 50%, it increases and decreases the ignition delay at low (T < 900 K) and high temperatures (T > 1000 K), respectively. For H₂ fractions above 80%, the ignition is influenced more strongly by H₂ chemistry rather than by i-C₈H₁₈ chemistry, and does not exhibit the NTC behavior. Nevertheless, the addition of a relatively small amount of i-C₈H₁₈ (a low cetane number fuel) can significantly enhance the ignitability of H₂-air mixtures at NTC temperatures, which are relevant for HCCI and PCCI dual fuel engines. The CO addition seems to have a negligible effect on the ignition of i-C₈H₁₈/H₂/air mixtures, indicating that the ignition of i-C₈H₁₈/syngas blends is essentially determined by i-C₈H₁₈ and H₂ oxidation chemistries. The sensitivity and reaction path analysis indicates that i-C₈H₁₈ oxidation is initiated with the production of alkyl radical by H abstraction through reaction: i-C₈H₁₈ + O₂ = C₈H₁₇ + HO₂. Subsequently, the ignition chemistry in the NTC region is characterized by a competition between two paths represented by reactions R2 (C₈H₁₇ + O₂ = C₈H₁₇O₂) and R8 (C₈H₁₇ + O₂ = C₈H₁₆ + HO₂), with the R8 path dominating, and increasing the ignition delay. As the amount of H₂ in the blend becomes significant, it opens up another path for the consumption of OH through reaction R36 (H₂ + OH = H₂O + H), which slows down the ignition process. However, for T > 1100 K, the presence of H₂ decreases ignition delay primarily due to reactions R31 (O₂ + H = OH + O) and R35 (H₂O₂ + M = OH + OH + M).     

The simulation and analysis of leakage and explosion at a renewable hydrogen refuelling station

The number of hydrogen refuelling stations (HRSs) is steadily growing worldwide. In China, the first renewable hydrogen refuelling station has been built in Dalian for nearly 3 years. FLACS software based on computational fluid dynamics approach is used in this paper for simulation and analysis on the leakage and explosion of hydrogen storage system in this renewable hydrogen refuelling station. The effects of wind speed, leakage direction and wind direction on the consequences of the accident are analyzed. The harmful area, lethal area, the farthest harmful distance and the longest lethal distance in explosion accident of different accident scenarios are calculated. Harmful areas after explosion of different equipments in hydrogen storage system are compared. The results show that leakage accident of the 90 MPa hydrogen storage tank cause the greatest harm in hydrogen explosion. The farthest harmful distance caused by explosion is 35.7 m and the farthest lethal distance is 18.8 m in case of the same direction of wind and leakage. Moreover, it is recommended that the hydrogen tube trailer should not be parked in the hydrogen refuelling station when the amount of hydrogen is sufficient.     

Experimental investigation on unconfined hydrogen explosion with different ignition height

Experimental research is performed to investigate the effects of ignition height on explosion characteristics in a 27 m³ hydrogen/air cloud. With the ignition height decreasing, the flame propagation velocity increases gradually. The flame travels in oscillating mode and the average oscillating frequency lies between 145Hz and 155Hz. An original parameter τ, which involves flame scale and flame propagation velocity, is proposed to measure the effect of buoyancy. The higher the value of τ, the more obvious the buoyancy effect. As the ignition height increases, the critical flame scale for flame deceleration increases. The middle ignition height in the gas cloud causes the highest overpressure peak, overpressure impulse, overpressure rising and decreasing rate. As the ignition point approaches the initial gas boundary, the explosion intensity would decrease gradually. For the open space outside the flame, overpressure peak for the lower space is higher, while, the middle space experiences higher overpressure impulse.     

A large-scale study on the effect of ambient conditions on hydrogen recombiner-induced ignition

Hydrogen recombiners (known in the nuclear industry as passive autocatalytic recombiners-PARs), in general, can be utilized for mitigation of hydrogen in controlled areas where there is potential for hydrogen release and ventilation is not practical. Recombiners are widely implemented in the nuclear industry, however there are other applications of recombiners outside the nuclear industry that have not yet been explored practically. The most notable benefit of recombiners over conventional hydrogen mitigation measures is their passive capability, where power or operator actions are not needed for the equipment to remove hydrogen when it is present.One of most significant concerns regarding the use of hydrogen recombiners in industry is their potential to ignite hydrogen at elevated concentrations (>6 vol%). The catalyst, heated by the exothermal H₂–O₂ reaction, is known to be a potential ignition source to cause hydrogen burns. An experimental program utilizing a full-size PAR at the Large-Scale Vented Combustion Test Facility (LSVCTF) has been carried out by Canadian Nuclear Laboratories (CNL) to investigate and understand the behaviour of hydrogen combustion induced by a PAR on a large-scale basis. A number of parameters external to the PAR have been explored including the effect of ambient humidity (steam) and temperature. The various aspects of this investigation will be discussed in this paper and examples of results are provided.     

An experimental study on ignition timing of hydrogen Wankel rotary engine

The hydrogen-fueled Wanke rotary engine is a promising power system that has both high power and eco-friendly properties. This work investigated the effect of ignition timing on a dual-spark plugs synchronous-ignition hydrogen-fueled Wankel rotary engine under low speed, part load and lean combustion. The results show that with delaying the ignition timing, CA0-10 is shortened first and then lengthened and CA10-90 is consistently shortened. When the CA50 is located between 35 and 40°CA ATDC, the maximum brake torque can be realized. Besides, the selection of ignition timing needs to consider the “trade-off” relationship between the combustion phase and corresponding in-cylinder pressure. The maximum brake torque ignition timing is between 5 and 10°CA ATDC. And there is also a “trade-off” relationship between stability and thermal load when ignition timing is selected. In addition, HC and NO emissions will not become the problem limiting the power performance of hydrogen-fueled Wankel rotary engine under this operating condition.     

Ignitability and mixing of underexpanded hydrogen jets

Reliable methods are needed to predict ignition boundaries that result from compressed hydrogen bulk storage leaks without complex modeling. To support the development of these methods, a new high-pressure stagnation chamber has been integrated into Sandia National Laboratories’ Turbulent Combustion Laboratory so that relevant compressed gas release scenarios can be replicated. For the present study, a jet with a 10:1 pressure ratio issuing from a small 0.75 mm radius nozzle has been examined. Jet exit shock structure was imaged by Schlieren photography, while quantitative Planar Laser Rayleigh Scatter imaging was used to measure instantaneous hydrogen mole fractions downstream of the Mach disk. Measured concentration statistics and ignitable boundary predictions compared favorably to analytic reconstructions of downstream jet dispersion behavior. Model results were produced from subsonic jet dispersion models and by invoking self-similarity jet scaling arguments with length scaling by experimentally measured effective source radii. Similar far field reconstructions that relied on various notional nozzle models to account for complex jet exit shock phenomena failed to satisfactorily predict the experimental findings. These results indicate further notional nozzle refinement is needed to improve the prediction fidelity. Moreover, further investigation is required to understand the effect of different pressure ratios on measured virtual origins used in the jet dispersion model.     

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.     

Simulation of hydrogen leak and explosion for the safety design of hydrogen fueling station in Korea

Hydrogen has been used as chemicals and fuels in industries for last decades. Recently, it has become attractive as one of promising green energy candidates in the era of facing with two critical energy issues such as accelerating deterioration of global environment (e.g. carbon dioxide emissions) as well as concerns on the depletion of limited fossil sources. A number of hydrogen fueling stations are under construction to fuel hydrogen-driven vehicles. It would be indispensable to ensure the safety of hydrogen station equipment and operating procedure in order to prevent any leak and explosions of hydrogen: safe design of facilities at hydrogen fueling stations e.g. pressurized hydrogen leak from storage tanks. Several researches have centered on the behaviors of hydrogen ejecting out of a set of holes of pressurized storage tanks or pipes. This work focuses on the 3D simulation of hydrogen leak scenario cases at a hydrogen fueling station, given conditions of a set of pressures, 100, 200, 300, 400 bar and a set of hydrogen ejecting hole sizes, 0.5, 0.7, 1.0 mm, using a commercial computational fluid dynamics (CFD) tool, FLACS. The simulation is based on real 3D geometrical configuration of a hydrogen fueling station that is being commercially operated in Korea. The simulation results are validated with hydrogen jet experimental data to examine the diffusion behavior of leak hydrogen jet stream. Finally, a set of marginal safe configurations of fueling facility system are presented, together with an analysis of distribution characteristics of blast pressure, directionality of explosion. This work can contribute to marginal hydrogen safety design for hydrogen fueling stations and a foundation on establishing a safety distance standard required to protect from hydrogen explosion in Korea being in the absence of such an official requirement.     

An experiment on the ignition of hydrogen injected into a high temperature oxidizer

Experiments were performed to investigate the combustion process which occurs when hydrogen is injected into a high temperature oxidizer. Hydrogen was injected through a circular nozzle into an oxygen-argon mixture in a temperature range 950–1500 K produced behind a reflected shock wave. The effects of oxidizer temperature, oxygen concentration and size of nozzle diameter on the ignition delay were studied. Also, the flow field was observed with a conventional schlieren system and a high speed camera. Consequently it has been found that the ignition delay is influenced mainly by the oxidizer temperature, that the minimum ignition temperature is around 980 K irrespective of the oxygen concentration and the size of the nozzle diameter, that the ignition takes place at a position near the nozzle exit and that the resulting flame propagates toward the tip of the jet. This is followed by a discussion of the ignition process and the applicability of hydrogen to diesel engines based on the present experiment results.     

Computational fluid dynamics modeling of hydrogen ignition in a rapid compression machine

In modeling a rapid compression machine (RCM) experiment, a zero-dimensional code is commonly used along with an associated heat loss model. However, the applicability of such a zero-dimensional modeling needs to be assessed over a range of accessible experimental conditions. It is expected that when there exists significant influence of the multidimensional effects, including boundary layer, vortex roll-up, and nonuniform heat release, the zero-dimensional modeling may not be adequate. In this work, we simulate ignition of hydrogen in an RCM by employing computational fluid dynamics (CFD) studies with detailed chemistry. Through the comparison of CFD simulations with zero-dimensional results, the validity of a zero-dimensional modeling for simulating RCM experiments is assessed. Results show that the zero-dimensional modeling based on the approach of “adiabatic volume expansion” generally performs very well in adequately predicting the ignition delay of hydrogen, especially when a well-defined homogeneous core is retained within an RCM. As expected, the performance of this zero-dimensional modeling deteriorates with increasing temperature nonuniformity within the reaction chamber. Implications for the species sampling experiments in an RCM are further discussed. Proper interpretation of the measured species concentrations is emphasized and the validity of simulating RCM species sampling results with a zero-dimensional model is assessed.     

Combined effect of ignition position and equivalence ratio on the characteristics of premixed hydrogen/air deflagrations

Premixed hydrogen/air deflagrations were performed in a 100 mm × 100 mm × 1000 mm square duct closed at one end and opened at the opposite end under ambient conditions, concerning with the combined effect of ignition position IP and equivalence ratio ?. A wide range of ? ranging from 0.4 to 5.0, as well as multiple IPs varying from 0 mm to 900 mm off the closed end of the duct were employed. It is indicated that IP and ? exerted a great impact on the flame structure, and the corresponding pressure built-up. Except for IP₀, the flame can propagate in two directions, i.e., leftward and rightward. A regime diagram for tulip flames formation on the left flame front (LFF) was given in a plane of ? vs. IP. In certain cases (e.g. the combinations of ? = 0.6 and IP₅₀₀ or IP₇₀₀), distorted tulip flames were also observed on the right flame front (RFF). Furthermore, the combinations of IP and ? gave rise to various patterns of pressure profiles. The pressure profiles for ignition initiated at the right half part of the duct showed a weak dependence on equivalence ratio, and showed no dependence on ignition position. However, the pressure profiles for ignition initiated at the left half part of the duct were heavily dependent on the combination of IP and ?. More specifically, in the leanest (? = 0.4) and the richest (? = 4.0–5.0) cases, intensive periodical oscillations were the prime feature of the pressure profiles. With the moderate equivalence ratios (? = 0.8–3.0), periodical pressure oscillations were only observed for IP₉₀₀. The maximum pressure peaks P_max were reached at ? = 1.25 rather than at the highest reactivity ? = 1.75 irrespective of ignition position. The ignition positions that produced the worst conditions were different, implying a complex influence of the combination of IP and ?.     

Control of the ignition possibility of hydrogen by electrostatic discharge at a ventilation duct outlet

To control the ignition possibility of hydrogen by electrostatic discharge at a ventilation duct outlet, we investigated the effect of the outlet shape. Four types of outlets were designed; outlet A (6.35 mm pipe), outlet B (12.7 mm pipe), outlet C (25.4 mm pipe) and a tapered porous outlet (called TP outlet in this paper). Iron (III) oxide particles were used as the model dust.     

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.     

Study on the mechanism of the ignition process of ammonia/hydrogen mixture under high-pressure direct-injection engine conditions

As environmental problems and energy crisis become more serious, ammonia is one of the potential alternative fuels. In order to better use ammonia as fuel in power equipment, the ignition process was studied under high-pressure direct-injection engine condition. In the paper, the Homogeneous model in Chemkin package was selected for numerical calculation. In the six cases with different hydrogen mixing ratios, the effect of initial temperature, pressure, equivalence ratio and hydrogen mixing ratio on ignition delay time (IDT) were studied. It conducted that IDT could be effectively reduced when adding 10–50% hydrogen to ammonia. Then, after sensitivity analysis of NH₃/H₂ mixtures, the key equations and free radicals affecting combustion characteristics were found. The rate of production (ROP) of the key radicals were carried out. It was found that the hydrogen provided the initial concentration of H radical before the start fire, which greatly improved the ROP of OH radical of R1(H + O₂=O + OH) compared to the original H needed to break the N–H chemical bond in pure ammonia. And the OH radical was related to the consumption of NH₃ by R31(NH₃+OH=NH₂+H₂O).     

Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures

Measurements on ignition delay times of n-butane/hydrogen/oxygen mixtures diluted by argon were conducted using the shock tube at pressures of 2, 10 and 20 atm, temperatures from 1000 to 1600 K and hydrogen fractions (${\mathrm{X}}_{{\mathrm{H}}_2}$) from 0 to 98%. It is found that hydrogen addition has a non-linear promoting effect on ignition delay of n-butane. Results also show that for ${\mathrm{X}}_{{\mathrm{H}}_2}$ less than 95%, ignition delay time shows an Arrhenius type dependence and the increase of pressure and temperature lead to shorter ignition delay times. However, for ${\mathrm{X}}_{{\mathrm{H}}_2}$ = 98% and 100% mixtures, non-monotonic pressure dependence of ignition delay time were observed. The performances of the Aramco2.0 model, San Diego 2016 model and USC2.0 model were evaluated against the experimental data. Only the Aramco2.0 model gives a reasonable agreement with all the measurements, which was conducted in this study to interpret the effect of pressure and hydrogen addition on the ignition chemistry of n-butane.