Hydrogen-air Explosion Research Articles

Experimental and numerical investigation of premixed hydrogen-air explosion suppression by heptafluoropropane and carbon dioxide mixtures

Experimental and numerical investigation of premixed hydrogen-air explosion suppression by heptafluoropropane and carbon dioxide mixtures

CFD Analysis of the Effects of a Barrier in a Hydrogen Refueling Station Mock-Up Facility during a Vapor Cloud Explosion Using the radXiFoam v2.0 Code

A CFD (computational fluid dynamics) analysis to investigate the effects of the installation of a barrier in a hydrogen refueling station (HRS) mock-up facility, with a dummy vehicle and dispensers in the vapor cloud region, during a hydrogen-air explosion using a gas mixture volume of 70.16 m3 was conducted to determine whether the radXiFoam v2.0 code with the established analysis methodology to predict the peak overpressure can be utilized to evaluate the safety of a HRS with such a barrier installed in a large city in the Republic of Korea. The radXiFoam v2.0 code was developed on the basis of the XiFoam solver in the open-source CFD software OpenFOAM-v2112 by modifying C++ source codes in several libraries and governing equations so as to ensure effective calculations of the hydrogen-air chemical reaction and radiative heat transfer through water vapor in a humid air environment and to remove unnecessary warning messages that arise when using the radXiFoam v1.0 code. First, we conducted a validation analysis on the basis of measured overpressure datasets from a near field to a far field of a vapor cloud explosion (VCE) site in the HRS mock-up facility to evaluate the uncertainty in prediction datasets by radXiFoam v2.0. After this validation analysis, we undertook CFD sensitivity calculations by installing barriers with heights of 2.1 m and 4.2 m at a horizontal distance of 2.3 m from the VCE region in the grid model used for the validation analysis to assess the effects of these barriers on reducing the peak overpressure of the blast wave. From these calculations, we judged that the radXiFoam v2.0 code can accurately simulate the effects of the barrier during a VCE, as the calculated overpressure reduction values according to the barrier height are reasonable on the basis of previous validation results from Stanford Research Institute’s explosion test with such a barrier. The results herein imply that the radXiFoam v2.0 code is feasible for use in HRS safety when barrier installation must meet the technical regulations of the Korea Gas Safety Corporation in a large city.

Study on the hydrogen-air premixed flame propagation characteristics in semi-open space with obstacle

In semi-open space, obstacles have the potential to accelerate flame propagation and increase hydrogen-air deflagration pressure. Therefore, this paper is dedicated to exploring the influence of obstacles on the premixed hydrogen deflagration, which is crucial for enhancing the safety of industrial production and energy utilization. By using Large Eddy Simulation (LES) model in OpenFOAM, this study investigates the deflagration characteristics of premixed hydrogen in the presence of three different shaped obstacles. The analysis results reveal that under obstacle conditions, the flame shape can be categorized into four phases: the hemispherical phase, finger-shaped phase, jet phase, and vortex phase. The velocity of the flame front is nearly same for elliptical and rectangular obstacle condition, but it is 36% higher compared to triangular obstacle condition. The impact of triangular and rectangular obstacles on explosion overpressure is less than that of triangular obstacles on explosion overpressure, but it is 16% higher than that of elliptical obstacle. Analyzing the vorticity generated by different obstacle reveals that the vorticity produced by rectangular obstacle is twice as much as that produced by elliptical obstacle, whereas the vorticity produced by triangular obstacle is 2.4 times greater than that produced by elliptical obstacle. The acceleration of hydrogen-air explosion process occurs due to the narrow space created by obstacle and pipeline walls, and the shape of obstacle significantly influences this acceleration.

Effects of Vent Size and Pressure on Hydrogen Explosion Dynamic Characteristics.

Explosion venting is an effective method to reduce the explosion damage; in order to study the mechanism of an explosion venting process in internal and external space, this paper investigates the influence of vent parameters on hydrogen-air explosion in a rectangular duct through numerical simulation. The model including the internal and external space is first constructed, and then the explosion dynamic behaviors of the full flow field are analyzed under different vent pressures and sizes. The study aims to reveal the coupling effect of the flame, pressure, and flow field on hydrogen explosion venting. The results indicate that the explosion intensity increases with the growth of the vent pressure and the reduction of the vent size. The maximum external overpressure increases to 2.6 and 2.3 times as the vent pressure increased to 10 times or vent size reduced by 90%. The flame and combustible gas mixture evolve from a mushroom cloud into a jet form as the vent size decreases, and vortexes formed at the flame front suppress flame propagation. However, the flame speed increases significantly as the flame passes the vent under the impact of larger pressure gradient, which results in a more violent turbulence intensity and secondary external explosion.

Effects of hydrogen concentration, number of obstacles, and blockage ratio on vented hydrogen-air explosions and its consequence analysis

With the swift advancement of commercial hydrogen fuel cell vehicles (HFCVs), accident risks of hydrogen explosions are heightened, especially in enclosed or semi-enclosed areas such as tunnels and underground parking lots. To prevent and control these accidents, experiments are conducted in a 1 m-long cylindrical tube at 1 atm and 286 K to study the impacts of hydrogen concentration, number of obstacles, and blockage ratios on vented hydrogen-air explosions. As the number of obstacles rises, there is a gradual rise in the maximum internal overpressure for a hydrogen concentration of 20 vol%. However, a reverse trend is observed for hydrogen concentrations of 30 vol% and 50 vol%. The size of the external flame gradually expands as the hydrogen concentration increases, but diminishes gradually with increasing in the number of obstacles and blockage ratio. The maximum external overpressure decreases as the distance to the vent increases. The maximum temperature experiences an increase with increasing in hydrogen concentration and blockage ratio but undergoes a decrease with an increase in the number of obstacles. Furthermore, the safety radii are determined using thermal radiation criteria to ensure the absence of thermal radiation hazards for humans, and PROBIT equations to ensure the absence of overpressure and impulse hazards for humans and buildings. The safety radii for humans based on thermal radiation criteria are smaller compared to those based on the corresponding overpressure and impulse hazards.

Comparative effectiveness of C3-Hydrocarbons and their mixtures on suppression of hydrogen-air explosions

The next-generation energy economy relies heavily on hydrogen. Handling hydrogen during production, storage, and transportation requires risk mitigation. In this regard, the application of halide-free additives is very crucial. The present study experimentally and theoretically investigates the individual and combined inhibiting effect of propane and propene on hydrogen-air explosions. The experiments are carried out at 298 K and 1 bar, with additives ranging from 0.5 to 5% for the individual and 1–2% each for the combined effect in the 20–50% hydrogen-air mixture. Experimentally, the additive content in the stoichiometric and rich mixture decreases the explosion pressure, maximum pressure rise (MPR) rate, laminar burning velocity (LBV), and flame temperature, while increasing minimum ignition energy. For instance, the addition of 2% propane in a 50% hydrogen-air mixture, reduces the MPR rate by 98%, whereas such an effect is observed with 2.5% propene. Theoretical study shows that 3% equivalent additive augments inhibiting reactions, countering the main flame-promoting reaction R1 (H + O2O + OH) and thereby reducing LBV. Due to its low activation energy, propene consumes the active radicals twice the rate of propane, while propane not only scavenges the active radicals but also promotes oxygen consumption in the prior phase of combustion, weakening flame propagation. Overall, this approach exhibits a significant step forward in harnessing the potential of hydrogen while mitigating the safety risks associated with its use.

Flame kinetic behavior of premixed hydrogen-air explosion in an obstructed channel

Flame kinetic behavior of premixed hydrogen-air explosion in an obstructed channel

Thermal radiation hazards of the external flow field for vented hydrogen–air explosion: Effect of nitrogen fraction

An experimental investigation is conducted to examine the impact of nitrogen fraction on vented explosions of hydrogen–air–nitrogen mixtures in a 1-m-long cylindrical duct at 1 bar and 281 K. The study employs high-speed shadowgraph imaging, a pressure testing system, and infrared thermal imaging to record the venting process. The results showed that as the nitrogen fraction increases, the rupture time of the vent cover gradually increases. However, the internal peak overpressure exhibits an opposite trend, with P2 (caused by the ignition of unburned gas outside the tube by the escaping flame) near the vent consistently dominating. The frequency of Helmholtz oscillations initially decreases and then increases with increasing nitrogen fraction. Higher nitrogen fractions are associated with a greater likelihood of lower external peak overpressure but a decreasing trend in maximum external impulse. The maximum temperature shows an initial decrease followed by an increase with the addition of nitrogen fraction. This trend is also observed for flame length, flame width, high-temperature duration (>500 °C), and heat energy. Furthermore, the study determined safety zones that are free from thermal radiation damage.

Cellular instabilities of outwardly propagating spherical hydrogen-oxygen flames using a soap bubble method

Explosion hazards associated with hydrogen-oxygen mixtures remain a significant challenge for the energy industry, despite hydrogen garnering attention as a carbon emissions-free alternative energy source. While several studies have been conducted on hydrogen-air explosions, research on hydrogen-oxygen explosions has been limited. In the current study, we employed a spherical soap bubble method to experimentally investigate the flame propagation behavior of an unconfined hydrogen-oxygen explosion. The experimental values of the laminar burning velocity were in agreement with its calculated values and were notably higher than those observed hydrogen-air mixtures. In the case of hydrogen-oxygen mixtures, a spherical expanding flame is accelerated due to the presence of Darrieus-Landau and diffusive-thermal instabilities. Furthermore, we compared the correlation between the critical Pѐclet number and the critical Karlovitz number with the Markstein number for hydrogen-oxygen mixtures to results obtained with propane-oxygen and methane-oxygen mixtures.

Coupling effects of venting and inerting on hydrogen-air explosions at elevated static activation pressures

The effect of varying CO2 addition ratios (λ) on the internal and external pressure characteristics of vented hydrogen-air explosions with an equivalent ratio of 1.5 at elevated static activation pressures (Pstat) of 0.5–2.5 bar was experimentally investigated. The results showed that the inerting effect of CO2 improved the effectiveness of the venting and allowed equilibrium venting at small vent areas and elevated Pstat, further improving the stability and reliability of the hydrogen-air explosive venting technique. However, it was recognized that increasing the CO2 content beyond 10–15% tended to decrease the effectiveness of the maximum pressure rise rate reduction. For external pressure, the peak external explosion pressures decreased significantly as the λ increased. At a fixed λ, the peak external explosion pressure increased and then decreased as the Pstat increased, reaching a peak at 2.0 bar. Based on the laminar flame venting mechanism, a predictive model for the equilibrium venting area with coupled inerting and venting effects was developed. The pseudo-source theory was introduced to establish a model that allowed estimating the peaks of external explosion pressures at elevated Pstat.

Inhibition of hydrogen-air mixtures by using chemical vapor additives

Various approaches are being used for the minimization of risk related to hydrogen hazards. The present study highlights the suppression of hydrogen-air explosion using additives (bromoethane and bromopropane) through experimental and theoretical methods. The experiments were carried out with different equivalence ratios (Ф = 0.5–2) and additive contents (3–15%) at 298 K and 1 bar initial conditions. The results indicate the influence of additives on the maximum pressure-rise, minimum ignition energy and burning velocity. Theoretical study was conducted using CHEMKIN and determined the sensitivity analysis, species profile and reaction path analysis. After the explosion, pressure increases at lower additive fractions for fuel-lean and stoichiometric mixtures and decreases at higher additive contents. In fuel-rich mixtures, pressure rapidly decreases with the increasing additive contents. In the presence of additives, the relative pressure-rise rate decreases, while the time to maximum pressure-rise and ignition energy increase are irrespective of Ф. Moreover, the maximum declination of burning velocity reflects at Φ = 2 and 3% of either additive reduces the burning rate almost by half. The sensitivity coefficient decreases for the flame-promoting reactions and increases for the flame-inhibiting reactions. In such reactions, the inhibiting radicals capture the free radicals and cease the flame propagation, resulting in flame suppression. The reactions R788 (C2H5Br + OH = C2H4Br + H2O) and R834 (C3H7Br + OH = C3H6Br + H2O) have a negative reaction sensitivity coefficient that reflects the effect of suppression. The enhancement of the mole fraction of hydrogen bromide (HBr) with additive content signifies flame suppression. The effect of additives on the explosion characteristics indicates that bromopropane is more effective than bromoethane in minimizing hydrogen hazards.

Effect of nitrogen-modulated laminar burning velocity on duct-vented hydrogen-air explosions

To investigate the combined effect of venting and dilution on flame behavior and pressure evolution, nitrogen gas was used to modulate the laminar burning velocity (LBV) of stoichiometric hydrogen/air mixtures. The results reveal that the coupling of the relief duct and LBV affects the flame dynamic, e.g., at LBV = 114 cm/s, the flame front becomes planar at the end of the enclosure. Lowering the LBV extends the time of the flame front into the vent, and the second peak pressure in the enclosure decreases with longer time intervals. There is a notable reduction in both the duration and magnitude of the negative pressure at the lower LBVs. The original equation is modified to achieve a greater degree of applicability. At lower LBVs, the interaction between the maximum pressure in the duct and the maximum pressure in the enclosure is weak, the correlation is probably variable for different fuel types.

Pore-scale resolved simulation of quenching, acceleration, and transition to detonation of hydrogen explosions by metal foams

Pore-scale simulations of quenching, acceleration, and transition to detonation of gaseous hydrogen explosions by metal foams were performed. The model solves the three dimensional compressible reactive Navier-Stokes equations with detailed chemistry in a closed square channel with a metal foam placed inside. The structures of the foam were fully resolved using an immersed boundary method. Stoichiometric mixtures of hydrogen-oxygen and hydrogen-air were considered. An explosion in the channel was initiated with a high-pressure and high-temperature spark. The simulation results show that the explosion can be quenched, accelerate, or transition to detonation depending on the foam pores-per-inch (PPI). Hydrogen-air explosions were quenched with foams of ∼39 PPI or higher due to heat losses from the flame, which is consistent with experimental observations. The explosion was not quenched by foams with low PPI. Instead the low PPI foams exacerbated the explosion by causing flame acceleration. Worse, a 78 PPI foam caused a stoichiometric H2 and O2 mixture to transition to detonation. Hydraulic resistance due to products venting away from the flame surface was found to produce strong pressure gradients within the foam. This causes a self-pressurizing effect that grows as the flame propagates further into the foam. The pressure within the foam grows and steepens into a shock. The shock initiated the detonation by a Mach reflection, which is similar to deflagration-to-detonation transition mechanisms in traditional obstacle-laden channels. Statement of SignificanceMetal foam structures are often used as devices to inhibit flames in scenarios where explosion hazards are a concern. Direct numerical simulations were performed to increase understanding of how flames and explosions interact with metal foams. The simulation results show that high pores-per-inch (PPI) foams quench the explosion by removing heat from the flame. Lower PPI foams can have the opposite effect and exacerbate the explosion by promoting flame acceleration or even transition to detonation. Flames self-pressurize as they propagate through low PPI foams due to hydrodynamic friction and gas-dynamic effects from flowing product gases. In extreme cases, the pressure waves near the flame can steepen into a shock and initiate a detonation. These results indicate that improperly selected metal foam flame arresters can exacerbate the explosion.

Flameless venting characteristics of hydrogen explosion under the coupling of carbon dioxide and metal foam

To avoid the hazards of vented flame and external explosion caused by hydrogen-air explosion venting, the effects of metal foam and carbon dioxide on the characteristics of vented flame, reduced overpressure, and external pressure are investigated. The results indicate that a combination of inerting (such as adding CO2) and venting technology is recommended to achieve the quenching of vented flames without increasing the reduced overpressure of hydrogen explosions. The effect of the thickness of metal foam and the concentration of CO2 on the length of vented flame is significant, but it only has a minor impact on the width. The laminar burning velocity of hydrogen decreases with the increase of the volume ratio of CO2, which weakened the chemical reaction and reduced overpressure and explosion severity (KG). The characteristics of the external explosion are revealed with the assistance of BOS images. When the venting diameter is 80 mm, the flameless venting efficiency is approximately at 53.33%. Finally, the criterion for heat transfer quenching of vented flames is discussed. The quenching mechanism during flameless venting of hydrogen explosions is analyzed, providing a basic reference for the safety design of hydrogen explosion protection.

Exploring the effects of turbulent field on propagation behaviors in confined hydrogen-air explosion using OpenFOAM

Explosion always happens in turbulent field, which is hard to measure in reality. This paper explores the influence of turbulent kinetic energy on the propagation behaviors of flame and pressure of stoichiometric hydrogen-air explosion in confined vessel, using large eddy simulation. The momentum equation and progressive variable equation are adapted to consider interaction of turbulence and flame. The results reveal that with initial turbulent kinetic energy increasing, its promoting effects on flame and pressure propagation weaken. For flame at low initial turbulent kinetic energy and low initial pressure, it undergoes three propagating stages: acceleration, steady propagation and deceleration. In addition, flame deceleration is caused by rapid decrease of flow velocity which falls faster with initial turbulent kinetic energy increasing. Furthermore, explosion pressure propagates faster with initial turbulent kinetic energy increasing. Sharp fluctuation of flow velocity appears in the flame zone where big pressure gradient exists.

Effect of N2 and CO2 on explosion behavior of hydrogen-air mixtures in non-premixed state

In this paper, the suppression effects of non-premixed inert gas (N2, CO2) on hydrogen-air explosion behaviors are experimentally studied. The effects of N2 and CO2 addition on flame propagation, explosion pressure and flame length are analyzed. The results show that N2 and CO2 can effectively reduce the explosion hazard in non-premixed state. The mixing process of non-premixed gases leads to the development of turbulent flame, but the suppression of inert gas on explosion exceeds the promotion of turbulent flame. Under the action of N2 and CO2, the energy of the flame decreases, and the front contour becomes blurred. When the fuel zone is only set at the ignition end, with the suppression of N2 and CO2, the maximum explosion pressure is reduced by 35.1% and 50.6%, respectively. It is worth noting that the high-frequency periodic oscillation and pressure soaring appear in the pressure histories, which are attributed to the rapid phase transition of water. Furthermore, the two inert gases can significantly reduce the chemical reaction rate and heat release rate during the explosion, which is reflected in the decrease of the maximum flame length. It is found that N2 and CO2 have different explosion suppression effects and mechanisms, but CO2 has better suppression performance on hydrogen-air explosion.

Chemical inhibition of hydrogen-air explosions: Literature review, simulations and experiments

This paper summarises the first results from the joint industry project (JIP) ‘Risk-reduction for hydrogen installations by partial suppression of explosions’ (HyRISE). The main objective of the project is to develop fundamental knowledge that can support the development of practical solutions for mitigating hydrogen explosions in congested and confined environments by means of active systems for chemical inhibition. The aim is not to extinguish the incipient flame, but rather to lower the reactivity of the fuel-air mixture prior to ignition, and thereby reduce the consequences of an accidental explosion to a tolerable level. The paper consists of three parts: a review of previous work on chemical inhibition of premixed hydrogen-air flames, simulations with the chemical kinetics software Cantera, and experiments conducted in a 20-litre explosion vessel. The results demonstrate that chemical inhibition can be effective for hydrogen-air mixtures. However, it remains to develop efficient inhibitors and practical systems that can be implemented in industrial facilities, with satisfactory performance over a wide range of hydrogen concentrations.

Experimental study on the multi-level suppression of N2 and CO2 on hydrogen-air explosion

The multi-level suppression of the hydrogen-air explosion by non-premixed N2 and CO2 in a closed duct was experimentally investigated. The multi-level suppression effect of inert gas on hydrogen-air explosion was discussed by comparing explosion parameters such as explosion pressure, the rate of pressure rise, combustion duration and flame propagation. The results showed that the inert gas adjacent to the explosion zone played a major role in explosion suppression. Compared to single-level explosion suppression, the multi-level explosion suppression was not obvious for inhibitors on the same side, while the multi-level explosion suppression was most efficient for inhibitors on different sides. Moreover, the turbulent development of the flame would exacerbate the explosive reaction, which could not be avoided. The experiment in this paper had great practical significance, and the experimental results could provide data support and theoretical guidance for the safe utilization of hydrogen and the effective suppression of hydrogen-air explosion.

Visualization of the external flow field during a vented explosion for hydrogen-air mixtures: Effects of hydrogen concentrations and vent areas

To study the effects of hydrogen concentrations and vent areas on the vented hydrogen-air explosion, series of experiments were performed in a cylindrical tube under the conditions of 1 atm and 10 ℃. The pressure oscillation frequency is proportional to the hydrogen concentration, while the Helmholtz oscillation disappeared gradually with the increase of Kv. The position inside the container which was farther away from the vent cover would experience the higher dynamic loading. The volume concentration of the hydrogen at the external field (cExt) was always lower than the initial hydrogen concentration, that increased with the increase of the initial concentration, while it was zero at some concentrations with the increase of Kv. The external flame velocity of rich fuel was always larger than that of lean fuel. The deflagration to detonation (DDT) in the outer field was easier to achieve for the concentration near to 50 vol% and for lower Kv. The secondary explosion wave (PExt) increased firstly and then decreased with the increase of hydrogen concentration, which decreased with the increase of Kv in general. The maximum surface temperature rose with the increase of concentration and dropped with the increase of Kv in general. The high-temperature field was closer to the vent for lower concentration and higher Kv.

Vented hydrogen-air explosions at elevated static activation pressures

The flame behaviors and pressure characteristics of vented hydrogen-air explosions at elevated static activation pressures (Pstat) were investigated using a 20 L spherical vessel. For vented flame, it was found that the maximum flame lengths and widths raised with the increase in venting diameters (Dv) and equivalent ratios (Φ) by dominating the concentration of vented unburned gas, while they were insensitive to the change of Pstat. Based on the variation law, non-dimensional formulas for predicting maximum flame length and width were established and the relative error was within 50 %. For internal pressure, the pressure curves were characterized with notably oscillations and an empirical formula coupled with NFPA 68 was proposed with a relative error within 25 %. For external pressure, the mechanism of the external explosion was revealed with the assistance of BOS images. The intensity of external explosion mainly depended on the external flow field intensity and the concentration of the unburned gas. The peaks of external explosion pressure and the pressure recession indices reached their maximum at Dv = 125 mm due to the conflicting effect of the increased Dv.