Effects of mesh aluminium alloys and propane addition on the explosion-suppression characteristics of hydrogen-air mixture

Accidents involving hydrogen explosions occur frequently, yet systematic methods of explosion suppression have not been investigated and applied. Therefore, this paper studied the deflagration characteristics in hydrogen with the addition of propane in the tube filled with mesh aluminium alloys (MAAs). The effects of different propane contents and different filling densities (the mass of MAAs per unit volume in a vessel) on the explosion suppression of the premixed gas are examined. The results show that propane and MAAs can effectively suppress the hydrogen-air explosion. However, MAAs have multiple suppression/promotion effects on the propane-hydrogen explosion. Based on the mathematical model, the dominant effect of MAAs changes abruptly toward the promoting effect when the hydrogen content exceeds 72.26% of the premixed gas stoichiometric concentration. It is also found that an increase in filling density would have a beneficial effect on explosion suppression. The study results provide references for preventing hydrogen and hydrogenated hydrocarbon fuels explosions and optimizing the performance of MAAs.     

Propagation of blast waves from a bursting vessel with internal hydrogen-air deflagration

Most studies on blast waves generated by gas explosions have focused on gas explosions occurring in open spaces. However, accidental gas explosions often occur in confined spaces and the blast wave generates from a bursting vessel as a result of an increase in pressure caused by the gas explosion. In this study, blast waves from bursting plastic vessels in which gas explosions occurred are investigated. The flammable mixtures used in the experiments were hydrogen-air mixtures at several equivalence ratios and a stoichiometric methane-air mixture. The overpressures of the blast waves were generated by venting high-pressure gas in the enclosure and volumetric expansion with a combustion reaction. The measured intensities of the blast waves were greater than the calculated values resulting from high-pressure bursting without a combustion reaction. The intensities of the blast waves resulting from the explosions of hydrogen-air mixtures were much greater than those of the methane-air mixture.     

Explosion characteristics and suppression of hybrid Mg/H2 mixtures

In the production of magnesium hydride, hybrid Mg/H₂ mixtures has a risk of explosion; therefore, systematic experiments were conducted to study its explosion characteristics and suppression. According to the results, the maximum rate of explosion pressure rise (dP/dt)_max of hybrid Mg/H₂ mixtures was more the twice than that of pure Mg. Five common explosion suppressants were individually added to hybrid Mg/H₂ mixtures to inhibit dust explosion. Na₂CO₃, CaCO₃, and (NH₄)₂HPO₃ exhibited great inhibitory abilities for hybrid Mg/H₂ mixtures explosion; however, melamine–cyanurate acid (MCA) and melamine polyphosphate (MPP) could stimulate the explosion. The mechanisms underlying MCA and MPP promotion of the hybrid Mg/H₂ mixtures explosion were determined, namely the high concentration of flammable gases, such as ammonia, released after their thermal decomposition. These gases can ignite and intensify the explosion of hybrid Mg/H₂ mixtures. This study provides a reference for reducing the risk in the H₂ storage system of Mg.     

Numerical investigation of the mechanism behind the deflagration to detonation transition in homogeneous and inhomogeneous mixtures of H2-air in an obstructed channel

The accidental release of hydrogen into enclosures can result in a flammable mixture with concentration gradients and possible deflagration-to-detonation transition (DDT). This numerical study aims to investigate the effect of obstacle spacing and mixture concentration on the DDT in a homogeneous and inhomogeneous hydrogen-air mixture. The paper focuses on the mechanisms behind the DDT in two mixtures with an average hydrogen concentration of 15% and 30%. Unlike the near-stoichiometric mixture, in the lean mixture, DDT only occurs in the inhomogeneous mixture. Depending on obstacle spacing, three different regimes of DDT were observed in the near-stoichiometric inhomogeneous mixture: i) Detonation was ignited when a strong Mach stem formed and propagated between the obstacles; ii) two explosion centers appeared when incident shock and Mach stem reflected from upper and lower obstacles, respectively; iii) Mach stem did not form but DDT occurred behind the flame front at the top of the obstacle.     

Scaling-effect of explosion in H2/CH4/air mixtures

The scaling-effect of mixture explosion is an unresolved issue in explosion science. In this work, we carry out experimental measurements of explosion characteristics using hydrogen/methane/air (H₂/CH₄/air) mixtures in two tubes with lengths of 1.5 m and 60 m. The explosion overpressure of the mixtures increases exponentially with hydrogen mole fractions in the small tube, as expected. In contrast, explosion overpressure increases rapidly, causing detonation when hydrogen is added to the mixtures. Comparing measurements in both tubes, the explosion overpressure exhibits a clear scaling-effect dependence on the tube size. The scaling-effect cannot be explained by the aspect ratio (AR) of the tube. The analysis of the hotspot size, which is correlated with the ignition delay time of mixtures, is the critical factor governing the scaling-effect of explosion seen in a large tube.     

A safe and clean way to produce H2O2 from H2 and O2 within the explosion limit range

The direct synthesis of hydrogen peroxide (DSH) from hydrogen and oxygen is an attractive production route due to its green nature. However, it faces multiple technical challenges, the biggest being the explosion risk of the flammable gas mixture. Herein we have used microreactors to perform the reaction in an inherently safer way which allows the hydrogen concentration to fall within the explosion limit range. For the first time, we have studied the flame propagation phenomena inside a microreactor to determine the optimum channel dimension for DSH. A mechanism of “fast synthesis and slow destruction” has been proposed via investigation on the influence of channel length and liquid flow rate. Besides, a variety of reaction parameters including gas flow rate, oxygen: hydrogen ratio, catalyst composition and gas pressure have been studied carefully. The successful employment of a microreactor in this case has indicated the potential of using microreactors to inhibit the explosion risks of hazardous processes.     

Vented confined explosions involving methane/hydrogen mixtures

The EC funded Naturalhy project is assessing the potential for using the existing gas infrastructure for conveying hydrogen as a mixture with natural gas (methane). The hydrogen could then be removed at a point of use or the natural gas/hydrogen mixture could be burned in gas-fired appliances thereby providing reduced carbon emissions compared to natural gas. As part of the project, the impact on the safety of the gas system resulting from the addition of hydrogen is being assessed. A release of a natural gas/hydrogen mixture within a vented enclosure (such as an industrial housing of plant and equipment) could result in a flammable mixture being formed and ignited. Due to the different properties of hydrogen, the resulting explosion may be more severe for natural gas/hydrogen mixtures compared to natural gas. Therefore, a series of large scale explosion experiments involving methane/hydrogen mixtures has been conducted in a 69.3 m³ enclosure in order to assess the effect of different hydrogen concentrations on the resulting explosion overpressures. The results showed that adding up to 20% by volume of hydrogen to the methane resulted in a small increase in explosion flame speeds and overpressures. However, a significant increase was observed when 50% hydrogen was added. For the vented confined explosions studied, it was also observed that the addition of obstacles within the enclosure, representing congestion caused by equipment and pipework, etc., increased flame speeds and overpressures above the levels measured in an empty enclosure. Predictions of the explosion overpressure and flame speed were also made using a modified version of the Shell Global Solutions model, SCOPE. The modifications included changes to the burning velocity and other physical properties of methane/hydrogen mixtures. Comparisons with the experimental data showed generally good agreement.     

Minimum ignition energy of hydrogen-air and methane-air mixtures at temperatures as low as 200 K

The primary objective of this study is to measure the minimum ignition energy (MIE) of methane-air and hydrogen-air mixtures at low temperatures and atmospheric pressure. Initial fuel-air mixture temperatures as low as 200 K were considered, for a constant equivalence ratio of 1.0 for methane-air and 0.16 for hydrogen-air. The ignition source was a spark, generated by a high-voltage pulse of 100 μs duration, applied on two pin electrodes of 0.1-mm diameter, separated by a gap distance of 1 mm. The experimental methodology was validated by comparing the results obtained with those from previous studies available in the literature. First, for methane-air mixtures, the MIE as a function of the equivalence ratio followed the same trend at 295 K and 255 K, i.e., its lowest value was obtained for a stoichiometric mixture. Second, when the temperature of the mixture was decreased, the MIE increased linearly for both fuels. The rate at which the MIE changed was higher for hydrogen-air (?7.9 μJ/K) than for methane-air (?3.4 μJ/K). Overall, this study provides valuable information on the MIE of methane-air and hydrogen-air mixtures at low temperatures, which can be useful for the design of cryogenic fuel storage systems.     

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.     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 1: Effects of increase in congestion

Experimental studies were conducted with the objective of gaining a better understanding of the potential explosion hazard consequences that could be associated with a high-pressure leak from a hydrogen vehicle refuelling system. The first part of the study, described in this paper, was a series of experiments designed to establish hydrogen–air explosion overpressures in a well-defined and well understood 3 m × 3 m x 2 m (high) repeated pipe congestion. The results of the experiments are discussed in terms of the conditions leading to the greatest overpressures. It is concluded from the study that stoichiometric ratio in the range of 1.2–1.3 gives highest overpressure. Moreover, it was observed that increasing the congestion from 4-gate to 9-gate congestion leads to significant increase in the overpressure. In addition, it was concluded that, explosion in a hydrogen-air mixture is significantly more severe than the explosion in an ethane-air, methane-air or propane-air mixtures. This is attributed to higher laminar flame speed of hydrogen-air mixtures.     

Study on leakage and explosion consequence for hydrogen blended natural gas in urban distribution networks

It appears to be the most economical means of transporting large quantities of hydrogen over great distances by the existing natural gas pipeline network. However, the leakage and diffusion behavior of urban hydrogen blended natural gas and the evolution law of explosion characteristics are still unclear. In this work, a Computational Fluid Dynamics three-dimensional simulation model of semi-confined space in urban streets is developed to study the diffusion process and explosion characteristics of hydrogen-blended natural gas. The influence mechanism of hydrogen blending ratio and ambient wind speed on the consequences of explosion accident is analyzed. And the dangerous area with different environmental wind effects is determined through comparative analysis based on the most dangerous scenarios. Results indicate that the traffic flow changes the diffusion path of the jet, the flammable gas cloud forms a complex profile in many obstacles, high congestion level lead to more serious explosion accidents. Wind effect keeps the flammable gas cloud near the vehicle flow, the narrow gaps between the vehicles aggravate the expansion of the flammable gas cloud. When the wind direction is consistent with the leakage direction, hydrogen blended natural gas is gathered in the recirculation zone due to the vortex effect, which results in more serious accident consequences. With the increase in hydrogen blending ratio, the higher content of H and OH in the gas mixture significantly increases the premixed burning rate, the maximum overpressure rises rapidly when the hydrogen blend level increases beyond 40%. The results can provide a basis for construction safety design, risk assessment of leakage and explosion hazards, and emergency response in hydrogen blended natural gas distribution systems.     

Explosion venting of rich hydrogen-air mixtures in a small cylindrical vessel with two symmetrical vents

The safety issues related to explosion venting of hydrogen-air mixtures are significant and deserve more detailed investigations. Vented hydrogen-air explosion has been studied extensively in vessels with a single vent. However, little attention has been paid to the cases with more than one vent. In this paper, experiments about explosion venting of rich hydrogen-air mixtures were conducted in a small cylindrical vessel with two symmetrical vents to investigate the effect of vent area and distribution on the pressure buildup and flame behavior. Experimental results show that venting accelerates the flame front towards the vent but has nearly no effect on the opposite side. The maximum internal overpressure decreases while the maximum external flame length increases with the increase of the vent area. Two pressure peaks can be identified outside the vessel, which correspond to the external explosion and the following gas jet, respectively. Compared with the case of single vent, the use of two vents with same total vent area leads to nearly unchanged maximum internal and external overpressure but much smaller external flame length.     

Explosion characteristics of n-decane/hydrogen/air mixtures

Hydrogen can be used in conjunction with aviation kerosene in aircraft engines. To this end, this study uses n-decane/hydrogen mixtures to investigate the explosion characteristics of aviation kerosene/hydrogen in a constant volume combustion chamber with different hydrogen addition ratios (0, 0.2, 0.4), wide effective equivalence ratios (0.7–1.7), an initial temperature of 470 K, and initial pressures of 1 and 2 bar. The results show that the explosion pressure and explosion time decrease linearly with increasing hydrogen addition ratio. The effect of initial pressure is also discussed. A comparison of the adiabatic explosion pressures indicates that the hydrogen addition effect varies at different initial pressures and effective equivalence ratios owing to heat loss. In addition, the maximum pressure rise rate and deflagration index increase with increasing hydrogen concentration, which is more obvious for rich mixtures and high hydrogen concentrations.     

Effect of concentration,obstacles, and ignition location on the explosion overpressure of hydrogen-air in a closed-vessel

The report deals with the investigation of explosion safety parameters of hydrogen-air mixtures in a 17.17 L cylindrical closed-vessel with different concentrations, obstacles, and ignition locations. The experimental data including the maximum explosion pressure, laminar burning velocity, and corresponding flame radius were confirmed by using GASEQ code and theoretical calculation, respectively. The report shows the orifice plate reduced the maximum explosion pressure of the low-concentration hydrogen (φ＜20% v/v), while the maximum explosion pressure of high-concentration hydrogen (φ＞20% v/v) was increased, and the oscillation of the explosion pressure in the closed-vessel was obvious. The effect of the ignition location on the maximum explosion pressure was related to the interaction between the flame instability and the orifice plate for the φ = 30% v/v hydrogen-air mixture.     

Experimental and theoretical study on the suppression effect of CF3CHFCF3 (FM-200) on hydrogen-air explosion

This study experimentally and numerically determined the effect of FM-200 on H₂/air explosion. Firstly, the explosion pressure was investigated to evaluate the suppression efficiency. The results indicated that the effect of FM-200 on H₂/air explosion was quite different for various equivalence ratios. FM-200 could enhance the explosion at lean mixture, but suppress the explosion at rich mixture. Then, the burning velocity, heat production and temperature free radicals were investigated. The results also demonstrated that FM-200 exhibited stronger suppression effect in rich explosion. In addition, the increase of free radicals indicated the enhancement effect of FM-200 at lean explosion. Last, the analysis of sensitivity and reaction path was performed to understand the suppression kinetics. It was shown that R1466 and R1468 could suppress explosion at Φ = 1.3 and 1.6, however, they changed into promoting explosion at Φ = 0.8 and 1.0. Moreover, the reaction path analysis indicated that CHF:CF₂→CHF:O→CO could enhance explosion at Φ = 0.8. For CHF:CF₂→CH₂F→HF, it played an important role in scavenging H to suppress explosion at Φ = 1.6. Furthermore, it was indicated that there was a competition between the enhancement and suppression effect at Φ = 1.3.     

Vented deflagration of a hydrogen-air mixture in a rectangular vessel with a hinged aluminum vent panel

Experiments on explosion venting of a stoichiometric hydrogen-air mixture ignited near the top vent of a 1-m³ rectangular vessel with a hinged aluminum vent panel were performed to investigate the effect of the panel area density on the pressure build-up and flame behavior. When using aluminum panels, three pressure peaks could be distinguished in the pressure-time histories. The first pressure peak, which increases with the panel area density, is the dominant one. However, the second and the third pressure peaks, with magnitudes ranging from 5 to 10 kPa, are independent of the panel area density. The use of aluminum panels weakens the external explosion because the gas mixtures were vented laterally shortly after the vent panel was opened. Panel inertia has a negligible effect on the final stage of the downward propagating flame. The maximum external flame length decreases with the increase in panel area density.     

Effects of hydrogen addition on the explosion characteristics of n-hexane/air mixtures

To study the effects of hydrogen addition on the explosion characteristics (the explosion pressure and maximum rate of pressure rise) of n-hexane/air mixtures, experiments were performed in a cylindrical vessel at 100 kPa, 353 K, with equivalence ratios of 0.8–1.7 and hydrogen addition range from 0% to 80%. Concurrently, flame images were captured by high-speed schlieren photography to study the burning performance. The results indicate that both the explosion pressure and maximum pressure rise rate increase with the increase in hydrogen addition in terms of the lean n-hexane/hydrogen/air mixtures. With respect to the richer mixtures, however, the inverse tendency is observed. With increasing hydrogen fractions, the explosion pressure and maximum pressure rise rate decrease. The peak values of the explosion pressure and maximum pressure rise rate shift to the leaner mixture with increased hydrogen proportion. Moreover, the laminar burning velocities of n-hexane/hydrogen/air mixture were also obtained via the expanding spherical method and the pressure-time histories, respectively. Variation of laminar burning velocity with hydrogen proportion from both methods were studied as well, and the results show that the laminar burning velocity changes significantly under different hydrogen addition.     

A review of hydrogen-air cloud explosions: The fundamentals,overpressure prediction methods,and influencing factors

Hydrogen is one of the most promising renewable energies that has been observing rapid development over the past years. Recent accidental explosion incidents and the associated damages have demonstrated the importance of hydrogen safety against potential explosions. This article presents a systematic review on hydrogen explosions. Potential explosion scenarios including the existence of impurities and rich-oxygen environment in the production, storage with extreme-high pressure and ultra-low temperature, transportation, and consumption processes are reviewed. Different types of hydrogen-air cloud explosion include expansion and deflagration, detonation, and deflagration-to-detonation transition (DDT). Existing studies on hydrogen explosion covering laboratory and field blasting test, numerical simulation utilizing various computational approaches, and theoretical derivation are reviewed and summarized. CFD modeling is currently one of the main research methods because of its cost effectiveness, though challenges existing in simulation hydrogen-air cloud detonation comparing with testing results. Apart from the properties of hydrogen-air cloud such as concentration, size and heterogeneity, environmental factors such as ignition, ventilation and obstacle are found to strongly influence the loading characteristics of hydrogen-air cloud explosion. Existing prediction approaches for estimating blast loading from hydrogen-air cloud explosion including the TNT equivalent method (TNT-EM), TNO multi-energy method (TNO MEM), and Baker-Strehlow-Tang method (BST) are primarily empirical based. Because of the inherited difference of hydrogen-air cloud from solid explosives and conventional flammable gases, the accuracies of these approaches are still doubtable, which requires further study.     

Numerical and experimental study of the influence of CO2 dilution on burning characteristics of syngas/air flame

In this study, effect of carbon dioxide dilution on explosive behavior of syngas/air mixture was investigated numerically and experimentally. Explosion in a 3-D cylindrical geometry model with dimensions identical to the chamber used in the experiment was simulated using ANSYS Fluent. The simulated results showed that after ignition, the flame front propagated outward spherically until it touched the wall, like the propagating flame observed in the experiment. Both experimental and simulated results presented a same trend of decreasing the maximum explosion pressure and prolonging the explosion time with CO₂ dilution. The results showed that for CO₂ additions, the maximum explosion pressure decreased linearly and the explosion time increased linearly, while the maximum rate of pressure rise decreased nonlinearly, which can be correlated to an exponential equation. In addition, both results showed a good agreement for syngas/air flame with CO₂ addition up to 20% in volume. However, larger discrepancies were observed for higher levels of CO₂ dilutions. Of the three diluents tested, carbon dioxide displayed the strongest effect in reducing explosion hazard of syngas/air flame compared to helium and nitrogen. Chemical kinetic analysis results showed that maximum concentration of major radicals and net reaction rates of important reactions drastically decreased with CO₂ addition, causing a reduction of laminar flame speed.     

Experimental studies on the explosion indices in turbulent stoichiometric H2/CH4/air mixtures

Explosion characteristics of the stoichiometric hydrogen/methane/air mixtures with different hydrogen fractions (λ) and different turbulent intensities (u'_rms) in a fan-jet-stirred spherical explosion vessel. From the experimental results, it could be clearly found that both the maximum explosion overpressure (p_max) and the maximum rise rate of overpressure rose with the increase of u'_rms, but the major reasons to such rising were not totally the same. In turbulence, with the increase of λ, p_max declined but (dp/dt)_max rose, and such behaviours were mainly attributed to the completion on the variations between propagation speed and adiabatic explosion pressure. The explosion duration (t_c) was also measured, it rose with the increase of u'_rms and/or λ for the enhancement on propagation albeit such enhancement was attributed to different mechanism for different influence factors. The variations of deflagration index (K_G) indicated that the hazardous level of stoichiometric hydrogen/methane mixtures would become more hazardous in the presence of turbulence. Furthermore, the heat loss during the explosion also was calculated and analysed. The results reported in this article could provide more basic but important information to practical utilizations of hydrogen/methane blended fuels, especially on the safety protection strategies.