Detonation performance of cylinder TNT-RDX explosives wrapped by spiral aluminum tube storing high-pressure active gases

Cylinder TNT-RDX explosives were wrapped by a spiral aluminum tube that stored high-pressure active gases (H₂, O₂, or CO₂) to improve the detonation performance. Their detonation performance was investigated by underwater explosion and air explosion systems. The underwater explosion results show that hydrogen participating in the detonation reaction of explosives mainly improves the peak overpressure, and simultaneous fragmentation and combustion of the spiral aluminum tube mainly contribute to the improvement of energy parameters. In the air explosion, the hydrogen utilizes ambient oxygen to further improve the peak overpressure of composite explosives. The increase amplitude of energy parameters in the air explosion is much lower than that in the underwater explosion due to the air explosion with a higher attenuation speed of shock waves and detonation temperature. The increase amplitude of oxygen to the peak overpressure (6.429%) is less than that of hydrogen to the peak overpressure (8.066%). The energy improvement of oxygen on composite explosives is best due to changing the object that reacts with the aluminum fragments. The average fragment length of aluminum tube calculated by the Grady model and the Goloveshkin model indicates that aluminum fragments formed by the aluminum tube increase the characteristic length of aluminum that can participate in the reaction.     

瓦斯爆炸冲击波在并联巷道中传播特性的数值模拟

运用AutoReaGas软件模拟了煤矿井下掘进工作面发生瓦斯爆炸时冲击波向临近采煤工作而的传播规律.研究结果表明：冲击波在巷道中传播时,超压峰值和最高温度不断减小,到回风巷交叉口时,两条相向传播的冲击波产生叠加效应,使超压和温度明显增大;最高温度沿着巷道的变化规律与超压峰值基本相同,两个并联采煤工作而对应测点的超压峰值...     

Effects of combined obstacles on deflagration characteristics of hydrogen-air premixed gas

To study the mechanism by which an increase in the number of obstacles affects the propagation of hydrogen-air premixed gas explosions under a constant overall volume of obstacles, a large eddy simulation method was used to carry out numerically simulate configurations with different distribution modes of combined obstacles. The study focused on the flame structure, evolution process of overpressure dynamics, and flame-flow coupling relationship. The results showed that the flame propagation velocity and flame front area are increased during the conversion of the combined obstacles from 1-30 mm to 4–7.5 mm, while the flame front area logarithmically depends on the number of obstacles. The flames gradually develop from “corrugated flamelets” to “thin reaction zones” in different distribution modes. In addition, the results showed that although increasing dispersion increases the explosion overpressure, a critical number of obstacles likely exist. Beyond the critical point, explosion overpressure peak no longer strongly varies with the number of obstacles. Furthermore, for working configurations with different numbers of obstacles, an increase in the overall number of obstacles before reaching the same number of obstacles weakly affects the flame shape and flow rate of the flame front. This study provides theoretical guidelines for safety designs to prevent hydrogen-air premixed gas explosion in obstructed spaces.     

Flame dynamics in a vented vessel connected to a duct: 1. Mechanism of vessel-duct interaction

The paper presents a parametric experimental study of explosion initiated in a vessel and vented through a duct. The aim is to clarify the mechanism of the vessel-duct mutual interaction during explosion and its role in determining the overpressure in the vessel. For a vessel of fixed size, the chosen parameters are the diameter and length of the discharge duct and the profile of area change between the vessel and the duct.Results with a stoichiometric propane-air mixture demonstrate that the change of the combustion regime inside the vessel, responsible for augmented overpressure, is driven by an impulse generated in the initial part of the duct (during an explosion-like combustion) shortly after the flame penetrates into it. The origin of the impulse is discussed. Increasing the duct/vessel diameter ratio or using the profiled vessel-duct passage weakens the generated impulse (decreases its pressure and velocity amplitudes) but at the same time generates higher subsequent pressure rise in the vessel. This implies that enhancement of the explosion overpressure in the vessel is unavoidable in this type of venting.     

Exploration of critical hydrogen-mixing ratio of quenching methane/hydrogen mixture deflagration under effect of porous materials in barrier tube

To improve the safety of the methane/hydrogen mixture pipeline network, The experimental deflagration quenching behavior of porous materials on hydrogen mixed methane in barrier tubes was studied, the influence of the hydrogen mixing ratio on the quenching results of porous materials and the transient change of overpressure was discussed, the critical quenching hydrogen mixing ratio of porous materials was explored. Results show that the hydrogen mixing ratio has a significant effect on the quenching results of porous materials. According to the different quenching results of porous materials under different hydrogen mixing ratios, the successful quenching zone (φ<19%) and the quenching failure zone (φ ≥ 19%) can be divided. It can be determined that the critical quenching hydrogen mixing ratio is φ = 19%. The critical quenching speed is 33.0 m/s. When the porous material is coupled with hydrogen mixing, the pressure curve appears as a “multi-peak” phenomenon, and the maximum pressure peak is generated by the “multi-peak” game. If the hydrogen mixing ratio is greater than the critical quenching hydrogen mixing ratio, it may bring about the uncertainty of the maximum pressure peak and increase the unpredictability of the explosion hazard to the gas pipeline network. Therefore, reasonable hydrogen mixing is conducive to improving the safety of methane/hydrogen mixture pipeline network transportation. The research results could provide an important reference for the engineering application of methane/hydrogen mixture flame arrester design and the selection of safe hydrogen concentration.     

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.     

燃料电池船舶舱内氢气泄漏爆炸的数值模拟研究

以燃料电池客船“Water-Go-Round”号为对象,利用FLUENT软件模拟燃料电池客船舱内管道发生氢气泄漏并引发爆炸的情况,研究不同舱室氢气点火爆炸事故的影响规律。结果表明：可燃氢气云被点燃后,爆炸超压波自点火位置向四周迅速传播,点火位置对超压波的分布影响较大;控制舱爆炸时,超压强度最大,对船体超压危害最大;乘客舱爆炸强度最小,但超压中心分布在乘客舱,超压对乘客造成的危害最大;船舶舱室燃烧火焰温度主要由可燃氢气云的分布决定,燃料电池舱的火焰衰减趋势基本相同;乘客舱受到的高温危害较低,船艏舱无燃烧火焰的高温危害。     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 2: Effects of increase in hydrogen concentration in hydrogen-methane-air mixture

Hydrogen is seen as an important energy carrier for the future which offers carbon free emissions. At present it is mainly used in refueling hydrogen fuel cell cars. However, it can also be used together with natural gas in existing gas fired equipment with the benefit of lower carbon emissions. This can be achieved by introducing hydrogen into existing natural gas pipelines. These pipelines are designed, constructed and operated to safely transport natural gas, which is mostly methane. Because hydrogen has significantly different physical and chemical properties than natural gas, any addition of hydrogen my adversely affect the integrity of the pipeline network, increasing the likelihood and consequences of an accidental leak. Since it increases the likelihood and consequences of an accidental leak, it increases the risk of explosion. In order to address various safety issues related to addition of hydrogen in to a natural gas pipeline a EU project NATURALHY was introduced. A major objective of the NATURALHY project was to identify how much hydrogen could be introduced into the natural gas pipeline network. Such that it does not adversely impact the safety of the pipeline network and significantly increase the risk to the public. This paper reports experimental work conducted to measure the explosion overpressure generated by ignition of hydrogen-methane-air mixture in a highly congested region consisting of interconnected pipes. The composition of the methane/hydrogen mixture used was varied from 0% hydrogen (100% methane) to 100% hydrogen (0% methane) to understand its effect on generated explosion overpressure. It was observed that the maximum overpressures generated by methane-hydrogen mixtures with 25% (by volume) or less hydrogen content are not likely to be significantly greater than those generated by methane alone. Therefore, it can be concluded that the addition of less than 25% by volume of hydrogen into pipeline networks would not significantly increase the risk of explosion.     

Experimental study on the propagation characteristics of hydrogen/methane/air premixed flames in a narrow channel

This work is focused on the explosion characteristics of premixed gas containing different volume fractions of hydrogen in a narrow channel (1000 mm × 50 mm × 10 mm) under the circumstance of stoichiometric ratio. The ignition positions were set in the closed end and the middle of the pipeline respectively. The results showed that when the gas was ignited at the pipeline closed end, the propagating flame was tulip structure for different premixed gas. When the hydrogen volume fraction was less than 40%, the flame propagation speed increased significantly with the rise of hydrogen volume fraction, and the overpressure peak also appeared obviously in advance. However, when the volume fraction of hydrogen was more than 40%, the increase of flame propagation speed and the overpressure peak occurrence time varied slightly. Furthermore, when the ignition position was placed in the middle of the pipeline, the flame propagation speed propagating to the opening end was much faster than that propagating to the closing end, and there was no tulip shape when the flame propagates to the opening end. The flame propagating to the closed end appeared tulip shape under the influence of airflow, and high-frequency flame oscillation occurred during the propagation. This work shows that the hydrogen volume fraction and ignition position significantly affected the flame structure, flame front speed, and explosion overpressure.     

Rethinking “BLEVE explosion” after liquid hydrogen storage tank rupture in a fire

The underlying physical mechanisms leading to the generation of blast waves after liquid hydrogen (LH2) storage tank rupture in a fire are not yet fully understood. This makes it difficult to develop predictive models and validate them against a very limited number of experiments. This study aims at the development of a CFD model able to predict maximum pressure in the blast wave after the LH2 storage tank rupture in a fire. The performed critical review of previous works and the thorough numerical analysis of BMW experiments (LH2 storage pressure in the range 2.0–11.3 bar abs) allowed us to conclude that the maximum pressure in the blast wave is generated by gaseous phase starting shock enhanced by combustion reaction of hydrogen at the contact surface with heated by the shock air. The boiling liquid expanding vapour explosion (BLEVE) pressure peak follows the gaseous phase blast and is smaller in amplitude. The CFD model validated recently against high-pressure hydrogen storage tank rupture in fire experiments is essentially updated in this study to account for cryogenic conditions of LH2 storage. The simulation results provided insight into the blast wave and combustion dynamics, demonstrating that combustion at the contact surface contributes significantly to the generated blast wave, increasing the overpressure at 3 m from the tank up to 5 times. The developed CFD model can be used as a contemporary tool for hydrogen safety engineering, e.g. for assessment of hazard distances from LH2 storage.     

Comparison of the explosion characteristics of hydrogen,propane, and methane clouds at the stoichiometric concentrations

Numerical simulations were performed to study explosion characteristics of the unconfined clouds. The examined cloud volume was 4 m × 4 m × 2 m. The build-in obstruction inside the cloud was the 8 × 8 × 4 perpendicular rod array. The obstacle volume blockage ratio was 0.74. Three gases were considered: hydrogen/air at the stoichiometric concentrations, propane/air at the stoichiometric concentrations, and methane/air at the stoichiometric concentrations. The hydrogen/air cloud explosion has higher peak overpressure and the overpressure rises locally at the nearby region of the cloud boundary. The explosion overpressures of both methane/air and propane/air are lower, compared with the hydrogen/air, and decreases with distance. The maximum peak dynamic pressure is reached beyond the original cloud, which is clearly different from the explosion peak overpressure tends. Furthermore, dynamic pressure of a cloud explosion is of the same order as overpressure. The explosion flame region for the hydrogen/air cloud is approximately 1.25 times of the original width of the cloud. The explosion flame regions for propane/air or methane/air clouds are approximately 1.4 times of the original width of the cloud. Unlike the explosion overpressures, the explosion temperatures have little difference between the three mixture examined in this study. The higher energy of explosive mixture generates a high temperature hazard effect, but the higher energy of explosive mixture may not generate a larger overpressure hazard effect in a gas explosion accident.     

Effects of hydrogen concentration and film thickness on the vented explosion in a small obstructed rectangular container

The explosion venting is an effective way to reduce hydrogen-air explosion hazards, but the explosion venting has been less touched in an obstructed container. The present study mainly focused on the effects of hydrogen concentration and film thickness on the explosion venting in a small obstructed rectangular container. High speed schlieren photography was employed to obtain the flame fine structure and velocity. Pressure transducers were used to measure the overpressure nearby the obstacle. The experimental results show that the obstacle has a significant effect on the flame shape, tip speed and overpressure. In the process of flame evolution, the flame surface becomes more wrinkled with time after the tulip flame. Compared with the cases without the obstacle, the flame surface becomes more distorted and wrinkled downstream of the obstacle under the influence of obstacle enhanced turbulence and flow instability. Upstream of the obstacle, the lower part of the flame surface becomes concave while the upper part shows convex. The pressure histories show that the maximum overpressure increases with the hydrogen concentration in the range of 11.8%–23.7%. Two main pressure peaks were observed for all hydrogen concentrations in the presence of the obstacle. The Helmholtz oscillations appear after the second pressure peak and its duration increases slightly when the hydrogen concentration increases. The combined effect of the obstacle and hydrogen concentration on the second peak overpressure is more significant than on the first peak overpressure. Moreover, the maximum overpressure shows a monotonic increase with the film thickness.     

A Study of the Impulse Wave Discharged from the Exits of Two Parallel Tubes

The twin impulse wave leads to very complicated flow fields, such as Mach stem, spherical waves, and vortex ring. The twin impulse wave discharged from the exits of the two tubes placed in parallel is investigated to understand the detailed flow physics associated with the twin impulse wave, compared with those in a single impulse wave. In the current study, the merging phenomena and propagation characteristics of the impulse waves are investigated using a shock tube experiment and by numerical computations. The Harten-Yee's total variation diminishing (TVD) scheme is used to solve the unsteady two-dimensional compressible Euler equations. The Mach number Ms of incident shock wave is changed below 1.5 and the distance between two-parallel tubes, L/d, is changed from 1.2 to 4.0. In the shock tube experiment, the twin impulse waves are visualized by a Schlieren optical system for the purpose of validation of computational work. The results obtained show that on the symmetric axis between two-parallel tube     

Explosion venting of hydrogen-air mixtures from a duct to a vented vessel

Experiments were conducted on the vented explosion of hydrogen–air mixtures from a 150-cm-long duct to a cylindrical vessel with a vent at the center of its side wall to investigate the effects of vent burst pressure and an obstacle in duct on the process of explosion venting. Turbulent pressure oscillation owing to a pressure wave moving back and forth in a duct and vessel was observed for unvented explosions. For explosion venting from duct to vessel, flame acceleration in duct much increases the explosion overpressure in vessel. The maximum explosion in duct is always higher than that in vessel, and both of them increase with an increase in the vent cover thickness. An obstacle installed in duct significantly affected the explosion overpressure, which first increased and then decreased with an increase in the blockage ratio. Three pressure peaks were distinguished in the external pressure-time histories, which were resulted form different pressure waves formed outside the vessel.     

Ceiling-vented deflagrations of a hydrogen-air mixture in a 75 m3 container

A series of hydrogen explosions were conducted in a real scale container with and without vents. The effects of hydrogen concentration, ignition location, obstacles on the vented hydrogen-air explosions were investigated. Hydrogen explosions with concentration of 12% and 16% were conducted in constant volume container, and the maximum peak overpressure can reach 45 kPa and 175 kPa, respectively. During the vented hydrogen explosions, three overpressure peaks generated by the vent rupture, Helmholtz oscillation, and the thermo-acoustic-vibration coupling, respectively, were recorded. The maximum peak overpressure is about several kilopascal, the pressure reduction can reach 97.1% by comparison with peak overpressure developed in closed container. The obstacles change the way that the flame travel inside the container, and resultantly the flame propagates vertically and increases the flame area, which promotes the reaction and increases the peak overpressure, which also increases with the hydrogen concentration. Three engineering models used to depict the relationship between the vent area and the maximum reduced explosion pressure were assessed. Results show that the these models over-predict the maximum reduced explosion pressure for high-reactivity mixtures. However, for low reactivity, the Molkov’ models show a scatter while the NFPA68 gives a better prediction.     

Effects of nitrogen addition on vented hydrogen–air deflagration in a vertical duct

In this paper, experiments were performed to investigate the coupling effects of venting and nitrogen addition ratio (χ) on flame behavior and pressure evolution during hydrogen–air deflagration within and outside a 1-m-high vertical duct with a vent on its top. Experimental results reveal that χ has significant effects on the pressure–time histories in the duct. Helmholtz oscillations of the internal overpressure were observed in all tests, and acoustic type oscillations appears in the tests only for χ = 25% and 30%. For a certain χ, the maximum overpressure (Pmax) increased with the distance to the vent, i.e., the highest overall explosion overpressure was attained near the duct bottom; however, the difference in Pmax between various measuring points decreases with an increase in χ. In all tests, a pressure peak in the duct was observed shortly after external explosion. The maximum internal and external overpressure decreased as χ was increased.     

Effects of the geometry of downstream pipes with different angles on the shock ignition of high-pressure hydrogen during its sudden expansion

To investigate the effects of the geometry of downstream pipes on the shock ignition and the formation of the shock waves during high-pressure hydrogen sudden expansion, a series of bench-mark experiments were designed and high-pressure hydrogen were released into five types of pipes with different angles (60, 90, 120, 150 and 180°). It was found that the geometry of downstream pipes had a significant influence on the shock ignition of hydrogen. The incident shock wave would be reflected at the corner of the pipes with angles of 60, 90, 120 and 150°. The intensity of the reflected shock wave is higher if the angle is smaller. In addition, the average velocity of the leading incident shock wave would decrease when it passed the corner of the pipe. Using a pipe with smaller angle significantly increases the likelihood of shock ignition and lowers the minimal required burst pressure for shock ignition. The overpressure of the incident shock waves inside the exhaust chamber (for the cases with the angles of 60, 90, 120 and 150°) decreases sharply. There are three flame propagation behaviors inside the exhaust chamber: flame quenching, flame separation and no flame separation. The results of this study have implications concerning designs for storage safety of hydrogen energy and may help get better understanding of shock ignition mechanism of high pressure hydrogen and effect of pipeline geometry on ignition.     

ON INERTIA EFFECTS IN THERMOELASTICITY AND THERMAL SHOCK PARAMETERS

A material parameter R_d is presented, called the “dynamic thermal shock resistance,” which measures the resistance to thermal stress waves induced by very rapid body heating, e.g. by radiation, of both slender and massive bodies. This dynamic parameter is the complementing counterpart to the well-known “quasi-static thermal shock resistance” R_s, which is the classical measure for the resistance to thermal stresses due to temperature gradients induced by ordinary contact (or surface) heating when inertia effects may be neglected. The two material parameters. R_d and R_s, give a very different ranking of materials. Both parameters have an important physical significance: they place a restriction, solely defined in terms of material properties, on the maximum value of the product of a characteristic structural dimension and the specific thermal power or heating.     

Mechanism of indirect initiation of detonation

Based on the first-order Arrhenius kinetics of chemical reaction and hydrodynamics, we proposed a mechanism to interpret the physical process of detonation onset. In the proposed mechanism, all the movements of chemical mixture are described by the characteristic waves of hyperbolic system. Each wave in different manner contributes to the transition from deflagration to detonation. The triggered detonation is the result of interaction of the multiple waves, more accurately, is a direct result of the re-ignition in the gaseous explosive in the unreacted zone by the reaction-released energy that is transferred in the form of the characteristic wave. This mechanism provides a complete and theoretic explanation to “explosion in the explosion” observed in experiments. It associated with the traditional ignition theory may be used to build up the criterion for deflagration-to-detonation transition (DDT). The mechanism is further verified by our numerical solutions to the mathematic model.     

External overpressure of vented hydrogen-air explosion in the tube

Vented explosion experiments involving hydrogen-air mixtures are performed in a 2 m-long cylindrical tube under the influences of the hydrogen concentration and vent burst pressure. Photos of the external flame shot by a high-speed camera show that the jet flame was expelled outside the vessel, and the relation between the flame propagation and external overpressure is summarized. The internal peak overpressure increases and then decreases with increasing hydrogen concentration. In contrast, the external peak overpressure exhibits the opposite correlation in comparison with the internal peak overpressure. The variations in the pressure peaks of the internal pressure curves are also discussed. When the hydrogen concentration is lower than 40 vol %, the second pressure peak plays a more dominant role than the other pressure peaks. However, when the hydrogen concentration is higher than 40 vol %, the third pressure peak becomes more dominant.