Precursor Shock Research Articles

Gas explosion overpressure loads in utility tunnels under different pipe support spacing

To investigate the effects of pipe support system characteristics on gas explosion overpressure loads in utility tunnel gas compartments, a 200 m × 2 m × 3 m model of a typical Chinese utility tunnel was constructed using CFD dynamics. Analysis of spatial and temporal distributions of explosion overpressure load under different support spacings. The results indicated that a short support spacing (L = 5 m) contributes to a more uniform distribution of the gas cloud and enhanced the average flame propagation velocity, whereas a medium support spacing (L = 15 m, 25 m) significantly increased the peak overpressure and instantaneous flame velocity. The explosion overpressure curve showed a multipeak structure, with peak P1 caused by the precursor shock wave and peaks P2–P4 caused by the reflected wave. Within 70 m, the secondary reflected wave P3 dominated, and within 130–200 m, the first reflected wave, P2, played a major role. The peak overpressure loads at different support spacings decreased and subsequently increased along the length of the utility tunnel, and the maximum peak overpressure load was reached at the end. The peak overpressure in the same cross-section is unevenly distributed; the closer to the far-end wall, the more significant the difference, and the maximum overpressure usually appears near the top of the tunnel. According to the quantitative relationship between the peak explosion overpressure and support spacing, the optimised design of the pipe support spacing should not be less than 25 m.

Effect of hydrogen ratio on leakage and explosion characteristics of hydrogen-blended natural gas in utility tunnels

Based on computational fluid dynamics technology, the effect of hydrogen blending ratio (HBR) on the concentration distribution of leaked gas diffusion in utility tunnels was investigated, and based on this, the explosion characteristics of hydrogen-blended natural gas (HBNG) were studied in an inhomogeneous concentration field. The results showed that the concentration field of HBNG gradually stabilized in the downstream of the leak site, and the peak concentration increased linearly with HBR. When HBR≤5%, a high concentration region above the upper explosive limit is formed inside the gas cloud. This characteristic of the distribution induces secondary combustion in utility tunnels. The maximum flame velocity and steady acceleration distance increase with the increase of HBR. The flame shape is significantly affected by the concentration field distribution. The explosion overpressure induced by the real concentration field created multiple peak structures that exacerbated the explosion disaster complexity. The first overpressure peak at the ends was formed by the superposition of the precursor shock wave and its reflected wave, and those at other locations are caused by the shock waves. However, all maximum overpressure peaks are formed by the superposition of multiple reflections of the explosion wave. The maximum peak overpressure at both ends of the tunnel is higher than that in the middle. The explosion overpressure induced by the inhomogeneous gas cloud is lower than that caused by homogeneous gas cloud under the same volume. The research results can provide an effective reference for the anti-explosion design and risk assessment of utility tunnel.

Flame acceleration, detonation limit and heat loss for hydrogen-oxygen mixture at cryogenic temperature of 77 K

Experimental investigations have been conducted in hydrogen-oxygen mixtures with equivalence ratio of 1.5 at cryogenic temperature (77 K) and initial pressure (P0) from 0.2 to 0.5 atm. The pressure sensors and optical fibers are used to measure the overpressure and flame velocity, respectively. The precursor shock wave is formed by the combination of the 1st and 2nd shock waves. When flame catches up with the precursor shock wave, detonation occurs. Strong flame acceleration was observed in all tested conditions, which can be well characterized and predicted by the Zel'dovich number and expansion ratio. The stuttering and galloping modes were observed at 0.5 and 0.3 atm, respectively. With the decrease of the initial pressure to 0.20–0.25 atm, detonation could not occur, only the deflagration mode was observed. The stability of the mixture can be indicated by the parameter χ, and the improvement of stability of mixture will shorten the initial pressure range where the galloping mode occurs. The heat loss effect on detonation limit has subsequently been examined for the present experiments as well as those with equivalence ratio of 2.6 reported in our previous study [Shen, X. et al., Proceedings of the Combustion Institute 2022]. For equivalence ratio of 2.6, the heat loss is found to have dominant effect on the appearance of detonation limit. Its influence can be quantitatively characterized by critical ratio of heat loss to heat release. However, for equivalence ratio of 1.5, the heat loss effect is found to have only relatively small effect on detonation limit.

Accelerated ignition-shock coupling and deflagration to detonation transition by ozone kinetic enhancement of dimethyl ether mixture

This study aims to understand the effect of kinetic enhancement by ozone addition on the Deflagration to Detonation Transition (DDT) and ignition-shock coupling in a microchannel using dimethyl ether (DME). Simultaneous high-speed chemiluminescence and shadowgraph imaging are carried out to analyze the flame front propagation, shock wave structures, autoignition, and ignition-shock coupling during the DDT process. The experimental results show that for all cases, with and without ozone kinetic enhancement, autoignition and DDT always occur in a reactivity gradient region with oblique shock cluster between the flame front and the flame acceleration-induced precursor shock. The observed DDT occurs via the Zeldovich hot spot gradient mechanism through the following stages: (1) Oblique shock cluster formation and acoustic compression between the flame front and the precursor shock, (2) autoignition initiation in the oblique shock cluster region, (3) spontaneous ignition wave propagation, and (4) ignition-shock coupling. It is found that autoignition location, oblique shock strength, induction zone length, ignition-shock coupling, and DDT onset flame velocity are very different from ozone kinetic enhancement. As the ozone addition is higher, the autoignition is advanced and the ignition location moves farther ahead of the flame front at a lower flame front propagation speed and weaker oblique shock clusters. Two different ignition-shock coupling modes are identified depending on ozone concentration. Without ozone kinetic enhancement, DDT occurs via ignition wave coupling with the precursor shock. However, ozone addition can accelerate the autoignition and initiate a direct ignition-shock coupling for DDT without interaction with the precursor shock. The present results show that kinetic enhancement in autoignition using an ignition enhancer such as ozone or plasma is critical and effective in accelerating and controlling DDT.

Experimental characterization of a modified high-speed plasma synthetic jet actuator with oblique-slot exit

In the field of flow control research, oblique jets are known to offer several advantages over vertical jets. To gain a comprehensive insight into the flow field characteristics of a plasma synthetic jet actuator with an oblique-slot exit, the related experiments are conducted. The experiment employed high-speed schlieren imaging techniques and electrical parameter measurements to acquire the flow field characteristics and discharge properties of the oblique-slot actuator, followed by a comparative analysis with a vertical circular orifice actuator. The oblique-slot plasma synthetic jet exhibits a wall-attaching effect and asymmetric flow characteristics, which differ from those of the vertical circular orifice actuator. The actuator generates a wall jet with an initial velocity of 389.5 ± 15.08 m/s, effectively propelling the fluid within the boundary layer. The Mach number of the precursor shock wave in the direction of the jet reaches 1.59, but decreases to just 1.02 in the opposite direction. Over a period in the range of 10–70 μs, the Froude number of the plasma jet decreases from 1841 to 238. The dominant role of the inertial force gradually weakens, while the influence of buoyancy increases, causing the jet boundary to move upward. The oblique-slot jet configuration represents a typical planar jet, exhibiting superior flow control uniformity compared with the vertical circular orifice jet. The results indicate that the high-speed oblique-slot plasma synthetic jet actuator designed in this study possesses distinct advantages over vertical circular orifice actuators for high-speed fluid flow control.

Characteristics of flame acceleration and deflagration-to-detonation transition enhanced by SF6 jet-in-cross-flow/flame interaction

Jet-in-crossflow (JICF), as a turbulence generator, is proposed to promote the flame acceleration and deflagration-to-detonation transition (DDT) process. Previous work has suggested that JICF using CO2 or Ar was in favor of flame acceleration under optimal conditions. The competition mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative combustion-inhibition caused by non-reactive gas has been demonstrated. It is worth noting that the difference in molar weight between the medium of JICF and the combustible mixture may significantly impact deflagration propagation prior to DDT. Thus, in this work, to investigate the flame propagation perturbed by the enhancement on the difference in molar weight between the JICF medium and combustible mixture, an experimental study of the flame propagation perturbed by SF6 JICF is carried out. By adjusting the injection pressure (Pj) of the SF6 JICF, the characteristics of flame propagation and DDT are analyzed. The results showed that the flame can accelerate to the onset of detonation within a short run-up distance in the cases with SF6 JICF. Schlieren images of flame evolution show that the flame disturbed by SF6 JICF rapidly transitions to a wrinkled flame. It is suggested that the SF6 JICF-enhanced turbulence promotes flame acceleration and DDT. The effect of SF6 JICF on the flame characteristics can be concluded in two aspects: one is that the inhomogeneously distributed vortices that are induced by JICF cause a nonuniform diffusion of SF6 in the tube, resulting in a nonuniform dilution of the combustible mixture near the jet; another one is that the vortices induced by JICF stretch the flame surface to increase the flame area. Increasing the injection pressure could promote the formation of complex wrinkles in the flame, however, it shows few further improvements in promoting DDT. Different jet mediums are also tested, and the results show that SF6 JICF can shorten the run-up distance significantly compared with helium and nitrogen JICF. The small and densely distributed wrinkles significantly increase the flame area to enhance heat release. The flame disturbed by SF6 JICF could accelerate to the local sound speed of the unburnt mixture, which results in the facilitation of the precursor shock wave formation. After the flame passes through the SF6 JICF, it remains a larger acceleration rate and then transitions to quasi-detonation rapidly. The conclusion of this work could help to qualitatively understand the positive effect of JICF-induced turbulence on flame acceleration and DDT. It also provides a relatively efficient and safe technology to promote detonation initiation in the design of detonation engines.

Molecular mechanism of drought stress tolerance in barley (Hordeum vulgare L.) via a combined analysis of the transcriptome data

One of the main issues addressed by phytology in recent years has been plant tolerance mechanisms for abiotic stress. No combined analysis has been made to identify the genes involved in drought stress tolerance. The meta-analysis of microarray data related to drought stress was analysed by the R software packages and showed 3 029 upregulated genes and 3 017 downregulated genes. The upregulated genes were mostly related to the drought tolerance protein, abiotic stress response, and the Cys2His2 Zinc Finger Transcription Factor (C2H2 zinc finger TF). The downregulated genes were mainly related to the late embryogenesis abundant protein, abiotic stress response, and the basic leucine zipper (bZIP) TF. The common gene ontology (GO) terms in the upregulated and downregulated genes were mainly related to the metabolic process, response to stimulus, cellular metabolic process, and photorespiration. The up and down meta-differential expressed genes (meta-DEGs) mainly belonged to the those following Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways including: the biosynthesis of secondary metabolites, plant hormone signal transduction, mitogen-activated protein kinase (MAPK) signalling pathway, and RNA degradation. Moreover, in the upregulated and downregulated genes, the TFs with a high percentage mainly belonged to the Teosinte branched1/Cincinnata/proliferating cell factor (TCP), basic helix loop-helix (bHLH) and bZIP. Next, the hub upregulated genes were mainly related to the thiamine biosynthesis protein thiC, 4-hydroxyphenylpyruvate dioxygenase, ribose-5-phosphate isomerase precursor and heat shock protein. The hub downregulated genes were mainly associated with the elongation factor Ts, aldehyde dehydrogenase, and trigger factor. Finally, the data from the present meta-analysis were compared with previous studies on the qRT-PCR results and their up and down expressions were confirmed. Based on the findings of the current study, novel insights into the drought stress molecular response can be provided and various candidate genes can be introduced for barley drought stress tolerance breeding.

Phenomenon and analysis of direct initiation of detonation using multiple turbulent flame jets

This work reports experimental investigations on the direct initiation of detonation using multiple turbulent flame jets, with a special focus on the arrangement schemes and fundamental physics in the initiation processes. Results show that the direct initiation of detonation can be achieved using turbulent jets even when the jet tube diameter is much smaller than the empirical critical tube diameter due to flame–shock–wall interactions. Conspicuous evidence has been shown that the probability of the direct initiation increases significantly near the detonatability limit using multi-jets compared to a single jet. These results are found to be closely related to several new phenomena observed when using multiple jets to initiate the detonation. They are: (1) unexpected rapid promotion of the final-stage flame acceleration in ignition tubes by multiple jets, which is attributed to the fact that the expanding precursor shock waves propagate back into the adjacent tube and interact with the flame; (2) enhancement of hot spot generation by multiple jets due to the precursor shock intersection and the formation of an induction zone; (3) obvious velocity loss of impinging jets initiation as a result of induced hot spots propagation in the burned gases.

Effect of shock-flame interactions on initial damage characteristics in highway tunnel under hazmat tanker truck accident

Hazardous materials road transportation (HMRT) accidents are the main hazards of highway tunnels, and the damage evolution process subject to catastrophic consequences have gained significance due to increasing accidental events. This study presents the scene reconstruction referenced with actual case of methanol tanker truck explosion in highway tunnel and integrates it with effecting parameters such as location, causation, consequence, time series data, tunnel structure, hazmat properties, emergency. In this work, the damage characteristics caused by tanker truck explosion are discussed by comparing with different traffic situations inside the highway tunnel. The results indicate the strength of shock wave is enhanced as the traffic flow grows, especially in the ceiling and ground of tunnel, the peak overpressures are significantly higher than that in other areas. With the increasing of the traffic vehicles in tunnel, the process of the explosion overpressure changing in tunnel can be categorized in five stages. This study presents the predictive model relating with explosion specific impulse, which can be better presented in impulse changes in tunnel with the minimum degree of fitting is 0.9139. By comparison between different traffic situations in tunnel, it reveals the formation of precursor shock wave enhances the flame propagation, and the accelerated flame in turn strengthens the overpressure, indicating the underlying damage mechanism. This study contributes to better understanding multi-factor damage mechanism of HMRT accident in highway tunnel, and further optimizing HRMT accident detections and emergency plans.

Prediction of detonation initiation in an air-breathing pulse detonation combustor utilizing a simplified ethylene-air reaction mechanism

In order to predict the detonation initiations accurately with less computational effort, a reduced method was proposed to establish a three-step ethylene/air global reaction mechanism for detonation combustors. The activation energy of the three-step mechanism was set as a key parameter that varies with the initial state of the reactants. A database of detonation simulation activation energy was established in a range of the equivalence ratio of 0.7–2.3 and the pressure ahead of the precursor shock front of 0.2–2.0 atm. Compared with existing simplified chemical reaction mechanisms, the proposed mechanism was confirmed to better reproduce the results of GRI-Mech3.0 within the given applicability range. The developed mechanism was used to predict the lean and rich limits of detonation initiation for simulation successfully in an air-breathing pulse detonation combustor. The simulation results were consistent with the theoretical limits estimated by the detonation cell sizes. Stable operation of an air-breathing detonation combustor was achieved in the experiments. Moreover, a series of equivalence ratio ranges of detonation initiation and the initiation distances were obtained. The simulated limits fell within the two experimentally obtained ranges of the lean limit and the rich limit. This proved that the established three-step mechanism is appropriate for the simulations of detonation initiation.

On the influence of co-flow on the shocks and vortex rings in the starting phases of under-expanded jets

In the starting phases of continuously blowing under-expanded jets, this numerical study investigates the effect of co-flow (UaUj) (a) on the circulation and evolution of primary vortex ring (PVR) and (b) on the occurrence of Mach reflection, slipstream generation, and subsequent formation of counter rotating vortex rings (CRVRs). With increase in co-flow (UaUj), the PVR circulation gradually decreases. The size of supersonic PVR gradually decreases with increase in co-flow (UaUj), and at high magnitudes of co-flow (UaUj≳0.3), the supersonic PVR attains a circular shape. The strengths of embedded shock (ES) and vortex-induced shock are found to decrease with increase in co-flow (UaUj), and at high magnitudes of co-flow (UaUj≳0.3), these shocks may even cease to form inside the supersonic PVR. An increase in co-flow (UaUj) causes the expansion fan to become more and more narrow. This reduces the acceleration of the supersonic flow inside the inviscid core, thereby weakening the incident oblique shock (IOS), which in turn increases the pressure prevailing downstream of this shock inside the inviscid core. The increase in co-flow (UaUj) also leads to a simultaneous decrease in the pressures prevailing in front of the downstream marching PVR and Mach disk (MD) of the inviscid core due to the reduction in the strength of precursor shock. As the magnitudes of pressures prevailing in the upstream and downstream of Mach disk approach each other, hence, MD also weakens. This shows that with the increase in co-flow (UaUj), there is weakening of the different shocks (i.e., ES, IOS, and MD) involved in Mach reflection. This causes a reduction in the strength of the resulting slipstream, thereby affecting the formation of CRVR patterns.

Characteristics of Fragment Impacting Steel Cover at High Speed to Initiate Heated Explosives

AbstractThe temperature of the explosive has a great influence on its shock initiation sensitivity. The response characteristics of the heated explosive under fragment impact is a concern for the safety of the explosive. In order to study the initiation characteristics of heated explosives subjected to fragment impact, an experimental device that uses the shaped charge to generate the high‐velocity fragment (2599 m/s) to impact the cover to initiate the heated explosive was designed in this paper. This method achieves uniform temperature control of explosives via heating the upper and lower ends and preserves heat on the side of the explosive charge. We experimentally tested the method on hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX)‐based aluminized explosives (61 wt.% RDX, 30 wt.% Al, 9 wt.% binder) at different temperatures. The fragment impact behavior and explosive detonation wave growth were observed via X‐ray photography. A numerical model of fragment initiating explosive, which considered the temperature change of the explosive, was set up, a numerical simulation of the explosive charge initiated by fragment impacting the cover was carried out, and the reaction degree of explosive impacted by fragment and its change with temperature were analyzed. The study found that the precursor shock wave formed by the fragment impacting the cover initiated the explosive, and the explosive temperature greatly influences the fragment initiation process. Through the numerical simulation calculation of the explosive charge initiated by fragment impact under different cover thicknesses and temperatures, the relationship among the critical cover thickness, the run‐to‐detonation distance and explosive temperature was given. In the temperature range of 25 °C–111 °C, the shock sensitivity of RDX‐based aluminized explosives under fragment impact decreases with the increase of explosive temperature, while in the temperature range of 111 °C–170 °C, the sensitivity increases with the increase of explosive temperature.

Effects of jet/flame interaction on deflagration-to-detonation transition by non-reactive gas jet in a methane-oxygen mixture

Jet-in-crossflow (JICF) is a potential and reliable technique that can promote flame acceleration and deflagration-to-detonation transition (DDT). Most of the previous researches utilized combustible mixture or oxygen as the medium of JICF, therefore the detonation enhancement effect may be partly attributed to the chemical properties of the jet, and the purely turbulent effect induced by JICF on DDT has not been fully understood. In our previous work, the non-reactive gas jet affected flame propagation indirectly by separating the injection and ignition in the control sequence. A competing mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative non-reactive gas-inhibition effect on combustion has not been found, the non-reactive gas jet with high pressure and short duration time was also verified to be beneficial for precursor shock wave formation and flame acceleration. However, no detonation was observed in the work. In this paper, the non-reactive jet interacts with the flame propagation directly by overlapping the working time of the jet and ignitor partly, and the effect of jet parameters including injection pressure and time delay on DDT is investigated systematically. The results show that the flame affected by the non-reactive gas jet with a high-pressure ratio and a short time delay can transit to detonation in a short run-up distance successfully. The schlieren images of the flame evolution process illustrate that the flame interacted by the jet experiences four states: layered flame, wrinkled flame, cellular flamelets, and turbulent flame. Kelvin–Helmholtz (K-H) instability and Rayleigh-Taylor (R-T) instability dominate in the flame distortion. The characteristic timescales of the flow and reaction are computed in the four flame states, turning out that as the flame evolving, the turbulent intensity of the flame is enhanced. Damköhler number (Da) on the flame front eventually converges to [1,2] demonstrates the characteristic timescales of the turbulence and reaction are on the same order, and the turbulence dominates the flame evolution.

Investigation of the effect of turbulence induced by double non-reactive gas jet on the deflagration-to-detonation transition

Jet-in-crossflow (JICF) is proved to be advantageous to accelerate flame propagation and stimulate the deflagration-to-detonation transition (DDT) process in the recent decade. Studies focused on the performance of the jet of combustible mixture or reactive gas on facilitating the DDT process are carried out, and the results show that the combustible JICF can promote the flame transition to detonation efficiently. Although most investigations attribute the ability of the JICF on promoting DDT to the turbulent effect induced by the jet, the combustion-enhancement effect of the jet medium is seldom considered. In our previous work, it is proposed that the non-reactive gas jet in an optimal condition can accelerate flame propagation and promote the precursor shock wave formation. Hence, the performance of the multi-jet of non-reactive gas on facilitating the DDT process is worth investigating. In this paper, three different double jet placement methods are designed: parallel, staggered, and opposite positioned, and their performances on accelerating flame propagation and promoting the DDT process are investigated systematically. The results indicate that the staggered jet can accelerate the flame to detonation within a short run-up distance whereas the parallel jet and the opposite jet can only make the flame accelerate slightly. The schlieren images of flame evolution affected by double jet indicate that the complicated reactivity gradients generate on the flame surface while the flame is disturbed by the double jet. The interaction between the staggered jet and the flame induces local explosions, which results in a prominent increase of the flame area and a large volumetric burning rate. For the opposite jet, its negative effect of combustion-inhibition of the aggregated CO2 overwhelms the positive effect of turbulence-enhancement on the flame acceleration.

A quasi-one-dimensional study on global characteristics of tube train flows

The global characteristics of tube train flows under wide ranges of blockage ratios and train speeds are comprehensively investigated by quasi-one-dimensional numerical simulation and theoretical analysis. The established quasi-one-dimensional model for the tube train flow and the numerical method are both proven to be reliable and efficient. Investigation reveals the typical patterns of both inviscid and viscous flows. The transition limits for these flow patterns are determined. The flows tend to be unstarted when the train moves at a high subsonic speed or a low supersonic speed and tend to be started otherwise. The transition limits between started and unstarted modes do not vary much between inviscid and viscous flows, which allows the use of analytical inviscid criteria in approximate estimation of the start/unstart state of the viscous flow. Between the supersonic isentropic limit and the Kantrowitz limit lies a dual-solution area. Which solution the flow follows depends on how the attempted cruise speed of the train is reached; a large acceleration tends to induce a started flow. The unstarted inviscid tube train flow appears to be self-similar as the train suddenly moves at a constant speed. This flow involves a precursor shock wave in front of the train and a secondary shock wave on or behind the train at the same time. By contrast, the unstarted viscous flow eventually remains a steady laminar structure that moves with the train after a sufficiently long process of development. In the stabilized flow, the precursor shock wave only occurs in the supersonic case, and the secondary shock wave only occurs in the subsonic case. For both inviscid and viscous flows, the unstarted flow can be divided into two submodes according to whether the secondary shock wave detaches from the train. Aerodynamic drag on the train is strongly correlated with the flow modes. The drag in unstarted flow increases with train speed, while the drag coefficient peaks at the secondary shock attach–detach limit. The wall transport effects cause larger drag but do not change its overall features.

A self-trigger three-electrode plasma synthetic jet actuator

As a potential active flow control technology, plasma synthetic jet actuator (PSJA) has shown wide and promising application prospects in the high-speed flow. However, the application of PSJA is still limited by the low energy transfer efficiency and high driving voltage. This study presents a novel and practical self-trigger three-electrode plasma synthetic jet actuator (STPSJA), which is able to prolong the electrode distance with low breakdown voltage. A plasma jet is introduced to replace the high-voltage pulse of conventional three-electrode PSJA. The working process of STPSJA is investigated based on discharge waveforms, high-speed photography, and high-speed schlieren image system. Compared with the two-electrode actuator, the maximum precursor shock velocity can be increased from 449 m/s to 605 m/s. Furthermore, the discharge efficiency of STPSJA can be 68% when the electrode distance exceeds 10 mm and the breakdown voltage is only about 318 V. This study presents a promising step toward optimizing the design of plasma synthetic jet actuator.

Soap film catastrophes

Earlier systematic experimental studies of bursting soap films by McEntee & Mysels (J. Phys. Chem., vol. 73, 1969, pp. 3018–3028) revealed the existence of a precursor shock wave preceding the expanding hole in a punctured film, with a disturbed region of shrinking film material in between known as the ‘aureole’. In the present work we report and interpret new phenomena associated with the aureole – the formation of folds on the surface of soap films. In search of the theoretical explanation of the experimentally identified conditions under which the folds appear, we establish that they correspond to catastrophes of collapsing soap films.

Experimental study on the effect of modified attapulgite powder with different outlet blockage ratios on methane-air explosion

Through visual explosion experiment platform, this study conducted comparative experiments on methane-air premixed gas with a methane volume fraction of 9.5 % under different working conditions. The experiment studied the effect of active explosion suppression (spraying modified attapulgite powder) and explosion venting (setting the outlet blockage ratios) on methane explosion. The results showed that when the amount of powder injection was constant, the outlet blockage ratios of the pipeline was positively correlated with the peak explosion overpressure and the explosion index, but was negatively correlated with the peak flame propagation velocity. When the outlet blockage ratio was constant, the amount of powder spraying was negatively correlated with the peak value of explosion overpressure and the peak value of flame propagation velocity. When the outlet blockage ratio was 0, the explosion precursor shock wave broke through the PVC plastic film, which made the influx of a large amount of fresh air promote the generation of turbulence in the pipeline, and a “tulip-shaped” flame front appeared. The combination of active explosion suppression and explosion venting showed different effects on the explosion overpressure and flame propagation characteristics of methane explosion. Finally, this study conducted an in-depth discussion on the complex mechanism contained in it.

Modelling of a spherical deflagration at constant speed

We build in this paper a numerical solution procedure to compute the flow induced by a spherical flame expanding from a point source at a constant expansion velocity, with an instantaneous chemical reaction. The solution is supposed to be self-similar and the flow is split in three zones: an inner zone composed of burnt gases at rest, an intermediate zone where the solution is regular and the initial atmosphere composed of fresh gases at rest. The intermediate zone is bounded by the reactive shock (inner side) and the so-called precursor shock (outer side), for which Rankine-Hugoniot conditions are written; the solution in this zone is governed by two ordinary differential equations which are solved numerically. We show that, for any admissible precursor shock speed (and, in some cases, for a reaction heat large enough), the construction combining this numerical resolution with the exploitation of jump conditions is unique, and yields decreasing pressure, density and velocity profiles in the intermediate zone. In addition, the reactive shock speed is larger than the velocity on the outer side of the shock, which is consistent with the fact that the difference between these two quantities is the so-called flame velocity, i.e. the (relative) velocity at which the chemical reaction progresses in the fresh gases. Finally, we also observe numerically that the function giving the flame velocity as a function of the precursor shock speed is increasing; this allows to embed the resolution in a Newton-like procedure to compute the flow for a given flame speed (instead of for a given precursor shock speed). The resulting numerical algorithm is applied to stoichiometric hydrogen-air mixtures.

Three-Dimensional Numerical Investigation of Hypersonic Projectile Launched by Railgun on Transitional Ballistics

Supersonic accelerators use sabots to suppress bore balloting. However, sabot separation induces flight deviation in projectiles owing to their interactions with fluids and shock waves. A seamless calculation of the acceleration phase in the tube and the full separation process using 3-D computational fluid dynamics coupled with rigid body dynamics is presented in this study to investigate the shock-wave interactions around a railgun-launched projectile. The railgun acceleration induced a normal shock wave propagating ahead of the projectile; when the projectile reached the tube end, a substantial expansion at the muzzle generated two shock waves, namely, a spherical precursor shock wave and a Mach disk, outside the tube. Aerodynamic forces on the projectile/sabot decreased to almost zero just after the tube exit, with the ambient fluid around the tube being faster than the projectile by the substantial expansion. After exiting, the projectile penetrated the precursor shock wave at ; this increased the aerodynamic forces acting on the projectile, initiated sabot separation, and generated multiple shock-wave interactions, which induce unsteady aerodynamic loads on the projectile.