The damaging effects of gas explosions on the area outside the mine pithead incur more accident risks on the auxiliary facilities and related personnel.Combined with typical gas explosion cases and based on computational fluid dynamics technology, this paper studied the gas explosion and its evolution process in coal mines, and investigated the distributive characteristics of the shock wave overpressure and temperature field outside the pithead.The study shows that the main hazard of gas explosion incurs on the area outside the pithead originates from the explosion shock wave along the horizontal direction, and its peak overpressure and spread range increase with the gas quantity while, however, its high temperature hazard on the horizontal direction is not obvious.The conclusions provide an essential referential basis for the site surface layout for coal mining and its related risk assessment and accident investigation.

During electromagnetic launching, the current often concentrates on the surface of the armature due to the speed skin effect, which may reduce the service life of a launch system.To inhibit the concentration of the current density, a new type of aluminum alloy brush armature was proposed and its launching properties were studied.Results show that the main structure of the brush armature was intact within the scope of the experimental conditions.The contact pressure being kept constant, the mass loss of the brush armature at first increased with the increase of the current from 200 to 250 kA, then decreased with the further increase of the current from 250 to 350 kA.The mass loss of the brush armature decreased with the increase of the fiber diameter from 0.1 to 0.2 mm.An appropriate initial contact pressure could effectively reduce the mass loss of the armature.The brush armature with a different fiber diameter corresponded to a different optimal initial pressure.The microstructural observation results show that the main forms of damage of the aluminum fiber were mechanical wear and arc ablation.

With concrete regarded as a two-phase non-homogeneous composite material consisting of coarse aggregate and cement mortar and on the basis of Fuller gradation curve and Walaraven plane conversion formula, a two-dimensional circular aggregate random distribution program of the concrete was designed to simulate the dynamic response of the concrete subjected to impact loading.The influences of impact velocity, specimen dimension, size and distribution of coarse aggregate and volume fraction of coarse aggregate on the dynamic mechanical behavior of the concrete were analyzed, and the impact failure mode of the concrete was also discussed.The regulations of impact velocity, specimen dimension, size and distribution of coarse aggregate and volume fraction of coarse aggregate on the dynamic mechanical behaviors of the concrete were presented.Our numerical simulation shows that the peak stress of the concrete increases with the impact velocity, and thus the concrete is rate-dependent.With the enhancement of the specimen dimension, the peak stress of the concrete decreases, so the concrete produces an obvious size effect.With the volume fraction of coarse aggregate, the peak stress of the concrete increases at first and then declines.When the volume fraction of coarse aggregate equal to 40%, the peak stress of the concrete reaches its maximum value.With the increase of the minimum diameter of coarse aggregate, the peak stress of the concrete declines.However, with the increase of the maximum diameter of coarse aggregate, the peak stress of the concrete increases initially and then declines, which provides a theoretical guidance and technical support for the engineering application of the concrete.

Ignition energy plays an important role in combustible gas pipeline explosion.The experiments utilized sensors and the data acquisition system to determine measuring point pressures and explore pressure wave effects on the dynamic response of the thin-wall in the closed pipe. This study investigates the characteristics of the methane-air premixed gas flame propagation under different ignition-energy conditions, and carries on preliminary analyses of the dynamic strain of the thin-walled pipe. The results show that the increase of the ignition energy gives rise to the increase of explosive intensity, the maximum peak pressure and the dynamic strain of the thin-wall pipe in the whole process.In addition, there is still a good consistency between dynamic strain signals and pressure wave signals.These findings can provide a theoretical support for preventing gas explosion accidents in pipelines.

Carbon-encapsulated iron nanoparticles were formed using a gaseous detonation method in a mixture of hydrogen and oxygen in which the powder and gaseous forms of ferrocene were used as the raw materials.X-ray diffraction and transmission electron microscopy analysis shows that using either of the two different states of ferrocene yields carbon-encapsulated iron nanoparticles.The encapsulated particles were composed of iron or iron-carbon compounds as the core, and the outer shell was mainly composed of graphitic carbon.The sizes of most spherical nanoparticles varied between 5 and 30 nm.When using gaseous ferrocene in the detonation, the particle size distribution was narrower, the thickness of the shell layer was more uniform, and the particles had a higher sphericity.Using the iron-carbon alloy phase diagram, an analysis of the mechanism for gaseous detonation synthesis of carbon-encapsulated iron nanoparticles was carried out.The magnetic hysteresis loops of carbon-encapsulated iron nanoparticles were analyzed, which exhibit the dual natures of hard magnetic and paramagnetic.

Accurate measurement of the stresses and velocities at the boundaries of a dynamically loaded specimen may be obtained using split Hopkinson pressure bars (SHPB).However, the determination of a representative stress-strain curve based on these measurements can be challenging.According to the principle of SHPB technique, there are three groups of formulae to process the experimental data, and all the formulas are sensitive to time shifting.Due to transient effects and the error of time shifting, the stress-strain curves lack consistency.In order to solve the problem of time shifting, we introduce the three-wave mutual-checking method based on the conservation of momentum and the corresponding data processing program.It is shown that a more reliable stress-strain curve could be obtained using this method.In order to prove the correctness of this curve, numerical simulations of the SHPB experiments were performed.The results show that by using this program, the same result can be obtained by either the three-wave method or the two-wave method, and what matters more is that this method can avoid the error of time shifting which the traditional method cannot.

Dynamic behaviors of Sn material driven by collision of head-on sliding detonation waves were diagnosticated with pulsed X-ray radiography and DPS.The whole physical picture of the Sn sample, including the physical image of body fragmentation and the width of the free surface ejecta zone, was obtained.The results mark a great progress in the study of dynamic behaviors of metal materials driven by collision of head-on sliding detonation waves.

The dispersion principle and atomization process of four kinds of diesel fuel films induced by shock wave and high-speed airflow was studied, employing a vertical shock tube.The pressure and velocity of the shock wave, before and after reaching the diesel fuel films, were measured by a pressure acquisition system.The dispersion process of the diesel fuel films was recorded by a shadow-graph system.The experimental results show that the shock wave in 1.12 Mach was weakened to a high-speed airflow after impacting on the diesel fuel films.The atomization effect of diesel fuel film 1 and 2 is better, while its viscosity is low.In contrast, diesel fuel film 3 and 4 with a higher viscosity were broken up into big pieces and silks after the dispersion process.It can also be concluded that the diesel fuel films with a lower viscosity have a larger initial dispersion velocity, whereas it will be reduced faster.Apparently, the diesel fuel films with a higher viscosity have a smaller velocity because they have an obstructive effect on the aerodynamic force.Moreover, the corresponding dispersion distance and velocity decreased with the increase of the viscosity.

Computational design of the cavity in the enhanced hypervelocity launcher was carried out using Eulerian code Multi-Fluid Piecewise Parabolic Method (MFPPM).Three kinds of improved configurations of the cavity were presented with varied angles of the cavity.We numerically investigated the effects of the angle and the thickness of the TPX on the velocity, the velocity difference and the planarity of the flier.Our results show that with the given thicknesses and position of the target, the planarity and the velocity of the flier launched by the second kind of the improved configuration are significantly improved, but the range of the planarity is reduced and the velocity differences of the flier surface are increased so that the desirable planarity cannot be maintained.

Gas mixing is an interesting topic in industrial and scientific research fields.Based on the basic structure of an blasting vessel with a volume of 10 m³, this work designed an applicable gas-intake device and theoretically improved the gas intake technology.Then a series of experiments on the whole mixing process of methane and air were carried out in different conditions.According to the evaluation criteria on the gas mixing effect, specific sizes of the gas-intake device and relevant gas-intake conditions were determined.Results indicate that, when the hole diameter of gas-intake device is 1.5 mm and the separation distance is 100 mm, the gas mixing effect reach the optimal condition; and when the gas-intake velocity is fixed at 8 m³/h, and the pressure difference inside and outside the vessel is chosen as 0.04 MPa, the gas mixing effect are further optimized.After being compared with the original binary mixing result, the new system can shorten the mixing time by eleven times and make the mixing more homogeneous.The results in this study provide experimental data and basis for further study on gas mixing in a large scale confined vessel or space.

Space debris of different shapes, when impacting spacecraft protection structures, may produce debris clouds which in turn cause secondary damage on the spacecraft.Therefore, it is of great necessity to investigate the characteristics of debris clouds formed from hypervelocity impact by space debris of diverse shapes.We selected 2 aerospace materials, Al 2017-T4 and Al 2A12, respectively as the impacting projectile and the spacecraft shield material, and performed numerical simulations utilizing the smoothened particle hydrodynamics (SPH) technique with the nonlinear dynamic analysis software AUTODYN-2D.We investigated numerically the characteristics of debris clouds, which were generated by the hypervelocity impact of conical projectiles with the same mass and striking velocity but different length-radius ratios normally on the single protective plate in the conical bottom and tip directions.The results show that the axial velocity of the tip particle, the radial diameter and the axial length of debris clouds, and the diameter of the perforation hole are affected by the length-radius ratio and the impact direction of conical projectiles, and their changing regularities are established in the pres

Based on the modified analytic embedded atom method, we calculated the atomic force constants and the dynamical matrices of the metal molybdenum under different high pressures, and then reproduced the experimental results of the phonon dispersion in bcc molybdenum along three highly symmetrical directions [00ζ], [0ζζ] and [ζζζ] under pressures.In addition, we predicted the phonon dispersion curves of molybdenum under high pressures of 60, 80 and 100 GPa.The results show that our simulated results at high pressures of 0.1 MPa, 17 GPa and 37 GPa agree fairly well with the available experimental results, especially for lower frequencies rather than within the first Brillouin zone boundaries.The shapes of the dispersion curves predicted under high pressures of 60, 80 and 100 GPa are very similar to that under normal pressures.The vibration frequencies of molybdenum in all vibration branches under high pressures are all larger than the results achieved under normal pressures, and they increase along with the high pressures of 60, 80 and 100 GPa.