2022 Vol. 42, No. 1
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2022, 42(1): 012101.
doi: 10.11883/bzycj-2021-0191
Abstract:
In order to explore the flame dynamics of methane under oxygen-rich conditions, a series of explosion experiments were carried out in a small-scale square transparent pipe with a CH4/O2/CO2 premixed system as the research object. Through the analysis of explosion parameters, the influence of the fluctuation of initial ambient temperature on explosion intensity was revealed, and the micro-combustion mechanism of premixed system was discussed. The results show that under the ambient temperature of 273 K, the mixtures with the equivalence ratio\begin{document}$\varphi $\end{document} ![]()
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In order to explore the flame dynamics of methane under oxygen-rich conditions, a series of explosion experiments were carried out in a small-scale square transparent pipe with a CH4/O2/CO2 premixed system as the research object. Through the analysis of explosion parameters, the influence of the fluctuation of initial ambient temperature on explosion intensity was revealed, and the micro-combustion mechanism of premixed system was discussed. The results show that under the ambient temperature of 273 K, the mixtures with the equivalence ratio
2022, 42(1): 013101.
doi: 10.11883/bzycj-2021-0114
Abstract:
Based on the dynamic experimental results of concrete specimens under true triaxial confinement, the Holmquist-Johnson-Cook (HJC) model considering the strain rate effect and the Drucker-Prager (DP) model considering the hydrostatic pressure effect were employed for numerical analysis to explore the methods for studying the strain rate effects and inertia effects. In order to explore the relationship between the strain rate effect and the lateral inertia effect of concrete, the numerical simulation results of the HJC model were used to fit the parameters of the DP criterion, and the values of the parameters α and k at four strain rates were obtained. The relationship between the DP criterion parameters and the strain rate and hydrostatic pressure was comprehensively analyzed. The results of numerical analysis show that with the increase of strain rate, the strength of concrete increases, and this strength increase is partly due to the increase of the first stress invariant I1. It can be concluded that the strain rate and lateral inertia constraint of concrete specimens have a strong coupling effect. The relationship between the distribution characteristics of the transverse stress and the strain rate, hydrostatic pressure and specimen size under impact are analyzed theoretically and numerically. The results show that the amplitude of the transverse stress increases with the strain rate and hydrostatic pressure, but decreases with the sample size. In order to investigate the effect of lateral inertia on the strength improvement, a parameter ξ related to the maximum stress σx and the equivalent stress σe in the impact direction, defined as ξ=(σx−σe)/σx, was proposed. The relationship between the strain rate, hydrostatic pressure, specimen size and the parameter ξ was analyzed by the HJC model. It is found that this parameter has evident size effect, strain rate effect and hydrostatic pressure effect. However, the relationship between the parameter ξ and the stress triaxiality shows a strain rate independent characteristic. It can provide a new way for the investigation of strain rate effect.
Based on the dynamic experimental results of concrete specimens under true triaxial confinement, the Holmquist-Johnson-Cook (HJC) model considering the strain rate effect and the Drucker-Prager (DP) model considering the hydrostatic pressure effect were employed for numerical analysis to explore the methods for studying the strain rate effects and inertia effects. In order to explore the relationship between the strain rate effect and the lateral inertia effect of concrete, the numerical simulation results of the HJC model were used to fit the parameters of the DP criterion, and the values of the parameters α and k at four strain rates were obtained. The relationship between the DP criterion parameters and the strain rate and hydrostatic pressure was comprehensively analyzed. The results of numerical analysis show that with the increase of strain rate, the strength of concrete increases, and this strength increase is partly due to the increase of the first stress invariant I1. It can be concluded that the strain rate and lateral inertia constraint of concrete specimens have a strong coupling effect. The relationship between the distribution characteristics of the transverse stress and the strain rate, hydrostatic pressure and specimen size under impact are analyzed theoretically and numerically. The results show that the amplitude of the transverse stress increases with the strain rate and hydrostatic pressure, but decreases with the sample size. In order to investigate the effect of lateral inertia on the strength improvement, a parameter ξ related to the maximum stress σx and the equivalent stress σe in the impact direction, defined as ξ=(σx−σe)/σx, was proposed. The relationship between the strain rate, hydrostatic pressure, specimen size and the parameter ξ was analyzed by the HJC model. It is found that this parameter has evident size effect, strain rate effect and hydrostatic pressure effect. However, the relationship between the parameter ξ and the stress triaxiality shows a strain rate independent characteristic. It can provide a new way for the investigation of strain rate effect.
2022, 42(1): 013102.
doi: 10.11883/bzycj-2021-0074
Abstract:
Crystalline silicon has a complicated phase transition mechanism, which has received extensive attention in the research field of phase diagram, and the deformation mechanism of silicon crystals under dynamic loading is the current research hotspot. In order to reveal its deformation and phase transition behaviors under intensive dynamic loading, molecular dynamics method was used to simulate the shock compression behavior of single crystal silicon along the crystal directions [001], [110] and [111] at an initial ambient temperature of 300 K, respectively. All simulations were carried out basing on the classical open-source codes LAMMPS and a Tersoff interatomic potential was adopted to describe the material responses of silicon under dynamic compression. Before shock loading, periodic boundary conditions were applied along the three independent directions, and an NPT ensemble was used to equilibrate the systems; then shock compression was applied by using the piston method, where a virtual piston wall impinges the sample such that the particle velocity in the sample is the same as the piston speed after the shock reaches a steady state. The shock particle velocities varied from 0.3 km/s to 3.2 km/s, and a timestep of 0.001 ps was adopted. During the stress wave formation and propagation, the simulation system was in the NVE ensemble with the absence of temperature control. The loading method and effect are similar to typical plane impact experiments. The results show that with the increase of shock particle velocity, the shear stress of single crystal silicon increases gradually and then decreases sharply due to the structural phase change. Both the phase transition threshold and the phase transition mechanism are anisotropic. Among them, a variety of solid-solid phase transitions and solid-liquid phase transitions are observed under shock compression along the [001] crystal direction. The phenomenon of solid-liquid coexistence is highly consistent with the recent international experiments. The research results provides new nano-scale results to support the study of phase transition of crystalline silicon under dynamic loading.
Crystalline silicon has a complicated phase transition mechanism, which has received extensive attention in the research field of phase diagram, and the deformation mechanism of silicon crystals under dynamic loading is the current research hotspot. In order to reveal its deformation and phase transition behaviors under intensive dynamic loading, molecular dynamics method was used to simulate the shock compression behavior of single crystal silicon along the crystal directions [001], [110] and [111] at an initial ambient temperature of 300 K, respectively. All simulations were carried out basing on the classical open-source codes LAMMPS and a Tersoff interatomic potential was adopted to describe the material responses of silicon under dynamic compression. Before shock loading, periodic boundary conditions were applied along the three independent directions, and an NPT ensemble was used to equilibrate the systems; then shock compression was applied by using the piston method, where a virtual piston wall impinges the sample such that the particle velocity in the sample is the same as the piston speed after the shock reaches a steady state. The shock particle velocities varied from 0.3 km/s to 3.2 km/s, and a timestep of 0.001 ps was adopted. During the stress wave formation and propagation, the simulation system was in the NVE ensemble with the absence of temperature control. The loading method and effect are similar to typical plane impact experiments. The results show that with the increase of shock particle velocity, the shear stress of single crystal silicon increases gradually and then decreases sharply due to the structural phase change. Both the phase transition threshold and the phase transition mechanism are anisotropic. Among them, a variety of solid-solid phase transitions and solid-liquid phase transitions are observed under shock compression along the [001] crystal direction. The phenomenon of solid-liquid coexistence is highly consistent with the recent international experiments. The research results provides new nano-scale results to support the study of phase transition of crystalline silicon under dynamic loading.
2022, 42(1): 013103.
doi: 10.11883/bzycj-2021-0089
Abstract:
To investigate the mechanical characteristics of static fracture of marble subjected to dynamic damage after cyclic impacts, based on the modeling technology of the finite difference method (FDM) and the discrete element method (DEM) coupling, a three-dimensional numerical model of the split Hopkinson pressure bar (SHPB) was constructed, and the bar system and rock sample were modeled using FLAC3D and PFC3D programs, respectively. The numerical cyclic impact loading tests were carried out on notched semi-circular bend (NSCB) samples at a constant impact velocity, and then the static three-point bending fracture tests were simulated on these damaged samples. The coordinate data of the particles on fracture surfaces of the sample were extracted by compiling the Fish program, and then the fracture surface was reconstructed and the surface roughness was calculated quantitatively. The rationality and reliability of the numerical analysis were verified by comparison with the results of relevant laboratory tests. The results show that in the cyclic impact loading test, the stress-strain curve rebounds, resulting from the release of part of the stored strain energy during the unloading period. With the increase of the impact number n, the numbers of cracks and fragments generated increase. The connected force field becomes more and more chaotic and the number of broken force chains displays an increasing trend. The breakage of the force chains is the root cause of the deterioration of mechanical properties of the sample. The static fracture toughness of the sample after 5 times of impact is 53.35% lower than that of the natural sample, while the failure displacement increases. In the static loading process, more and more cracks and fragments generate as n increases. This is proof that the internal structure of the sample has been damaged in the cyclic impacts. The fracture surface roughness increases as the impact number increases. The research conclusions can provide certain guidance for the engineering practice.
To investigate the mechanical characteristics of static fracture of marble subjected to dynamic damage after cyclic impacts, based on the modeling technology of the finite difference method (FDM) and the discrete element method (DEM) coupling, a three-dimensional numerical model of the split Hopkinson pressure bar (SHPB) was constructed, and the bar system and rock sample were modeled using FLAC3D and PFC3D programs, respectively. The numerical cyclic impact loading tests were carried out on notched semi-circular bend (NSCB) samples at a constant impact velocity, and then the static three-point bending fracture tests were simulated on these damaged samples. The coordinate data of the particles on fracture surfaces of the sample were extracted by compiling the Fish program, and then the fracture surface was reconstructed and the surface roughness was calculated quantitatively. The rationality and reliability of the numerical analysis were verified by comparison with the results of relevant laboratory tests. The results show that in the cyclic impact loading test, the stress-strain curve rebounds, resulting from the release of part of the stored strain energy during the unloading period. With the increase of the impact number n, the numbers of cracks and fragments generated increase. The connected force field becomes more and more chaotic and the number of broken force chains displays an increasing trend. The breakage of the force chains is the root cause of the deterioration of mechanical properties of the sample. The static fracture toughness of the sample after 5 times of impact is 53.35% lower than that of the natural sample, while the failure displacement increases. In the static loading process, more and more cracks and fragments generate as n increases. This is proof that the internal structure of the sample has been damaged in the cyclic impacts. The fracture surface roughness increases as the impact number increases. The research conclusions can provide certain guidance for the engineering practice.
2022, 42(1): 013104.
doi: 10.11883/bzycj-2021-0008
Abstract:
Effective reduction of end surface friction is necessary to ensure the validity and accuracy of the split Hopkinson pressure bar (SHPB) experimental results. In order to study the effects of sample roughness and lubrication efficiency on the end surface friction and the final experimental results, copper was selected as a research material due to its steady mechanical properties and strain rate insensitivity of constitutive relation. In order to minimize the effects of end friction, all pressure bars with the diameter of 10 mm had a surface roughnesses of 0.8 μm. Copper samples of three typical surface roughnesses were prepared by mechanical processing and corrosion, then high precision repeat dynamic compression experiments by the SHPB were carried out under the conditions of full lubrication with MoS2 and complete non-lubrication, respectively. The results show that MoS2 can only play a good lubrication when the end roughness of the copper samples does not exceed 0.8 μm, then the lubricating efficiency of MoS2 decreases rapidly with increasing the end roughness of the copper samples, which results in a significant increase in the friction force and the dispersion of experimental data. MoS2 could not effectively reduce the friction force when the roughness of samples is 1.6 μm, and the lubricating efficiency was almost zero when the roughness is 3.2 μm, although the MoS2 has been believed to be an effective lubrication used in dynamic compression experiments by the SHPB for a long time. The end roughness of the pressure bars and samples should reach 0.8 μm when MoS2 is used as lubrication for the SHPB experiments, however, the end roughness of the samples treated by a corrosive solution is difficult to reach 0.8 μm. Therefore, it is necessary to lubricate the end of the samples better than MoS2, or to modify the experimental data by deducting the friction force to ensure the validity and accuracy of the SHPB experimental results.
Effective reduction of end surface friction is necessary to ensure the validity and accuracy of the split Hopkinson pressure bar (SHPB) experimental results. In order to study the effects of sample roughness and lubrication efficiency on the end surface friction and the final experimental results, copper was selected as a research material due to its steady mechanical properties and strain rate insensitivity of constitutive relation. In order to minimize the effects of end friction, all pressure bars with the diameter of 10 mm had a surface roughnesses of 0.8 μm. Copper samples of three typical surface roughnesses were prepared by mechanical processing and corrosion, then high precision repeat dynamic compression experiments by the SHPB were carried out under the conditions of full lubrication with MoS2 and complete non-lubrication, respectively. The results show that MoS2 can only play a good lubrication when the end roughness of the copper samples does not exceed 0.8 μm, then the lubricating efficiency of MoS2 decreases rapidly with increasing the end roughness of the copper samples, which results in a significant increase in the friction force and the dispersion of experimental data. MoS2 could not effectively reduce the friction force when the roughness of samples is 1.6 μm, and the lubricating efficiency was almost zero when the roughness is 3.2 μm, although the MoS2 has been believed to be an effective lubrication used in dynamic compression experiments by the SHPB for a long time. The end roughness of the pressure bars and samples should reach 0.8 μm when MoS2 is used as lubrication for the SHPB experiments, however, the end roughness of the samples treated by a corrosive solution is difficult to reach 0.8 μm. Therefore, it is necessary to lubricate the end of the samples better than MoS2, or to modify the experimental data by deducting the friction force to ensure the validity and accuracy of the SHPB experimental results.
2022, 42(1): 013301.
doi: 10.11883/bzycj-2021-0132
Abstract:
Based on the recently proposed Kong-Fang concrete material model and the fluid structure interaction (FSI) and restart algorithms available in the LS-DYNA, the damage and failure of concrete targets subjected to projectile penetration followed by explosion were numerically investigated. The numerical model, material models along with the corresponding parameters were firstly validated by comparing the numerical simulation results of the large-caliber projectile penetration experiment and the charge explosion test of a concrete target with a precast hole to the corresponding test data in terms of the penetration depth and scabbing depth, respectively. Then numerical simulations of the damage and failure in concrete targets struck by a typical warhead were conducted using three different modeling methods, i.e., charge explosion in a concrete target with a precast hole, charge explosions without and with projectile shell using the restart algorithm. The numerical results demonstrate that the crater diameter of the concrete target caused by explosion is only three times the projectile diameter when the pre-damage during the penetration process is not considered, and the damage and failure patterns are different from those using the other two methods. The numerically predicted crater diameter is very large when considering the pre-damage during the penetration process, as expected. However, the final crater diameter when the projectile shell is considered (about 14.5 times the projectile diameter) was slightly smaller than that without the consideration of projectile shell (around 16 times the projectile diameter), which mainly because part of the explosion energy is dissipated by the deformation and fracture of the projectile shell. The predicted crater depth with the consideration of projectile shell is increased by 5% compared with that ignoring the projectile shell, mainly due to the secondary penetration of the fragmentized warhead. The present numerical results can provide a reliable reference for further experimental investigation on the damage and failure of concrete targets subjected to projectile penetration followed by explosion.
Based on the recently proposed Kong-Fang concrete material model and the fluid structure interaction (FSI) and restart algorithms available in the LS-DYNA, the damage and failure of concrete targets subjected to projectile penetration followed by explosion were numerically investigated. The numerical model, material models along with the corresponding parameters were firstly validated by comparing the numerical simulation results of the large-caliber projectile penetration experiment and the charge explosion test of a concrete target with a precast hole to the corresponding test data in terms of the penetration depth and scabbing depth, respectively. Then numerical simulations of the damage and failure in concrete targets struck by a typical warhead were conducted using three different modeling methods, i.e., charge explosion in a concrete target with a precast hole, charge explosions without and with projectile shell using the restart algorithm. The numerical results demonstrate that the crater diameter of the concrete target caused by explosion is only three times the projectile diameter when the pre-damage during the penetration process is not considered, and the damage and failure patterns are different from those using the other two methods. The numerically predicted crater diameter is very large when considering the pre-damage during the penetration process, as expected. However, the final crater diameter when the projectile shell is considered (about 14.5 times the projectile diameter) was slightly smaller than that without the consideration of projectile shell (around 16 times the projectile diameter), which mainly because part of the explosion energy is dissipated by the deformation and fracture of the projectile shell. The predicted crater depth with the consideration of projectile shell is increased by 5% compared with that ignoring the projectile shell, mainly due to the secondary penetration of the fragmentized warhead. The present numerical results can provide a reliable reference for further experimental investigation on the damage and failure of concrete targets subjected to projectile penetration followed by explosion.
2022, 42(1): 013302.
doi: 10.11883/bzycj-2021-0007
Abstract:
With the advancement of hypervelocity weapons such as the “Rods-from-God”, the damage and failure in targets induced by the hypervelocity penetrators have been a topic of current research, which is still not fully understood. To address this issue, numerical investigation was carried out on ground shock induced by hypervelocity penetration of projectiles into limestone targets. As the material model and corresponding parameters are crucial for the accurate numerical predictions, the parameters for the p-α equation of state and the Kong-Fang material model recently proposed to describe the limestone were firstly calibrated based on a large amount of dynamic tests. The smooth particle hydrodynamics (SPH) method was employed for simulating the target and the axisymmetric numerical model was used to improve the computational efficiency. The calibrated parameters and numerical algorithm were validated by numerically simulating a series of penetration tests on limestone targets with a broad range of striking velocities. Then, based on the validated numerical model, the penetration of long-rod tungsten projectile into a limestone target was simulated and the mechanism of the corresponding ground shock was discussed. It was found that a high pressure in the target was induced by the hypervelocity impact of the projectile, which then propagated into the target as a stress wave, leading to the damage and failure in the target. The induced ground shock wave increased with the increase of the initial projectile velocity, especially when the initial projectile velocity is over 3.0 km/s. Finally, parametric study was conducted to investigate the effects of the parameters related to the projectiles and targets on the ground shock wave. The parameters related to the projectiles, for example, the length-to-diameter ratio and density, which can influence on the damage area of the targets by influencing the depth of penetration, has limited influence on the ground shock wave from the view of the relative depth (the ratio of the depth to the penetration depth). While the target parameters, especially the porosity which can affect the wave propagation, have a great effect on the ground shock wave.
With the advancement of hypervelocity weapons such as the “Rods-from-God”, the damage and failure in targets induced by the hypervelocity penetrators have been a topic of current research, which is still not fully understood. To address this issue, numerical investigation was carried out on ground shock induced by hypervelocity penetration of projectiles into limestone targets. As the material model and corresponding parameters are crucial for the accurate numerical predictions, the parameters for the p-α equation of state and the Kong-Fang material model recently proposed to describe the limestone were firstly calibrated based on a large amount of dynamic tests. The smooth particle hydrodynamics (SPH) method was employed for simulating the target and the axisymmetric numerical model was used to improve the computational efficiency. The calibrated parameters and numerical algorithm were validated by numerically simulating a series of penetration tests on limestone targets with a broad range of striking velocities. Then, based on the validated numerical model, the penetration of long-rod tungsten projectile into a limestone target was simulated and the mechanism of the corresponding ground shock was discussed. It was found that a high pressure in the target was induced by the hypervelocity impact of the projectile, which then propagated into the target as a stress wave, leading to the damage and failure in the target. The induced ground shock wave increased with the increase of the initial projectile velocity, especially when the initial projectile velocity is over 3.0 km/s. Finally, parametric study was conducted to investigate the effects of the parameters related to the projectiles and targets on the ground shock wave. The parameters related to the projectiles, for example, the length-to-diameter ratio and density, which can influence on the damage area of the targets by influencing the depth of penetration, has limited influence on the ground shock wave from the view of the relative depth (the ratio of the depth to the penetration depth). While the target parameters, especially the porosity which can affect the wave propagation, have a great effect on the ground shock wave.
2022, 42(1): 013303.
doi: 10.11883/bzycj-2021-0111
Abstract:
In order to study the initiation mechanisms of the explosive charge covered with a thick shell impacted by a high- velocity rod projectiles, the shock physical explicit Eulerian dynamic software SPEED was applied to numerically simulate the interactions beween the tungsten rod projectiles with different diameters and lengths and the Comp-B charge covered with a thick shell, the up-down method was used to obtain the critical impact velocity and the change of the detonation position, and the effects of the projectile diameter and length on the critical impact velocity were obtained. The initiation mechanisms of the Comp-B charge detonated by the projectile at the critical impact velocity were analyzed in depth, and the effects of the projectile impact velocity on the initiation mechanism and the detonation position were obtained. The research results show that the critical impact velocity decreases significantly as the projectile diameter increases, the critical impact velocity first decreases and then gradually changes as the projectile length increases. When the Comp-B charge is detonated by the projectile at the critical impact velocity, there are two initiation mechanisms, namely the macro-shear initiation mechanism after the projectile penetrates the shell and the low-velocity impact initiation mechanism without penetrating the shell. The mechanisms will transform as the projectile impact velocity continues to increase above the critical impact velocity. If the macro-shear initiation mechanism dominates when the Comp-B charge is detonated by the projectile at the critical impact velocity, it will transform into the high-velocity impact initiation mechanism; if the low-velocity impact initiation mechanism dominates at this time, it will first transform into the macro-shear initiation mechanism, and then transform into the high-velocity impact initiation mechanism. The detonation position is at a certain distance from the interface between the explosive and the shell, the distance decreases as the impact velocity of the projectile increases if the initiation mechanism remains the same.
In order to study the initiation mechanisms of the explosive charge covered with a thick shell impacted by a high- velocity rod projectiles, the shock physical explicit Eulerian dynamic software SPEED was applied to numerically simulate the interactions beween the tungsten rod projectiles with different diameters and lengths and the Comp-B charge covered with a thick shell, the up-down method was used to obtain the critical impact velocity and the change of the detonation position, and the effects of the projectile diameter and length on the critical impact velocity were obtained. The initiation mechanisms of the Comp-B charge detonated by the projectile at the critical impact velocity were analyzed in depth, and the effects of the projectile impact velocity on the initiation mechanism and the detonation position were obtained. The research results show that the critical impact velocity decreases significantly as the projectile diameter increases, the critical impact velocity first decreases and then gradually changes as the projectile length increases. When the Comp-B charge is detonated by the projectile at the critical impact velocity, there are two initiation mechanisms, namely the macro-shear initiation mechanism after the projectile penetrates the shell and the low-velocity impact initiation mechanism without penetrating the shell. The mechanisms will transform as the projectile impact velocity continues to increase above the critical impact velocity. If the macro-shear initiation mechanism dominates when the Comp-B charge is detonated by the projectile at the critical impact velocity, it will transform into the high-velocity impact initiation mechanism; if the low-velocity impact initiation mechanism dominates at this time, it will first transform into the macro-shear initiation mechanism, and then transform into the high-velocity impact initiation mechanism. The detonation position is at a certain distance from the interface between the explosive and the shell, the distance decreases as the impact velocity of the projectile increases if the initiation mechanism remains the same.
2022, 42(1): 014101.
doi: 10.11883/bzycj-2020-0207
Abstract:
Gas guns with a water tank assembly, were developed for launching projectiles into water obliquely and horizontally. The burst of gas guns was controlled by the quick valve and the piston valve. The cartridges and models in the one-stage gas gun were directly driven by high-pressure air. The heavy piston in the two-stage gas gun was driven by high-pressure air first, and then compressed the air in the gas-gathered chamber to drive the cartridges and models to the predetermined high-speed. By adjusting the angle between the water tank and the launching tube, the high-speed model can entry into water either obliquely or horizontally. The vertical gas gun with variable launch angles, is capable of launching a projectile with mass ranged from several to hundreds of grams at speed ranged from hundreds to thousands of meters per second. The horizontal gas gun can launch the projectile with mass ranged from several to tens kilograms at speed ranged from several to hundreds of meters per second. In contrast to a powder gas gun using small chamber filled with vitiated gas at high pressure and temperature, these gas guns are distinguished for the large volume air reservoir run at medium even low pressure, and characterized by a wide range of mass and speed of the projectile by adjusting the air pressure, which is close to isentropic expansion. Based on light reflection and beam on-off methodology, high-speed photography and shadowgraphy measurements, the results including the piston velocity in compression tube, the pressure time history at the end of the compression tube and the shadowgraph images of water entry and underwater navigation, were obtained. The results show that the piston velocities are in good agreement with the theoretical calculation before the diaphragm bursting, after which the difference increases. The high-speed shadowgraphs in the vertical gas gun clearly indicate the shock waves in the air and water generated by the oblique impacting of the projectile into the water, as well as the reflection of the shock wave on the gas-water interface, where the formation of cavitation, the break and splash of the interface are observed either. A gas bubble induced by water and enrolled air envelopes, which is extracted from the images in the horizontal gas gun, clearly indicates the fluctuation or instability along the gas-water interface close to the bubble rear. Compared with the numerical results by the commercial software fluent, the obtained bubble lineament basically coincides except at rear due to strongly wake turbulence attached to the projectile. In contrast to the water tunnel, the test rigs in this paper are superior in wide range of mass and speed, as well as reproducing real conditions such as impacting phenomenon and dynamic cavitation during the process of high-speed water entry.
Gas guns with a water tank assembly, were developed for launching projectiles into water obliquely and horizontally. The burst of gas guns was controlled by the quick valve and the piston valve. The cartridges and models in the one-stage gas gun were directly driven by high-pressure air. The heavy piston in the two-stage gas gun was driven by high-pressure air first, and then compressed the air in the gas-gathered chamber to drive the cartridges and models to the predetermined high-speed. By adjusting the angle between the water tank and the launching tube, the high-speed model can entry into water either obliquely or horizontally. The vertical gas gun with variable launch angles, is capable of launching a projectile with mass ranged from several to hundreds of grams at speed ranged from hundreds to thousands of meters per second. The horizontal gas gun can launch the projectile with mass ranged from several to tens kilograms at speed ranged from several to hundreds of meters per second. In contrast to a powder gas gun using small chamber filled with vitiated gas at high pressure and temperature, these gas guns are distinguished for the large volume air reservoir run at medium even low pressure, and characterized by a wide range of mass and speed of the projectile by adjusting the air pressure, which is close to isentropic expansion. Based on light reflection and beam on-off methodology, high-speed photography and shadowgraphy measurements, the results including the piston velocity in compression tube, the pressure time history at the end of the compression tube and the shadowgraph images of water entry and underwater navigation, were obtained. The results show that the piston velocities are in good agreement with the theoretical calculation before the diaphragm bursting, after which the difference increases. The high-speed shadowgraphs in the vertical gas gun clearly indicate the shock waves in the air and water generated by the oblique impacting of the projectile into the water, as well as the reflection of the shock wave on the gas-water interface, where the formation of cavitation, the break and splash of the interface are observed either. A gas bubble induced by water and enrolled air envelopes, which is extracted from the images in the horizontal gas gun, clearly indicates the fluctuation or instability along the gas-water interface close to the bubble rear. Compared with the numerical results by the commercial software fluent, the obtained bubble lineament basically coincides except at rear due to strongly wake turbulence attached to the projectile. In contrast to the water tunnel, the test rigs in this paper are superior in wide range of mass and speed, as well as reproducing real conditions such as impacting phenomenon and dynamic cavitation during the process of high-speed water entry.
2022, 42(1): 014201.
doi: 10.11883/bzycj-2021-0218
Abstract:
The split Hopkinson tensile bar is one of the most commonly used apparatuses to test the dynamic tensile mechanical properties of materials at the high strain rates from 102 s−1 to 103 s−1, in which the specimens with a dog-bone shape are usually used. The dimensions of the specimen used are critical to ensure the basic assumptions during dynamic tensile process, such as one-dimensional stress state and uniform deformation of the specimen etc. And whether these assumptions can be satisfied would affect the measurement accuracy of the dynamic tensile properties directly. So, it is urgent to study the influence of the specimen structural parameters on the stress and deformation states of the specimen during the dynamic tensile tests. At the same time, developing and establishing an effective method which can realize the global optimization of specimen structural parameters in the entire parameter space is crucial. In order to actualize the above research objectives, indicators which can quantify the measurement accuracy of the dynamic tensile tests were firstly proposed, namely the time required to reach the stress equilibrium, the deformation uniformity, the relative level of the non-axial stress, and the relative deformation of the transition zones. Orthogonal tests with 6 factors and 5 levels were then designed for the important structural parameters of the dog-bone shaped sheet tensile specimens. According to the rules of the orthogonal test design, 25 dynamic tensile specimens with different structural dimensions were obtained. The commercial finite element software ABAQUS/Explicit was used to establish a finite element model of the split Hopkinson tensile bar, and dynamic tensile test simulations were performed on the dynamic tensile specimens obtained from the orthogonal test design. An orthogonal test dataset with the specimen structural parameters as the input and the measurement accuracy indicators as the output was then constructed. Multi-objective orthogonal test matrix analysis was carried out on the orthogonal test dataset to obtain the influence order as well as the influence law of the structural parameters of the tensile specimens on the measurement accuracy indicators of tests. Taking the orthogonal test dataset as the training dataset, an artificial neural network (ANN) model was used to fit the nonlinear relationship which can predict the measurement accuracy indicators of the test by using the structural parameters of the specimen, and then the fitness function in the genetic algorithm (GA) was established by using this model. Finally, the structural parameters of the dynamic tensile specimen were optimized using the ANN-GA collaborative optimization method, and the optimal structural dimensions of the dynamic tensile specimen were obtained as the result of the optimization. Finite element simulation results show that the optimal structural dimensions obtained by the ANN-GA optimization method are valid. The results of this study demonstrate the practicability and effectiveness of the ANN-GA method in the structural optimization of dynamic tensile specimens. On the other hand, it can provide guidance for the specimen design in the dynamic tensile mechanical properties tests of materials, and can also provide a reference for the validity analysis of the experimentally measured mechanical properties.
The split Hopkinson tensile bar is one of the most commonly used apparatuses to test the dynamic tensile mechanical properties of materials at the high strain rates from 102 s−1 to 103 s−1, in which the specimens with a dog-bone shape are usually used. The dimensions of the specimen used are critical to ensure the basic assumptions during dynamic tensile process, such as one-dimensional stress state and uniform deformation of the specimen etc. And whether these assumptions can be satisfied would affect the measurement accuracy of the dynamic tensile properties directly. So, it is urgent to study the influence of the specimen structural parameters on the stress and deformation states of the specimen during the dynamic tensile tests. At the same time, developing and establishing an effective method which can realize the global optimization of specimen structural parameters in the entire parameter space is crucial. In order to actualize the above research objectives, indicators which can quantify the measurement accuracy of the dynamic tensile tests were firstly proposed, namely the time required to reach the stress equilibrium, the deformation uniformity, the relative level of the non-axial stress, and the relative deformation of the transition zones. Orthogonal tests with 6 factors and 5 levels were then designed for the important structural parameters of the dog-bone shaped sheet tensile specimens. According to the rules of the orthogonal test design, 25 dynamic tensile specimens with different structural dimensions were obtained. The commercial finite element software ABAQUS/Explicit was used to establish a finite element model of the split Hopkinson tensile bar, and dynamic tensile test simulations were performed on the dynamic tensile specimens obtained from the orthogonal test design. An orthogonal test dataset with the specimen structural parameters as the input and the measurement accuracy indicators as the output was then constructed. Multi-objective orthogonal test matrix analysis was carried out on the orthogonal test dataset to obtain the influence order as well as the influence law of the structural parameters of the tensile specimens on the measurement accuracy indicators of tests. Taking the orthogonal test dataset as the training dataset, an artificial neural network (ANN) model was used to fit the nonlinear relationship which can predict the measurement accuracy indicators of the test by using the structural parameters of the specimen, and then the fitness function in the genetic algorithm (GA) was established by using this model. Finally, the structural parameters of the dynamic tensile specimen were optimized using the ANN-GA collaborative optimization method, and the optimal structural dimensions of the dynamic tensile specimen were obtained as the result of the optimization. Finite element simulation results show that the optimal structural dimensions obtained by the ANN-GA optimization method are valid. The results of this study demonstrate the practicability and effectiveness of the ANN-GA method in the structural optimization of dynamic tensile specimens. On the other hand, it can provide guidance for the specimen design in the dynamic tensile mechanical properties tests of materials, and can also provide a reference for the validity analysis of the experimentally measured mechanical properties.
2022, 42(1): 014202.
doi: 10.11883/bzycj-2021-0106
Abstract:
Underwater blast shock wave is an important load in the evaluation of the impact resistance of ships, and it is also the key and basis for the fast prediction of the structure damage in underwater explosions. In the present study, a series of small equivalent underwater explosion experiments were carried out in the explosion tank. By comparing the theoretical predicted and experimental measured wall pressure characteristics, the applicability of the traditional Taylor formula in predicting the wall pressure of the underwater explosion shock wave was explored. It is found that the deviation of the Taylor plate theory in predicting the pulse width of the wall-pressure is mainly because the nonlinear variation of the shock wave velocity is not considered. Given this, a fitting formula of the shock wave velocity for 0.11 m/kg1/3≤R/W1/3≤5.30 m/kg1/3, where R is the detonation distance and W is the charge weight, is given to improve the traditional Taylor theoretical formula. The corrected theoretical values are in good agreement with the experimental values. For R/W1/3=0.11 m/kg1/3, the pulse-width deviation of the wall-pressure of the shock wave between the improved Taylor formula and the experimental result is reduced from 79.6% to 26.6%, and the deviation of the impulse is reduced from 119.3% to 58.4%. For R/W1/3≥0.21 m/kg1/3, the deviations of the pulse-width and the impulse of wall-pressure are both less than 12%. Moreover, in the simulation of the wall-pressure change at different distances by numerical method (e.g., finite element method), it is found that the numerical dissipation causes the plate to move in advance (before the arrival of the shock wave front), leading to a significant decrease in the peak of the wall-pressure when dealing with the near-field and far-field underwater explosion problems. Therefore, a feasible numerical strategy was proposed to eliminate the weakening effect caused by numerical dissipation. The improved numerical results are in great agreement with the improved Taylor plate theory, and the deviation of the wall-pressure peak is less than 9%. The improved theoretical formula and numerical method for the shock wave wall pressure can provide theoretical and technical supports for the field of explosion protection of ships.
Underwater blast shock wave is an important load in the evaluation of the impact resistance of ships, and it is also the key and basis for the fast prediction of the structure damage in underwater explosions. In the present study, a series of small equivalent underwater explosion experiments were carried out in the explosion tank. By comparing the theoretical predicted and experimental measured wall pressure characteristics, the applicability of the traditional Taylor formula in predicting the wall pressure of the underwater explosion shock wave was explored. It is found that the deviation of the Taylor plate theory in predicting the pulse width of the wall-pressure is mainly because the nonlinear variation of the shock wave velocity is not considered. Given this, a fitting formula of the shock wave velocity for 0.11 m/kg1/3≤R/W1/3≤5.30 m/kg1/3, where R is the detonation distance and W is the charge weight, is given to improve the traditional Taylor theoretical formula. The corrected theoretical values are in good agreement with the experimental values. For R/W1/3=0.11 m/kg1/3, the pulse-width deviation of the wall-pressure of the shock wave between the improved Taylor formula and the experimental result is reduced from 79.6% to 26.6%, and the deviation of the impulse is reduced from 119.3% to 58.4%. For R/W1/3≥0.21 m/kg1/3, the deviations of the pulse-width and the impulse of wall-pressure are both less than 12%. Moreover, in the simulation of the wall-pressure change at different distances by numerical method (e.g., finite element method), it is found that the numerical dissipation causes the plate to move in advance (before the arrival of the shock wave front), leading to a significant decrease in the peak of the wall-pressure when dealing with the near-field and far-field underwater explosion problems. Therefore, a feasible numerical strategy was proposed to eliminate the weakening effect caused by numerical dissipation. The improved numerical results are in great agreement with the improved Taylor plate theory, and the deviation of the wall-pressure peak is less than 9%. The improved theoretical formula and numerical method for the shock wave wall pressure can provide theoretical and technical supports for the field of explosion protection of ships.
2022, 42(1): 014203.
doi: 10.11883/bzycj-2021-0095
Abstract:
In order to estimate the underwater explosion shock wave pressure of slender cone-shaped charges and to study the characteristic of long duration shock waves, an engineering model based on the superposition principle was proposed. Cone-shaped charges are usually used to simulate the far-field shock wave of large equivalent explosives, and the wave strength is generally on the order of MPa, which can be regarded as a weak shock wave, so the problem can be simplified based on the acoustic approximation assumption. Based on the above analysis, the cone-shaped charge is divided into several small charges, and then the shock wave pressure generated by each small charge in the water is superimposed according to the propagation order of the detonation wave to obtain the shock wave pressure curve of the whole cone-shaped charge. The validity of the model was verified through experimental results. Then, the transmission characteristics and the pressure profile of the shock wave at different azimuths of the cone-shaped charge were analyzed. The results show that the shock wave is anisotropic around the charge. Long duration, low amplitude shock waves with a thick wave head are generated at the detonation end. Exponential decaying shock waves with high amplitude are formed on the side of the charge, while on the opposed side of the detonation end the amplitude and duration of the shock wave are between the former two. The differences in the shock wave distributions between the cone-shaped and spherical charges are related to their shapes and detonation methods. Due to the differences in the explosion initiation times of different parts of the explosive charge, the superimposition effect of shock waves at different azimuths is obviously different, which result in an anisotropic pressure field. The proposed method is in good agreement with the experimental and numerical simulation results, which can provide reference and basis for the power and damage assessment of the underwater explosion shock wave of the cone-shaped charges.s of cone-shaped charges.
In order to estimate the underwater explosion shock wave pressure of slender cone-shaped charges and to study the characteristic of long duration shock waves, an engineering model based on the superposition principle was proposed. Cone-shaped charges are usually used to simulate the far-field shock wave of large equivalent explosives, and the wave strength is generally on the order of MPa, which can be regarded as a weak shock wave, so the problem can be simplified based on the acoustic approximation assumption. Based on the above analysis, the cone-shaped charge is divided into several small charges, and then the shock wave pressure generated by each small charge in the water is superimposed according to the propagation order of the detonation wave to obtain the shock wave pressure curve of the whole cone-shaped charge. The validity of the model was verified through experimental results. Then, the transmission characteristics and the pressure profile of the shock wave at different azimuths of the cone-shaped charge were analyzed. The results show that the shock wave is anisotropic around the charge. Long duration, low amplitude shock waves with a thick wave head are generated at the detonation end. Exponential decaying shock waves with high amplitude are formed on the side of the charge, while on the opposed side of the detonation end the amplitude and duration of the shock wave are between the former two. The differences in the shock wave distributions between the cone-shaped and spherical charges are related to their shapes and detonation methods. Due to the differences in the explosion initiation times of different parts of the explosive charge, the superimposition effect of shock waves at different azimuths is obviously different, which result in an anisotropic pressure field. The proposed method is in good agreement with the experimental and numerical simulation results, which can provide reference and basis for the power and damage assessment of the underwater explosion shock wave of the cone-shaped charges.s of cone-shaped charges.
2022, 42(1): 015201.
doi: 10.11883/bzycj-2020-0436
Abstract:
Aiming at the problem that the free surface not only affected the blasting effect but also the blasting vibration effect, an analytical method of exploring the influence of free surface on the vibration attenuation law of underwater drilling blasting was proposed from the energy point of view. Taking the field monitoring data of the underwater blasting seismic wave between the river reach of the Three Gorges Dam and Gezhouba Dam as the research object, the characteristic information on time and frequency scales of the monitored signals was analyzed by wavelet transform. Similarly, the total energy of the blasting vibration signals, the energy distribution characteristics in variety of frequency band on different free surface, and the main frequency range were extracted. Combined with the method of the SPH-FEM simulation technology, the vibration velocity attenuation law of different numbers of free surfaces was verified. The results indicate that underwater drilling blasting vibration has the characteristic of low frequency, the short duration and the fast attenuation. The main frequency band of the underwater drilling blasting is focused on the frequency band of 15.625−31.250 Hz. Most of the explosion energy will be consumed as seismic energy in the underwater slotted blasting due to the restricted of a single free surface. The specific vibrational energy λE of the single free surface signal is 13.14 mm2/(kg·s2). However, with the increase of the number of free surfaces in subsequent excavation blasting, the λE of the double free surfaces and the three free surfaces decrease to 1.36 and 0.28 mm2/(kg·s2), and the reduction rates of the peak particle velocity (PPV) of the frequency band are 65% and 37%. Besides, the energy of the subsequent explosion will be used more for breaking and throwing the rock mass, and the main frequency of it will also develop from low frequency to high frequency band (31.25−62.50 Hz). Therefore, the influence of the number of free surfaces on the vibration energy distribution and attenuation law should be considered in the design of underwater controlled blasting. Using this regularity and characteristic, it is possible to more accurately determine the controlled explosive charge of each segment and reduce the resonance hazard to surrounding structures.
Aiming at the problem that the free surface not only affected the blasting effect but also the blasting vibration effect, an analytical method of exploring the influence of free surface on the vibration attenuation law of underwater drilling blasting was proposed from the energy point of view. Taking the field monitoring data of the underwater blasting seismic wave between the river reach of the Three Gorges Dam and Gezhouba Dam as the research object, the characteristic information on time and frequency scales of the monitored signals was analyzed by wavelet transform. Similarly, the total energy of the blasting vibration signals, the energy distribution characteristics in variety of frequency band on different free surface, and the main frequency range were extracted. Combined with the method of the SPH-FEM simulation technology, the vibration velocity attenuation law of different numbers of free surfaces was verified. The results indicate that underwater drilling blasting vibration has the characteristic of low frequency, the short duration and the fast attenuation. The main frequency band of the underwater drilling blasting is focused on the frequency band of 15.625−31.250 Hz. Most of the explosion energy will be consumed as seismic energy in the underwater slotted blasting due to the restricted of a single free surface. The specific vibrational energy λE of the single free surface signal is 13.14 mm2/(kg·s2). However, with the increase of the number of free surfaces in subsequent excavation blasting, the λE of the double free surfaces and the three free surfaces decrease to 1.36 and 0.28 mm2/(kg·s2), and the reduction rates of the peak particle velocity (PPV) of the frequency band are 65% and 37%. Besides, the energy of the subsequent explosion will be used more for breaking and throwing the rock mass, and the main frequency of it will also develop from low frequency to high frequency band (31.25−62.50 Hz). Therefore, the influence of the number of free surfaces on the vibration energy distribution and attenuation law should be considered in the design of underwater controlled blasting. Using this regularity and characteristic, it is possible to more accurately determine the controlled explosive charge of each segment and reduce the resonance hazard to surrounding structures.
2022, 42(1): 015401.
doi: 10.11883/bzycj-2021-0139
Abstract:
To investigate the explosion characteristics of oil shale dust, four kinds of oil shale dust from main mining areas such as Longkou (LK), Maoming (MM), Huadian (HD) and Fushan (FS) in China were chosen as experimental samples. A 20-litre explosion sphere vessel was used as the experimental device to carry out explosion experiments to systematically explore the influences of the parameters of the samples, including dust mass concentration, particle size, the content of volatile and ash, and oxygen content on the explosion characteristics of oil shale dust. The experimental results show that the higher the volatile content of the oil shale, the higher the maximum explosion pressure, the maximum rise rate of explosion pressure (dp/dt)max, and the lower the minimum explosion mass concentration; the volatile has a significant promoting effect on the explosion of the oil shale dust, while the ash has a significant inhibiting effect on it. In the dust particle size range from 37.52 microns to 106.43 microns, the magnitudes of pmax and (dp/dt)max of the four oil shale samples all decrease with the increase of particle size, and the time to reach pmax decreases gradually as the particle size becomes smaller. The smaller the particles are, the faster the volatiles are released, which can improve the reaction degree of the explosion. In the dust mass concentration range from 400 g/m3 to 2 500 g/m3, the magnitudes of pmax and (dp/dt)max of the four samples all took on a trend of increasing first and then decreasing with the increase of dust mass concentration. After the critical pressure concentration (1000 g/m3) was exceeded, the explosion intensity decreased slightly, but still maintained at a high level, and still had relatively strong destructive power. The magnitudes of pmax (0.61 MPa) and (dp/dt)max (29.32 MPa/s) of the LK sample are the highest in the four samples, which are at the same level as that of lignite with similar volatile content. The minimum explosion concentration (200 g/m3) of the LK sample is the lowest among the four samples, which is higher than that of the lignite with the similar volatile content. By using N2 as the inert gas, the oxidation, the heat released and the magnitudes of pmax and (dp/dt)max of the LK sample all decreased with the decrease of oxygen content. When the oxygen content was reduced to 15%, there were no more explosions in the system, and the limiting oxygen concentration was found to be 16%.
To investigate the explosion characteristics of oil shale dust, four kinds of oil shale dust from main mining areas such as Longkou (LK), Maoming (MM), Huadian (HD) and Fushan (FS) in China were chosen as experimental samples. A 20-litre explosion sphere vessel was used as the experimental device to carry out explosion experiments to systematically explore the influences of the parameters of the samples, including dust mass concentration, particle size, the content of volatile and ash, and oxygen content on the explosion characteristics of oil shale dust. The experimental results show that the higher the volatile content of the oil shale, the higher the maximum explosion pressure, the maximum rise rate of explosion pressure (dp/dt)max, and the lower the minimum explosion mass concentration; the volatile has a significant promoting effect on the explosion of the oil shale dust, while the ash has a significant inhibiting effect on it. In the dust particle size range from 37.52 microns to 106.43 microns, the magnitudes of pmax and (dp/dt)max of the four oil shale samples all decrease with the increase of particle size, and the time to reach pmax decreases gradually as the particle size becomes smaller. The smaller the particles are, the faster the volatiles are released, which can improve the reaction degree of the explosion. In the dust mass concentration range from 400 g/m3 to 2 500 g/m3, the magnitudes of pmax and (dp/dt)max of the four samples all took on a trend of increasing first and then decreasing with the increase of dust mass concentration. After the critical pressure concentration (1000 g/m3) was exceeded, the explosion intensity decreased slightly, but still maintained at a high level, and still had relatively strong destructive power. The magnitudes of pmax (0.61 MPa) and (dp/dt)max (29.32 MPa/s) of the LK sample are the highest in the four samples, which are at the same level as that of lignite with similar volatile content. The minimum explosion concentration (200 g/m3) of the LK sample is the lowest among the four samples, which is higher than that of the lignite with the similar volatile content. By using N2 as the inert gas, the oxidation, the heat released and the magnitudes of pmax and (dp/dt)max of the LK sample all decreased with the decrease of oxygen content. When the oxygen content was reduced to 15%, there were no more explosions in the system, and the limiting oxygen concentration was found to be 16%.
2022, 42(1): 015402.
doi: 10.11883/bzycj-2021-0064
Abstract:
To reveal the flame propagation mechanisms in the methane/coal dust hybrid explosions, the effects of coal type and methane concentration on the propagation characteristics of methane/coal dust hybrid explosion flame were experimentally investigated. Experiments were performed in a gas-solid hybrid explosion apparatus with methane concentrations below the lower explosive limit. The flame propagation images were captured by a high-speed camera and the flame temperature was recorded by a high-accuracy thermocouple. The results show that the volatile component is the dominant parameter in measuring the combustion characteristics of a certain coal type. With the increase of the volatile component, the combustion of the methane/coal dust flame gets enhanced. As a result, the flame propagation velocity increases and the flame temperature goes up. When the difference of the volatile component is small, due to the heat loss of the water evaporation, the combustion reaction of coal dust with lower water component is severer, the flame propagates more quickly. With the increase of the methane concentration, the combustion of the coal dust particle gradually transforms from the diffusion combustion of the released volatile components to the premixed combustion. The heat radiation and convection promotes the pyrolysis of the coal particle and the combustible substances are released, which maintains the continuous propagation of the hybrid flame. With the increase of the methane concentration, the hybrid explosion mechanism is transformed from the dust-driven type to the gas-driven type, and the combustion reaction gets enhanced. The methane/coal dust hybrid flame could be composed by five zones: unburned zone, preheated zone, gas combustion zone, multi-phase combustion zone and char combustion zone. Behind the flame front, large or the agglomerated particles could continue the combustion reaction, which is the multi-phase combustion zone. In addition, the combustion of the char develops the char combustion zone. The turbulent disturbance results in the distribution difference of the combustion materials, which leads to the interlacement of the different combustion zones.
To reveal the flame propagation mechanisms in the methane/coal dust hybrid explosions, the effects of coal type and methane concentration on the propagation characteristics of methane/coal dust hybrid explosion flame were experimentally investigated. Experiments were performed in a gas-solid hybrid explosion apparatus with methane concentrations below the lower explosive limit. The flame propagation images were captured by a high-speed camera and the flame temperature was recorded by a high-accuracy thermocouple. The results show that the volatile component is the dominant parameter in measuring the combustion characteristics of a certain coal type. With the increase of the volatile component, the combustion of the methane/coal dust flame gets enhanced. As a result, the flame propagation velocity increases and the flame temperature goes up. When the difference of the volatile component is small, due to the heat loss of the water evaporation, the combustion reaction of coal dust with lower water component is severer, the flame propagates more quickly. With the increase of the methane concentration, the combustion of the coal dust particle gradually transforms from the diffusion combustion of the released volatile components to the premixed combustion. The heat radiation and convection promotes the pyrolysis of the coal particle and the combustible substances are released, which maintains the continuous propagation of the hybrid flame. With the increase of the methane concentration, the hybrid explosion mechanism is transformed from the dust-driven type to the gas-driven type, and the combustion reaction gets enhanced. The methane/coal dust hybrid flame could be composed by five zones: unburned zone, preheated zone, gas combustion zone, multi-phase combustion zone and char combustion zone. Behind the flame front, large or the agglomerated particles could continue the combustion reaction, which is the multi-phase combustion zone. In addition, the combustion of the char develops the char combustion zone. The turbulent disturbance results in the distribution difference of the combustion materials, which leads to the interlacement of the different combustion zones.
