Initial Shock Wave Research Articles

Preliminary study on the tea dust explosion: the effect of tea dust size

Food-based dust is considered as combustible dust as they composed of distinct particles, regardless of the size or chemical composition and when suspended in air or any other oxidizing medium over a range of concentrations will present a fire or deflagration hazard. The explosion effect from food-based dust can cause catastrophic consequences because the initial shock wave from the explosion lift up more dust and triggers a chain reaction through the plant. One of the parameters that can enhance the explosion is the particle size of the dust. In this study, the effect of four different particle sizes of tea dust on the dust explosion severity was tested in a confined 20 L explosion bomb. Tea dust tends to explode due to its molecular structure which contains a carbon-hydrogen bond that can release the significant amount of thermal energy. The experimental results showed that the values of Pmax and (dP/dt)max of tea dust were more severe for the particle size of 160 ?m for which are 1.97 bar and 4.97 bar/s before drying and 2.09 bar and 7.01 bar/s after drying process. The finer dust reacted more violently than coarser ones. As particle size decreases, the rate of explosion pressure change increases, as long as the size is capable of supporting combustion.

Experimental investigation on multiple breakdown in water induced by focused nanosecond laser.

Multiple breakdowns in liquids still remains obscure for its complex, non-equilibrium and transient dynamic process. We introduced three methods, namely, plasma imaging, light-scattering technique, and acoustic detection, to measure the multiple breakdown in water induced by focused nanosecond laser pulses simultaneously. Our results showed that linear dependence existed among the cavitation-bubble lifetime, the far-field peak pressure of the initial shock wave, and the corresponding plasma volume. Such a relationship can be used to evaluate the ideal size and energy of each bubble during multiple breakdown. The major bubble lifetime was hardly affected by the inevitable coalescence of cavitation bubbles, thereby confirming the availability of light-scattering technique on the estimation of bubble size during multiple breakdown. Whereas, the strength of collapse-shock-wave and the subsequent rebound of bubbles was strongly influenced, i.e., the occurrence of multiple breakdown suppressed the cavitation-bubble energy being converted into collapse-shock-wave energy but enhanced conversion into rebound-bubble energy. This study is a valuable contribution to research on the rapid mixing of microfluidics, damage control of microsurgery, and photoacoustic applications.

Flame kernel evolution and shock wave propagation with laser ignition in ethanol-air mixtures

Flame kernel evolution and shock wave propagation in ethanol/air mixtures with laser ignition in a constant-volume chamber was investigated by means of high-speed Schlieren photography. At initial pressure of 0.1 MPa and temperature of 333 K, ignition was performed using a pulsed laser at the second harmonic wavelength with six different laser energies from 75 to 200 mJ, for the equivalence ratios ranging from 0.8 to 1.4. A cross-shaped plasma spot is captured ∼3.0 µs after the laser triggering, whereas the shape of the flame kernel is initially circular and then transforms to an ellipsoid structure. Following the contraction of the plasma, the flame kernel growth rate is initially very different in the four directions and then approaches a steady flame speed. The contraction of the plasma zone is accompanied by a rapid shock wave propagation where, the wave propagation speed is found to be negligibly influenced by the variations in either laser energy or mixture equivalence ratio. Under the test conditions in this study for ethanol/air mixtures, the initial shock wave speed is ∼480 m/s which decays to ∼380 m/s after 20 µs. The shock wave front propagates with a time dependency of about t0.6 when t < 20 µs, and then it becomes an approximately linear function of time.

Experimental observation of the erosion pattern, pits, and shockwave formation in a cavitating jet

In this study, the erosion pattern, pits, and shockwave formation of a cavitating jet were experimentally observed to gain an understanding of the erosion mechanism of a cavitating jet discharging from a cavitator nozzle into a still water environment. The erosion pattern and formation of pits were visualized by direct imaging on the eroded material surface, while the shockwave initiation points were detected using the cross-schlieren visualization method, combined with two orthogonal high-speed observations near the wall. The experimental results indicated that the radial distributions of the erosion depth, number of pits, and shockwave initiation points were highly correlated. These results provided direct evidence of the cavitation erosion caused by the formation of pits that resulted from the shockwave generated by the periodic cloud collapse event near the wall. The results also demonstrated that the growth of the cavitation erosion volume is highly correlated with the number of pits formed rather than the diameters of the pits.

DNS and LES of spark ignition with an automotive coil

The cycle to cycle combustion variability which is observed in spark-ignition engines is often caused by fluctuations of the early flame development. LES can be exploited for a better understanding and mastering of their origins. For that purpose appropriate models taking into account energy deposition, mixture ignition and transition to propagation are necessary requirements. This paper presents first DNS and LES of spark ignition with a real automotive coil and simplified pin-pin electrodes. The electrical circuit characteristics are provided by ISSIM while the energy deposition is modelled by Lagrangian particles. The ignition model is first evaluated in terms of initial spark radius on a pin-pin ignition experiment in pure air performed at CORIA and EM2C laboratories, showing that it pilots the radius of the torus formed by the initial shock wave. DNS of a quiescent lean propane/air mixture are then performed with this ignition system and a two-step mechanism. The impact of the modelled transferred energy during glow phase as well as the initial arc radius on the minimum ignition energy (MIE) are examined and compared to experimental values. Replacing the two-step chemistry by an analytically reduced mechanism leads to similar MIE but shows a different ignition kernel shape. Finally, LES of turbulent ignition using a Lagrangian arc model show a realistic prediction of the arc shape and its important role on the energy transfer location and thus on the flame kernel shape.

Formation of cylindrical shock wave in two-phase liquid layer with free surface

Numerical model for formation of 1D cylindrical shock-wave (SW) in a liquid layer with a free surface is considered. The liquid states are pure water and distilled water containing free micro-bubbles (1.5 I¼m, 106 cm-3). The SW initiation is performed on the axis by giving the pulse of mass velocity as the exponent for maximal amplitudes from 60 to 20 m/s. A two-phase mathematical model is the system of equation describing average pressure, velocity and density (including the Rayleigh-type equation). For pure water, the distributions of maximal amplitudes both from the axis along the radius for SW and from free surface up to the axis of symmetry for rarefaction wave (RW) were calculated. The distribution of maximal amplitudes of positive SW and negative RW along the radius appears to be completely symmetric. It was shown, the SW amplitude decreases proportionally to r -0.45 (within 3 cm from axis) and then asymptotics (r -0.72) is registered. The increase of RW amplitude during propagation to the axis is a cumulative effect. In two-phase model of distilled water the cavitation begins behind the RW front. Beginning from free surface, the volumetric gas concentration increases in 300 initial values (maximum velocity 60 m/s).Numerical model for formation of 1D cylindrical shock-wave (SW) in a liquid layer with a free surface is considered. The liquid states are pure water and distilled water containing free micro-bubbles (1.5 I¼m, 106 cm-3). The SW initiation is performed on the axis by giving the pulse of mass velocity as the exponent for maximal amplitudes from 60 to 20 m/s. A two-phase mathematical model is the system of equation describing average pressure, velocity and density (including the Rayleigh-type equation). For pure water, the distributions of maximal amplitudes both from the axis along the radius for SW and from free surface up to the axis of symmetry for rarefaction wave (RW) were calculated. The distribution of maximal amplitudes of positive SW and negative RW along the radius appears to be completely symmetric. It was shown, the SW amplitude decreases proportionally to r -0.45 (within 3 cm from axis) and then asymptotics (r -0.72) is registered. The increase of RW amplitude during propagation to the axis is ...

Mesoscopic effects on shock initiation of multi-component plastic bonded explosives

A series of one-dimensional Lagrangian tests have been performed to examine model parameters in the mesoscopic reaction rate model for shock initiation of multi-component plastic bonded explosives (PBXs) for two multi-component plastic bonded explosives PBXC03 (87% HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazoncine), 7% TATB (triaminotrinitrobenzene), and 6% binder by weight) and PBXC10 (25% HMX, 70% TATB, and 5% binder by weight). As the numerical results are in good agreement with experimental data, the model parameters have been used to predict the effects of variations in mesoscopic properties (the particle size, initial density, binder strength, and content) on the shock initiation characteristics of PBXC03 and PBXC10. It is found that the time to detonation for PBXC03 increases with all these mesoscopic properties, while the time to detonation for PBXC10 is basically independent of its mesoscopic properties. Thus, PBXC03 is sensitive to mesoscopic properties, but PBXC10 is not. Moreover, it is also found that the pressure-history curves behind the initial shock wave in PBXC03 have different trends from PBXC10, which implies different chemical reaction mechanisms. Further analysis reveals that it arises from the different hot spot ignition processes due to their different threshold initiation pressures. The hot spots are ignited gradually and almost simultaneously in PBXC03 and PBXC10, respectively.

Numerical study of three-dimensional developments of premixed flame induced by multiple shock waves

The three-dimensional interactions of a perturbed premixed flame interface with a planar incident shock wave and its reflected shock waves are numerically simulated by solving the compressible, reactive Navier–Stokes equations with the high-resolution scheme and a single-step chemical reaction. The effects of the initial incident shock wave strength (Mach number) and the initial perturbation pattern of interface on the interactions are investigated. The distinct properties of perturbation growth on the flame interface during the interactions are presented. Our results show that perturbation growth is mainly attributed to the flame stretching and propagation. The flame stretching is associated with the larger-scale vortical flow due to Richtmyer–Meshkov instability while the flame propagation is due to the chemical reaction. The mixing properties of unburned/burned gases on both sides of the flame are quantitatively analyzed by using integral and statistical diagnostics. The results show that the large-scale flow due to the vortical motion always plays a dominating role during the reactive interaction process; however, the effect of chemistry becomes more important at the later stage of the interactions, especially for higher Mach number cases. The scalar dissipation due to the molecular diffusion is always small in the present study and can be negligible.

Underwater explosion and its effects on nonlinear behavior of an arch dam

In the present paper, the behavior of the Karaj double curvature arch dam is studied focusing on the effects of structural nonlinearity on the responses of the dam body when an underwater explosion occurred in the reservoir medium. The explosive sources are located at different distances from the dam and the effects of the cavitation and the initial shock wave of the explosion are considered. Different amount of TNT are considered. Two different linear and nonlinear behavior are assumed in the analysis and the dam body is assumed with and without contraction joints. Radial, tangential and vertical displacements of the dam crest are obtained. Moreover, maximum and minimum principal stress distributions are plotted. Based on the results, the dam body responses are sensitive to the insertion of joints and constitutive model considered for the dam body.

Shock Wave and Detonation Properties of Pressed Hydrazine Nitrate

AbstractIn this work, the structure of the steady‐state detonation waves, the critical diameter, detonation parameters and the dependence of detonation velocity on the charge diameter for pressed hydrazine nitrate (HN) charges in the density range of 1.40–1.68 g/cm3 were investigated by VISAR interferometer. It was found that the critical diameter drops with the initial density decrease. Under steady‐state detonation conditions at a maximum initial density of 1.68 g/cm3, a high detonation velocity of 8.92 km/s is realized in HN charges. The change of the detonation velocity with variation of the initial density is not monotonic, but has a characteristic s‐shape. Experiments on the determination of Hugoniot data and initiation of detonation under shock wave action for HN were carried out with samples of maximum density. The results of the conducted experiments show the low shock wave sensitivity, which is much lower than that of TNT. A noticeable reaction rate behind the front of the initiating shock wave in charges of maximum density is observed only at a pressure above 20 GPa.

Developing a method to assess noise reduction of firearm suppressors for small-caliber weapons

Firearm suppressors have the potential to reduce the muzzle blast by diffusing the initial shock wave of a gunshot. Currently, the American National Standards Institute does not have any standards that specifically address suppressor measurements. A recent NATO standard, AEP 4875 has been proposed to characterize suppressor performance, but the scope of this standard does not include suppressor effects at the shooter’s ear. Additionally, the standard requires firing the weapon from an elevated platform 4 meters above the ground with microphones positioned with regular spacing of about 18 degrees at 5 meters from the muzzle. This study evaluated fourteen different firearms with and without a suppressor. Different loads of ammunition were used to vary the speed of the projectile. For ten of the guns, both supersonic and subsonic conditions were measured. Twelve microphones were positioned at 30-degree spacing in 3-meter ring at 1.5 meters above the ground. One microphone was positioned at 1 meter to the left of the muzzle and two microphones were positioned at 15 centimeters from the right and left ears. The suppressors were effective in reducing the peak sound pressure levels between 3 and 28 dB and 8-hour equivalent energy (LAeq8) between 2 and 24 dB.

The combustion of Al–CuO powder mixture under shock wave initiation of the reaction

The results of experiments on monitoring of manifestations of chemical transformation of Al–CuO powder mixture as a result of shock loading are given. Speeds of shift and expansion of chemical transformation area in free space are determined. The data about structure of combustion area of are received. The temperature of combustion area is measured. The duration of chemical transformation is determined.

Experimental research of shock wave processes influence on machineless gas flow energy separation effect

Experimental results for artificially initiated shock wave influence on machineless gas flow energy separation effect are presented. The working principle of the technique is based on interaction of supersonic and subsonic flows through the heat-conducting wall. In result at output there are two flows with different temperature – heated supersonic air flow and cooled subsonic one. Shock waves were initiated by conic ribs placed along the supersonic channel. During the research varied parameters included uni-flow and counter-flow air moving direction in subsonic and supersonic channels, subsonic flow rate divided by supersonic one (from 0 to 0.9), stagnation flow temperature (298, 313 and 343K) and initial Mach number (1.9, 2.5). The research was carried out with the use of infrared thermal imaging, thermocouples, total and static pressure probes, National Instruments automation equipment. Energy separation effect is increasing with the growth of Mach number and stagnation flow temperature. Rib placement in supersonic channel causes rise of static pressure and wall temperature and results in decreasing of energy separation effect at output of the device by less than 12%. Operability of the device with shock wave generation is remained.

Simulation of Underwater Explosions Initiated by High-Pressure Gas Bubbles of Various Initial Shapes

UNDerwater EXplosions (UNDEXs) are widely used in many areas of applied engineering including oil production and warship protection. However, the three-dimensional computations of UNDEXs, especially for explosives with complex initial shapes are still lacking, which is mainly due to the difficulty in capturing the multi-medium interface with high pressure ratio. In this study, we conducted a series of three-dimensional numerical simulations of UNDEXs with different initial shapes of a high-pressure gas bubble surrounded with water, to investigate the dynamics of the explosion caused by the shape change of the gas bubble. The movement of the interface was traced with the level-set method, and the conditions at the gas–water interface were treated using the Real Ghost Fluid Method (RGFM). As a result, the temporal evolution of the pressure field during the explosion and the pressure exerted at the boundaries of the computational domain in each simulation scenario were obtained. It was found that an initial shock wave is generated by the explosion and transmitted in the water, leading to an increase of the pressure and density. Meanwhile, inside the gas bubble, a rarefaction wave is formed, causing a pressure drop of the explosive gas. The results also show that if the initial shape of the bubble filled with the explosive gas is simple (e.g., spherical, cylindrical, cuboidal), the peak pressure of the shock wave is dominated by the cross-sectional area of the initial bubble along each direction. In addition, the duration of the high pressure phase of the shock wave is dictated by the thickness of the bubble. Moreover, the simulation of a bubble with an initially bullet-like shape revealed that this specific shape enables a concentration of the energy in a well-defined direction. The peak of the pressure generated by the gas bubble of this more complex shape is approximately twice than that of the other scenarios. However, the high pressure was found to drop more rapidly than that of the other cases, which might be attributed to the comparably small thickness of the initial bubble.

Dynamical behavior of the Richtmyer–Meshkov instability-induced turbulent mixing under multiple shock interactions

The dynamical behavior of Richtmyer–Meshkov instability-induced turbulent mixing under multiple shock interactions is investigated by large-eddy simulation. After the initial shockwave–interface interaction, the transmitted wave reverberates between the accelerated interface and the end-wall of the shock tube to form a process of multiple shock interactions. The turbulent mixing zone grows in a different manner under each of the impingements. After the initial shock, it grows as a power law of time. After the reshock and the impingement of the reflected rarefaction wave, it grows with time as a different negative exponential law. When the impingement of the reflected compression wave completes, it grows approximately in a linear fashion. The statistical quantities in the turbulent mixing zone evolve with time in a similar way under multiple impingements, and after the impingement of the reflected compression wave, they all decay asymptotically. Therefore, the turbulent mixing zone behaves in a statistically self-similar pattern. Even though the impingements of different waves result in different abrupt changes of the characteristic scale parameters of mixing turbulence, as a whole, the characteristic scales present a feature of growth, and the characteristic-scale Reynolds numbers present a feature of decay. The mixing flow is continuously anisotropic, yet the anisotropy weakens gradually. Therefore the development of turbulent mixing presents a trend of isotropy.

Nature of high reactivity of metal/solid oxidizer nanocomposites prepared by mechanoactivation: a review

In this review, the regularities of formation, structure and high reactivity of two types of energetic metal/solid oxidizer nanocomposites (Al(Mg)/X (X = MoO3, (–C2F4–) n )) prepared by mechanoactivation are examined. One reason for the high reactivity is an increase in contact surface between the components occurring after mechanoactivation. Two methods for determination of area of contact surface S C between the components are used, and the values of S C for all the systems are estimated. Considerable attention is paid to the role of highly reactive defects (grain sizes, dislocations and stacking faults, paramagnetic centers, “weakly bound” oxygen in MoO3, etc.), formed in the components under mechanical stress. For the Me/MeO3 systems, the formation of point defects in the oxide is an important factor. It was found that, after mechanoactivation, the evolution of O2 from MoO3 occurs at 230–450 °C. It is argued that this process is associated with the thermal destruction of “weak” Mo–O bonds in the “bridge” oxygen. It was suggested that the formation of defect structure in MoO3 and increasing of the oxygen mobility under heating give rise to a low-temperature peak in DSC curves and initiated self-ignition on the fuel–air mixture. For composites Mg/MoO3, self-ignition occurs at temperature 100 °C lower than that for Al/MoO3: The decreasing of temperature can be connected with larger S C in the first system. In the Mg/(–C2F4–) n system, the reactions of magnesium defects with (–C2F4–) n are accompanied by a weak heat evolution, too low to initiate ignition. In this case, the reaction is initiated by the thermal depolymerization of (–C2F4–) n , while a high values of S C provide a complete conversion. In the case of shock-wave initiation, defects in the components play only a minor role in the conversion, whereas the value of S C remains to be highly important.

Modeling and Simulation of Back Blast Flow Field of a Recoilless Gun Launching in Finite Space

A recoilless gun is a kind of weapon equipment for individual soldier and its back blast flow has certain influence on the shooter during the launch process. In order to study the launch safety of a recoilless gun firing in a finite space of 2.5×2.5×2m, numerical simulation of the back blast flow field was carried out based on the three-dimensional unsteady viscous compressible N-S equations. The result shows that a complex reflected shock wave is formed due to the reflection of the wall behind the nozzle. The reflected shock wave represents reciprocating motion in the finite space and its intensity is much higher than that of the initial shock wave. Critical damage will be imposed on the shooter if no safeguard procedure is adopted. The high temperature gas flow may ignite the inflammable gases in the finite space and cause secondary damage. The research results are significant for understanding the characteristics of the back blast flow of the recoilless gun launching in the finite space, predicting and taking effective action to reduce the damage on the shooter.

Effects of Al/O on pressure properties of confined explosion from aluminized explosives

Abstract Pressure histories were tested in a 500-L chamber to identify the pressure load in confined explosion from aluminized explosives. Different aluminized explosives with Al/O, ranging from 0.25 to 1.23, were used. The recorded pressure curves could express the reflection of initial shock wave and the after burning combustion of aluminum. As there is no objective way to gain quasi-static pressure (PQS), method of multipoint averaging was used in smoothing the original pressure curves to gain the PQS. The PQS, rising time of pressure (tQS) which stands for the duration of the initial reflected shock wave, and attenuation coefficient (ω) which stands for the supportive effects of the combustion of aluminum to the PQS are used to characterize the pressure load in the confined explosion from aluminized explosives. The research results showed that the Al/O significantly affected the three characteristic quantities. With the increase of Al/O, the PQS increased at first and decreased later, gaining maximum at Al/O = 0.99; the tQS sustained growth and the ω decreased at first and increased later, gaining minimum at Al/O = 0.99.

Development of explosive transformation of NCT energy-intensive metal complex during bridge initiation

This paper describes the results of the experimental study on the effect of structural parameters, such as diameter, height, and density, on the development of explosive transformation of an NCP energy-intensive metal complex during bridge initiation. The results are compared with those obtained in shock-wave initiation.

Effect of interfacial heat transfer on the critical conditions of shock-wave initiation of chemical reaction in porous energetic materials

This paper presents the results of numerical analysis of a viscoplastic model for hotspot formation based on the solid-state mechanism of hotspot ignition of an energetic porous material under shock-wave loading. The highly viscous pore collapse regime is considered, which is of great interest for theoretical studies of the shock-wave initiation of heterogeneous energetic materials. Interfacial heat transfer was described under the assumption that the gas is ideal and hence homobaric (uniform in pressure). Parametric analysis was conducted, and characteristic features of the effect of heat transfer and interfacial heat transfer on the critical conditions of shock-wave initiation of chemical reaction in the energetic porous material were determined.