On the effect of grain size on shock sensitivity of heterogeneous high explosives

Analysis of available data on dependence of the critical detonation diameter of various heterogeneous condensed explosives on mean size of grains and voids demonstrated that in many cases surprising correlations between and the initial specific surface area of heterogeneous explosives exist, namely, or . The run distance to detonation in wedge test with sustained strong shock of constant amplitude also linearly correlates with , i.e. . At the same time, the shock sensitivity reversal effect is often observed when grain size of HE is reduced. Apart from that Moulard (1989) found that detonation critical diameter of plastic bonded explosive with mono- and bimodal RDX grain size distribution depends nonmonotonously on mean grain size. Complicated dependence of shock sensitivity of heterogeneous explosives on their specific surface area can be explained based on comparison of the critical hot spot size at given characteristic pressure behind shock wave with the mean heterogeneity size . At high characteristic pressure (relative to the critical ignition pressure) is small compared with and all specific surface area of heterogeneous explosive is available for the hot spot growth process in accordance with the grain burn concept. However, when characteristic pressure of shock wave decreases, increases and can become comparable with . In this case only relatively large potential hot spots with size can result in self-supported hot spot growth process and shock sensitivity is controlled by the specific surface area which corresponds to only larger heterogeneities and can be significantly smaller than initial specific surface area. Received 18 July 1996 / Accepted 6 November 1996     

增强型喷射器对爆轰波DDT过程的影响

在激波、火焰及射流同时存在的流场中,组织燃烧转爆轰过程是脉冲爆震发动机实现点火、起爆的关键问题。设计一类喷射器,采用C_2H_2/O_2/Ar反应,数值验证了该喷射器能增强爆震室燃料燃烧转爆轰的可行性,并讨论了流场中热点的点火机制。结果显示:该装置在流场中可激发不稳定性,产生漩涡,加速能量、质量的交换。流场产生热点,促进火焰速度加快,追赶前导激波。喷射器位置影响前导激波的运动速度。在一定范围内,前导激波速度越大,碰撞产生的热点越容易激发燃烧转爆轰过程。     

On the propagation mechanism of a detonation wave in a round tube with orifice plates

This study deals with the investigation of the detonation propagation mechanism in a circular tube with orifice plates. Experiments were performed with hydrogen air in a 10-cm-inner-diameter tube with the second half of the tube filled with equally spaced orifice plates. A self-sustained Chapman–Jouguet (CJ) detonation wave was initiated in the smooth first half of the tube and transmitted into the orifice-plate-laden second half of the tube. The details of the propagation were obtained using the soot-foil technique. Two types of foils were used between obstacles, a wall-foil placed on the tube wall, and a flat-foil (sooted on both sides) placed horizontally across the diameter of the tube. When placed after the first orifice plate, the flat foil shows symmetric detonation wave diffraction and failure, while the wall foil shows re-initiation via multiple local hot spots created when the decoupled shock wave interacts with the tube wall. At the end of the tube, where the detonation propagated at an average velocity much lower than the theoretical CJ value, the detonation propagation is much more asymmetric with only a few hot spots on the tube wall leading to local detonation initiation. Consecutive foils also show that the detonation structure changes after each obstacle interaction. For a mixture near the detonation propagation limit, detonation re-initiation occurs at a single wall hot spot producing a patch of small detonation cells. The local overdriven detonation wave is short lived, but is sufficient to keep the global explosion front propagating. Results associated with the effect of orifice plate blockage and spacing on the detonation propagation mechanism are also presented.     

Effect of chemical reactivity on the detonation initiation in shock accelerated flow in a confined space

The interactions of a spherical flame with an in- cident shock wave and its reflected shock wave in a confined space were investigated using the three-dimensional reactive Navier-Stokes equations, with emphasis placed on the effect of chemical reactivity of mixture on the flame distortion and detonation initiation after the passage of the reflected shock wave. It is shown that the spatio-temporal characteristics of detonation initiation depend highly on the chemical reactivity of the mixture. When the chemical reactivity enhances, the flame can be severely distorted to form a reactive shock bifurcation structure with detonations initiating at different three-dimensional spatial locations. Moreover, the detonation initiation would occur earlier in a mixture of more enhanced reactivity. The results reveal that the detonations arise from hot spots in the unburned region which are initiated by the shock-detonation-transition mechanism.     

铸装TNT炸药表面热点温度的测量

非均匀炸药冲击起爆的热点理论一直是爆轰物理中,人们普遍关心的课题之一。本文对热点温度实验测量工作进行了尝试,叙述了用多通道光学高温计测量非均匀炸药表面热点温度的原理及方法。用实验方法得到在3.44GPa左右的冲击压力下,铸装TNT炸药表面热点温度约为3640K。     

坑道内爆炸条件下水的消波效应研究

内爆炸对坑道内的人员、装备、结构都具有巨大的毁伤效应,内爆炸的防护技术已成为研究的热点。本文就炸药在坑道内爆炸情况下水的消波效应,开展了坑道模型爆炸试验、实际坑道爆炸试验、数值模拟研究,结果表明:坑道内爆炸条件下水具有显著的消波效应。文中还给出了不同置水工况下坑道空气冲击波超压的衰减率范围,并对水消波效应的机理进行了初步探讨。     

Ignition and growth of a detonation by a high energy plasma

Many experimental and numerical studies have been achieved to describe the transition process of deflagration to detonation when a projectile impacts an explosive. Also a large work has been done for the determination of various parameters — such as the impact pressure, the efficiency factors, etc. of the laser — material interaction. When a laser beam impacts an explosive, the P² criterion, characteristic of shock initiated detonations, is no longer valid due to the generated hot plasma whose effect is to decrease the DDT (Deflagration to Detonation Transition) duration. The present paper deals with a modelling of the plasma-explosive medium allowing the determination of distances and times of the DDT process. The two phase modelling of the granular explosive takes into account the creation of hot spots. The pressure of the plasma is computed using a semi empirical model, while the temperature is obtained from Maxwell Boltzmann statistics. The authors focused their attention on the equation of state for the detonation products and the numerical process.     

PBX-9404的化学反应速率方程及起爆特性

通过对炸药化学反应速率方程的分析,建立了基于Kim弹粘塑性球壳塌缩热点模型原理的三项式化学反应速率方程模型。运用遗传算法确定了反应速率方程相关参数,通过与Forest-Fire反应速率模型数值模拟结果的对比对所建模型的合理性进行了验证。将所建反应速率方程模型嵌入有限元程序对PBX-9404炸药起爆过程进行数值模拟,根据模拟结果分析了炸药遭受冲击加载后炸药孔隙率、颗粒尺寸等变化对炸药冲击起爆过程的影响。     

圆球诱发斜爆轰波的数值研究

斜爆轰发动机是飞行器在高马赫数飞行条件下的一种新型发动机,具有结构简单、成本低和比冲高等优点.但是斜爆轰发动机的来流马赫数范围广,来流条件复杂,为实现斜爆轰波的迅速、可靠引发,采用钝头体来诱发.利用Euler方程和氢氧基元反应模型,对超声速氢气/空气混合气体中圆球诱导的斜爆轰流场进行了数值研究.不同于楔面诱发的斜爆轰波,球体首先会在驻点附近诱发正激波/爆轰波,然后在稀疏波作用下发展为斜激波/爆轰波.模拟结果显示,经过钝头体压缩的预混气体达到自燃温度后,会出现两种流场:当马赫数较低时,由于稀疏波的影响,燃烧熄灭,钝头体下游不会出现燃烧情况;而当马赫数较高时,燃烧阵面能传到下游.分析表明,当钝头体的尺度较小时,驻点附近的能量不足以诱发爆轰波,只会形成明显的燃烧带与激波非耦合结构;当钝头体的尺度较大时,流场中不会出现燃烧带与激波的非耦合现象,且这一特征与马赫数无关.通过调整球体直径,获得了激波和燃烧带部分耦合的燃烧流场结构,这一流场结构在楔面诱发的斜爆轰波中并不存在,说明稀疏波与爆轰波面的相互作用是决定圆球诱发斜爆轰波的关键.     

Re-initiation phenomenon of gaseous detonation induced by shock reflection

The two-dimensional, time-dependent, reactive Navier–Stokes equations including the effects of viscosity, thermal conduction and molecular diffusion were solved to reveal the wave evolution and chemical dynamics involved in the re-initiation process. The computation was performed for hydrogen–oxygen–argon mixtures at the low initial pressure (8.00 kPa), using detailed chemical reaction model. The results show that, the decoupled leading shock reflects on the right wall of the vertical branch. High temperature and pressure behind the reflected shock induce the generation of hot spots and local explosion. Therefore, the re-initiation of gaseous detonation occurs. In the re-initiation area, there exist very high OH concentration and no H ₂ concentration. However, in front of reflected shock, there exist relatively high H ₂ concentration and no OH radicals. Additionally, the shock–flame interaction induces RM instability. This results in the fast mixing between hot reacted gas mixture and the relatively cold unreacted gas mixture and accelerates the chemical reactions. However, the shock–flame interaction contributes much less to the re-initiation, in contrast with shock reflection. The transition of leading shock from regular reflection to Mach reflection happens during the re-initiation. The computed evolution of wave structures involved in the re-initiation is qualitatively agreeable with that from the experimental schlieren images.      

Numerical simulation of detonation reignition in H $_2$-O $_2$ mixtures in area expansions

Time-dependent, two-dimensional, numerical simulations of a transmitted detonation show reignition occuring by one of two mechanisms. The first mechanism involves the collision of triple points as they expand along a decaying shock front. In the second mechanism ignition results from the coalescence of a number of small, relatively high pressure regions left over from the decay of weakened transverse waves. The simulations were performed using an improved chemical kinetic model for stoichiometric H-O mixtures. The initial conditions were a propagating, two-dimensional detonation resolved enough to show transverse wave structure. The calculations provide clarification of the reignition mechanism seen in previous H-O-Ar simulations, and again demonstrate that the transverse wave structure of the detonation front is critical to the reignition process. Received 16 July 1998/ Accepted 17 November 1998     

A chemical shock tube driven by detonation

A chemical shock tube driven by a detonation driver is described in the present paper. This shock tube can produce a single controlled high-temperature pulse for studies of gas-phase reaction kinetics, but the difficulty associated with the timing for the rupture of diaphragms in the conventional chemical shock tube is overcome, because the detonation wave in the driver section can be predicted correctly and shows a good repeatability. In addition, this shock tube is capable of providing higher temperature conditions for the test gas than the conventional high-pressure shock tube, owing to the inherently high-driving capability of the detonation driver. The feasibility of this shock tube is examined by numerical simulations and preliminary experiments.     

含铝炸药能量释放的简化模型

为了在水下爆炸效应中反映出非理想爆轰特性的影响,建立了含铝炸药非理想爆轰能量释放的简化模型。该模型以CJ爆轰理论和二次反应理论为基础,把含铝炸药化学反应划分为快速反应和慢速反应,以释放的化学能和慢反应速率常数作为非理想特征参数,并应用于一维数值模拟。计算结果与基本实验结果一致,冲击波峰值的计算误差不大于10％,衰减时间常数的误差小于5％,冲击波能与实验值也具有良好的一致性。简化模型合理地描述了含铝炸药非理想爆轰的主要过程及非理想特性,可应用于含铝炸药的设计和爆炸效应的分析。     

Ablation combustion of secondary powder explosives

The explosive transformation in a passive charge initiated by an air shock wave was studied using synchrotron diagnostics of density. It was found that in the passive charge, a compression wave capable of forming effective hot spots does not arise and classical initiation conditions do not occur. The development of the reaction can be attributed to the ablation combustion observed in studies of the initiation of secondary powder explosives by a spark and gaseous detonation.     

Adaptive unstructured finite element method for two-dimensional detonation simulations

A non-equilibrium reacting flow methodology has been added to a conservative, monotonic, compressible flow solver to allow numerical simulations of gas detonations. This flow solver incorporates unstructured dynamically adaptive meshes with the Finite Element Method – Flux Corrected Transport (FEM-FCT) scheme, which has shown excellent predictive capability of various non-reacting compressible flows. A two-step induction parameter model was used to model the combustion of the gas phase coupled with an energy release equation which was simulated with a point implicit finite element scheme. This combustion model was then applied to a two-dimensional detonation test case of a hypothetical fuel:oxygen mixture. The detonation simulation yielded two transverse waves which continued to propagate. This feature and the detonation shock speed mean and fluctuations were found to be grid-independent based on a resolution of about twenty elements within the average induction length. The resolution was efficiently achieved with the unstructured dynamically adaptive finite elements, which were three orders of magnitude less in number then required for uniform discretization. Received 26 August 1996 / Accepted 31 March 1997     

A numerical study of the mechanisms of self-reignition in low-overdrive detonations

Below a threshold in overdrive, both stability analysis and numerical simulations predict that one-dimensional detonations in high activation energy mixtures behave as a chaotic sequence of failures followed by reignition. Instead, less chaotic, cellular detonations almost invariably occur in experiments. Numerical simulation, based on the Euler equations with single step chemistry, shows that a ZND detonation initially fails in that regime. The detonation splits into a weaker shock, a surface discontinuity separating reacted from unreacted fluid, and a rarefaction wave. However, the detonation is eventually reignited by the explosion of a small gas pocket, in a process reminiscent of deflagration to detonation transition. In the fluid heated by the leading shock, the chemical reaction occurs slowly at first, but becomes faster as heat is released, until the pocket explodes. Small differences in initial temperature result in large enough differences in reaction time sufficient for one pocket of fluid to explode. In two dimensions, the explosion occurs earlier because an oblique shock structure develops which unevenly heats the fluid that passes through the leading shock. Hence, pockets that underwent more heating will explode sooner. As it moves upstream, the two-dimensional explosion, meets the leading shock and the detonation quickly develops a transverse wave structure. An abridged version of this paper was presented at the 15th Int. Colloquium on the Dynamics of Explosions and Reactive Systems at Boulder, Colorado, from July 30 to August 4, 1995.     

CL-20基混合炸药的冲击起爆特征

为了研究CL-20基混合炸药的冲击起爆特征,深入分析冲击波作用下CL-20基混合炸药的爆轰成长规律,采用炸药驱动飞片冲击起爆实验方法,对CL-20、CL-20/NTO和CL-20/FOX-7三种压装混合炸药进行了冲击起爆实验,通过嵌入在炸药内部不同位置处的锰铜压力传感器,获得了炸药内部压力的变化历程。依据实验结果标定了混合炸药的点火增长模型参数,其中,利用反应速率方程中的两个增长项,分别模拟CL-20/NTO和CL-20/FOX-7混合炸药中两种组分的反应增长过程,得到这两种混合炸药的反应速率方程参数。并通过数值模拟的方法得到了三种炸药的临界起爆阈值和POP关系。研究结果表明：三种CL-20基混合炸药中,CL-20/NTO混合炸药具有更高的临界起爆阈值,而CL-20/FOX-7混合炸药具有更长的爆轰成长距离;此外,利用此套拟合双组分混合炸药反应速率方程的方法,可以对新型配方炸药的冲击起爆过程进行预测性计算

     

On the role of turbulence in detonation induced by Mach stem reflection

A series of experiments conducted by Chan has shown that while some shock waves may not be strong enough to induce detonation when they collide with an obstacle the resulting Mach stem will induce detonation if it collides with a subsequent obstruction. A series of numerical simulations, however, failed to demonstrate the expected results if either the Euler or laminar Navier-Stokes equations are solved. On the other hand, calculations using the Favre averaged Navier-Stokes equations with a k--F turbulence model are able to reproduce the experimental results, indicating that turbulent effects may play an important role in the ignition process. A detailed examination of the results shows that turbulence causes the formation of activated kernels in a similar process to that observed in deflagration-detonation transition. The simulations in this paper have been undertaken using a modern high resolution hydrocode and a reduced kinetics mechanism for hydrogen combustion. The paper describes the reduced mechanism, the solution methods employed in the hydrocode and discusses the results of the simulations and their implications. Received 28 October 1997 / Accepted 30 April 1998     

Turbulent jet initiation of detonation in hydrogen-air mixtures

Large scale experiments (50 m³) have been carried out on the initiation of detonation by means of a jet of hot combustion products. The effects of hydrogen concentration (18–30% vol.), jet orifice diameter (100–400 mm), and the mixture composition in constant volume explosion chamber (25–50%) were investigated. Both high enough hydrogen concentration and large enough jet size are necessary for detonation initiation. The minimum values are within the ranges of 20 to 25% vol. H₂, and of 100 to 200 mm correspondingly. A minimum ratio of jet size and mixture cell width 12–13 is required for detonation initiation.