基于三维离散元方法研究热点的位置和温度对奥克托今颗粒点火燃烧的影响

炸药的点火燃烧特性与炸药的安全和使用密切相关,其一直是研究者重点关注的问题之一.普遍认为,凝聚炸药起爆的关键是热点.基于此,本文采用三维离散元方法,以奥克托今(HMX)颗粒为研究对象,探究了HMX颗粒中不同位置和初始温度的热点对其燃烧过程的影响.结果发现,热点的不同初始温度和位置都会对HMX炸药颗粒的燃烧特性造成影响.由于处于炸药内部分散位置的热点表面积较大,其相比集中的热点更有利于HMX炸药颗粒的起爆.并且研究发现,并不是热点的温度越高越有利于炸药的起爆,这也取决于炸药颗粒的含量.本文的研究工作对炸药的实验研究以及军事应用提供了一定的参考.     

Effects of high activation energies on acoustic timescale detonation initiation

Acoustic timescale Deflagration-to-Detonation Transition (DDT) has been shown to occur through the generation of compression waves emitted by a hot spot or reaction centre where the pressure and temperature increase with little diminution of density. In order to compensate for the multi-scale nature of the physico-chemical processes, previous numerical simulations in this area have been limited to relatively small activation energies. In this work, a computational study investigates the effect of increased activation energy on the time required to form a detonation wave and the change in behaviour of each hot spot as the activation energy is increased. The simulations use a localised spatially distributed thermal power deposition of limited duration into a finite volume of reactive gas to facilitate DDT. The Adaptive Wavelet-Collocation Method is used to solve efficiently the 1-D reactive Euler equations with one-step Arrhenius kinetics. The DDT process as described in previous work is characterised by the formation of hot spots during an initial transient period, explosion of the hot spots and creation of an accelerating reaction front that reaches the lead shock and forms an overdriven detonation wave. Current results indicate that as the activation energy is raised the chemical heat release becomes more temporally distributed. Hot spots that produce an accelerating reaction front with low activation energies change behaviour with increased activation energy so that no accelerating reaction front is created. An acoustic timescale ratio is defined that characterises the change in behaviour of each hot spot.     

Calculations of the rate of growth of hot spots during detonation taking into account the turbulent mechanism of energy transfer

Hot spots important for the theory of detonation and the rate of their growth in a compressed cold explosive determining the induction period of detonation were studied. The numerical gas dynamics procedure based on the EGAK code was used to calculate the evolution of a hot spot taking into account turbulence and heat conductivity. The rate of growth obtained in the calculations, ～200 m/s, lends support to the hypothesis of a turbulent mechanism of energy transfer when hot spots grow during detonation initiation.     

Nonequilibrium Zeldovich-von Neumann-Doring theory and reactive flow modeling of detonation

This paper discusses the Nonequilibrium Zeldovich-von Neumann-Doring (NEZND) theory of self-sustaining detonation waves and the Ignition and Growth reactive flow model of shock initiation and detonation wave propagation in solid explosives. The NEZND theory identified the nonequilibrium excitation processes that precede and follow the exothermic decomposition of a large high explosive molecule into several small reaction product molecules. The thermal energy deposited by the leading shock wave must be distributed to the vibrational modes of the explosive molecule before chemical reactions can occur. The induction time for the onset of the initial endothermic reactions can be calculated using high pressure-high temperature transition state theory. Since the chemical energy is released well behind the leading shock front of a detonation wave, a physical mechanism is required for this chemical energy to reinforce the leading shock front and maintain its overall constant velocity. This mechanism is the amplification of pressure wavelets in the reaction zone by the process of de-excitation of the initially highly vibrationally excited reaction product molecules. This process leads to the development of the three-dimensional structure of detonation waves observed for all explosives. For practical predictions of shock initiation and detonation in hydrodynamic codes, phenomenological reactive flow models have been developed. The Ignition and Growth reactive flow model of shock initiation and detonation in solid explosives has been very successful in describing the overall flow measured by embedded gauges and laser interferometry. This reactive flow model uses pressure and compression dependent reaction rates, because time-resolved experimental temperature data is not yet available. Since all chemical reaction rates are ultimately controlled by temperature, the next generation of reactive flow models will use temperature dependent reaction rates. Progress on a statistical hot spot ignition and growth reactive flow model with multistep Arrhenius chemical reaction pathways is discussed. The text was submitted by the authors in English.     

Explosive compositions based on the mechanoactivated metal-oxidizer mixtures

Detonation-like regimes in mechanoactivated energetic composites (MAECs) were experimentally studied. The test MAECs consisted of layers of a metal (Al, Mg) and Teflon mixed at the submicron and nano levels. The systems reacted to form solid final products. MAECs are appreciably superior to ordinary mixtures in chemical transformation rate. The burning of MAECs occurs in an explosive regime at a velocity of 300–400 m/s, with the temperature of the products being as high as 4000 K. When initiated with a HE charge, porous MAECs detonate in the steady regime. Depending on the composition and density of the samples, the detonation velocity varies from 700 to 1300 m/s, values markedly higher than the speed of sound in the initial mixture. Detonation is controlled by a hot spot mechanism, more specifically, by relay reaction propagation by jets of products.     

氢氧爆轰波在激波管中的成长机制研究

 针对气相爆轰波成长机制研究,采用压力传感器和高速摄影技术,测试了氢氧混合气体在点火后的火焰波、前驱冲击波以及爆轰波的成长变化过程,计算了冲击波过程参数和气体状态参数,分析了火焰加速机制。实验结果表明,APX-RS型高速摄影系统可用于拍摄气相爆轰波的成长历程;氢氧爆轰波的产生是由于湍流火焰和冲击波的相互正反馈作用,导致反应区内多处发生局部爆炸,爆炸波与冲击波相互耦合,最终成长为定常爆轰波。     

Low-velocity detonation modes of grained pyroxylin powder

It is well known that low-velocity detonation excited by the explosion of a thin layer of a plastic explosive within charges of grained pyroxylin powder is propagated with a velocity that is practically constant along the charge. However, it varies depending on the power of the initiating pulse. The present paper is devoted to the elucidation of the mechanism of this unusual feature of detonation process. Experiments were carried out on charges of VTM grade grained single-channel powder with different initial density and were added by numerical modeling. It is shown that the property studied is the consequence of the relatively low intensity of the chemical transformation and the limited charge length (120 mm in the experiment and calculations). The reaction zone of the detonation wave has no time to form completely under these conditions, and the development process is interrupted at a stage when the wave characteristics change actively. The wave evolution was distinctly revealed on pressure profiles; however, the front trajectory, if excluding the initiation area, has an almost linear form. The wave velocity, close to constant, corresponds to this. To form a stationary wave with characteristics that are not dependent on the initiation conditions in a range corresponding to lowvelocity detonation mode, charges with much greater length are necessary. As regards the mechanism of the excitation of chemical transformation in the wave front, as numerical modeling showed, high-porosity charges operate by the gas-phase mechanism (the compression and heating in the high-speed gas flow in pores). In the case of compacted charges with a porosity of 0.2 and lower, heating and ignition of the powder occur by the solid-phase mechanism (because of dissipation at plastic deformations of the porous layer). Details of both mechanisms are considered.     

180°圆弧形钝感炸药的爆轰波传播

 采用贴体坐标下与Level Set方法相结合的爆轰冲击波动力学（DSD）计算方法,研究了180°圆弧形钝感炸药中非理想爆轰波的传播过程。通过数值模拟计算和实验测量的对比分析,得到了180°圆弧形炸药中爆轰波传播的一些规律：圆弧形钝感炸药可以实现定常爆轰,即在极坐标中整个爆轰波以固定角速度转动。这种定常阵面的形状和角速度与圆弧的外半径无关,定常体系依赖于圆弧形炸药的内半径和覆盖圆弧的外壳物质。对描述圆弧形炸药中爆轰波传播规律的经验公式进行了研究,结果表明这些经验公式能够准确描述爆轰波速度的变化,在实验测量和预估方面具有一定的参考价值。     

Detonation development in PRF/air mixtures under engine-relevant conditions

The development of advanced boosted internal combustion engines (ICEs) is constrained by super-knock which is closely associated with end gas autoignition and detonation development. The present study numerically investigates the transient autoignition and detonation development processes under engine-relevant conditions for primary reference fuel (PRF) consisting of n-heptane and isooctane. The effects of PRF composition are systematically examined. By considering the transient local sound speed rather than its initial value, a new non-dimensional parameter is proposed to assess the transient chemical-acoustic interaction and to quantify the autoignition modes. Two detonation sub-modes, normal and over-driven detonation, are identified and the corresponding mechanisms are interpreted. For the over-driven detonation, there exist two developing regimes with weak/strong chemical-acoustic coupling and slow/rapid pressure enhancement. It is found that the maximum pressure caused by autoignition decreases with the blending ratio of isooctane, mainly due to the increase in excitation time. Besides, the strongest detonation induced by hot spot usually occurs within the over-driven detonation sub-regime. Its condition can be well quantified by the new non-dimensional parameter proposed in work and its strength is determined by the ratio of hot spot acoustic time to excitation time. The deviation of transient autoignition front propagation from prediction based on homogenous ignition is mainly attributed to the non-uniform compression effect caused by gradually enhanced pressure wave, while the influence of heat conduction and mass diffusion is negligible. The initial expansion stage dominating the induction period of local autoignition is greatly influenced by the compression of pressure wave. Therefore, the continuously enhanced pressure wave non-uniformly changes the local ignition delay (i.e. reduces its spatial gradient) within the hot spot and thereby accelerates the autoignition front propagation. The relationship among the parameters quantifying the detonation propensity is assessed and interpreted. The present study provides helpful understanding of detonation development under engine conditions.     

Density-based kinetics for mesoscale simulations of detonation initiation in energetic materials

In this work we present one- and two-dimensional mesoscale simulations of detonation initiation in energetic materials. We solve the reactive Euler equations, with the energy equation augmented by a power deposition term. The reaction rate at the mesoscale is modelled using a density-based kinetics scheme, adapted from standard ‘Ignition and Growth’ models. The deposition term is based on previous results of simulations of void collapse at the microscale, modelled at the mesoscale as hot spots. For an isolated hot spot in a homogeneous medium, it is found that a critical size of the hot spots exists. If the hot spots exceed the critical size, initiation of detonation can be achieved. For sub-critical hot-spot sizes, we show that it takes a collection of hot spots to achieve detonation. We also carry out two-dimensional mesoscale simulations of random packs of HMX crystals in a binder, and show that the transition between no detonation and detonation depends on the number density of the hot spots, the initial radius of the hot spot, the post-shock pressure of an imposed shock, and the amplitude of the power deposition term.     

冲击引起石墨→金刚石相转变机理的探讨

 通过对冲击引起石墨转变为金刚石两种相转变模型的比较和冲击波在疏松介质中传播特性的分析,研究了在冲击波阵面内形成热斑的机制和引起原子发生异常迁移的机制,进而探讨了在冲击波阵面内,在热斑区,石墨→金刚石的相变过程,以及卸载时金刚石的石墨化过程。     

The mechanism of hot spot formation in condensed explosives

The hypothesis of an electrical mechanism of formation of detonation initiation zones (hot spots) in condensed explosives is considered. The hypothesis is based on the generation of electric fields and the appearance of shock-induced conductivity in the propagation of a shock wave (compression wave) in condensed dielectric media, including explosives. These physical phenomena can cause a local electrical breakdown, whose channel is identified with a hot spot. The available experimental data are analyzed from the point of view of the hypothesis suggested, and a procedure for verifying this hypothesis is outlined.     

猛炸药RHT-901和钝感炸药IHE-2的爆轰波直角绕射

 采用高速转镜分幅相机和电探针技术研究了猛炸药RHT-901和钝感炸药IHE-2的爆轰波直角绕射图像和不同位置上的爆轰波传播时间。从研究得出，两种炸药都在拐角顶点附近绕射，爆轰波传播时间增长，爆速变小。但是两种炸药绕射爆轰波的状态不一样，钝感炸药IHE-2中爆轰波绕过直角时，在拐角顶点附近约10 mm范围内炸药未完全反应，猛炸药RHT-901中爆轰波绕过直角时未出现类似现象。两者相比，钝感炸药中绕射爆轰波速度变化大，波阵面曲率半径小，而猛炸药的绕射爆轰波速度变化小，波阵面曲率半径大。这说明炸药的爆轰波绕射与炸药的冲击感度、反应区宽度有关。     

非均匀炸药冲击起爆和起爆后的行为

 介绍并分析了Campbell 等人及其他作者研究非均匀炸药冲击起爆和起爆后行为所获得的实验结果,但不涉及其冲击起爆条件。足够强的冲击波进入非均匀炸药后,爆轰将瞬时（指不经过感应时间）且直接（指不经过其他过程,如爆燃）被引发;非均匀炸药起爆后,其中传播的自始至终是一个不断增长的爆轰波,直至发展为正常爆轰,整个过程都是爆轰的增长（新定义）过程。不存在由反应冲击波不断增长并转变为爆轰波的所谓向爆轰的增长。所谓向爆轰的增长,实际上是爆轰的增长（按新定义）的初期;Craig原定义的爆轰的增长,实际上是爆轰的增长（按新定义）的后期;而所谓反应冲击波,实际上是增长中的初期爆轰波。爆轰的增长（按新定义）是所有猛炸药的特性,炸药反应不充分并逐渐趋于充分是爆轰的增长的化学机制。     

Formation of double front detonations of a condensed-phase explosive with powdered aluminium

The performance characteristics of aluminised high explosive are considered by varying the aluminium (Al) mass fraction in a hybrid non-ideal detonation model. Since the time scales of the characteristic induction and combustion of high explosives and Al particles differ, the process of energy release behind the leading detonation wave front occurs over an extended period of time. Two cardinal observations are reported: a decrease in detonation velocity with an increase in Al mass fraction and a double front detonation (DFD) feature when anaerobic Al reaction occurs behind the front. In order to simulate the performance characteristics due to the varying Al mass fraction, the tetrahexamine tetranitramine (HMX) is considered as a base high explosive when formulating the multiphase conservation laws of mass, momentum, and energy exchanges between particles and HMX product gases. While experimental studies have been reported on the effect of Al mass fraction on both gas-phase and solid-phase detonations, the numerical investigations have been limited to only gas-phase detonation for the varying Al particles in the mixture. In the current study, a two-phase model is utilised for understanding the volumetric effects of Al mass fraction in condensed phase detonations. A series of unconfined and confined rate sticks are considered for characterising the performance of aluminised HMX with a maximum Al mass fraction of 50%. The simulated results are compared with the experimental data for 5–25% mass fractions, and the higher mass fraction behaviours are consistent with the experimental observations.     

Detonation of explosives containing heavy inert particles

The physical and mathematical aspects of the theory of a detonation wave containing heavy inert particles are considered. The detonation wave intensity and structure are determined by the relaxation of velocities of both the reactive explosive and the inert admixture. The generalized Jouguet condition is formulated for the velocity of a self-sustained detonation wave. The results of analytical treatment and the model numerical solutions of the problem of the detonation wave velocity selection and the wave structure determination are presented as a function of the ratio of the characteristic times of the heat evolution and the two-component flow velocity relaxation. A limiting case of the fast particle drag is represented by the shock wave structure determined by relaxation of the two-component flow velocity.     

Experimental observations of detonation in ammonium-nitrate-fuel-oil (ANFO) surrounded by a high-sound-speed, shockless, aluminum confiner

Detonations in explosive mixtures of ammonium-nitrate-fuel-oil (ANFO) confined by aluminum allow for transport of detonation energy ahead of the detonation front due to the aluminum sound-speed exceeding the detonation velocity. The net effect of this energy transport on the detonation is unclear. It could enhance the detonation by precompressing the explosive near the wall. Alternatively, it could decrease the explosive performance by crushing porosity required for initiation by shock compression or destroying confinement ahead of the detonation. At present, these phenomena are not well understood. But with slowly detonating, non-ideal high explosive (NIHE) systems becoming increasing prevalent, proper understanding and prediction of the performance of these metal-confined NIHE systems is desirable. Experiments are discussed that measured the effect of ANFO detonation energy transported upstream of the front by a 76-mm-inner-diameter aluminum confining tube. Detonation velocity, detonation front-shape, and aluminum response are recorded as a function of confiner wall thickness and length. Detonation shape profiles display little curvature near the confining surface, which is attributed to energy transported upstream modifying the flow. Average detonation velocities were seen to increase with increasing confiner thickness, while wavefront curvature decreased due to the stiffer, subsonic confinement. Significant radial sidewall tube motion was observed immediately ahead of the detonation. Axial motion was also detected, which interfered with the front-shape measurements in some cases. It was concluded that the confiner was able to transport energy ahead of the detonation and that this transport has a definite effect on the detonation by modifying its characteristic shape.     

Detonation development in hydrogen/air mixtures inside a closed chamber: role of a cold wall

Detonation development from a hot spot has been extensively studied, where ignition occurs earlier than that in the surrounding mixtures. It has also been reported that a cool spot can induce detonation for large hydrocarbon fuels with Negative Temperature Coefficient (NTC) behavior, since ignition could happen earlier at lower temperatures. In this work we find that even for hydrogen/air mixtures without NTC behaviors, a cold wall can still initiate and promote detonation. End-wall reflection of the pressure wave and wall heat loss introduce an exothermic center outside the boundary layer, and then autoignitive reaction fronts on both sides may evolve into detonation waves. The right branch can be further strengthened by appropriate temperature gradient near the cold wall, and exhibits different dynamics at various initial conditions. The small excitation time and the large diffusivity of hydrogen provide the possibility for detonation development within the limited space between the autoignition kernel and the cold wall. Moreover, detonation may also develop near the flame front, which may or may not co-exist with detonation waves from the cold wall. Correspondingly, wall heat flux evolution exhibits different responses to detailed dynamic structures. Finally, we propose a regime diagram describing different combustion modes including normal flame, autoignition, and detonation from the wall and/or the reaction front. The boundary of normal flame regime qualitatively agrees with the prediction by the Livengood-Wu Integral method, while the detonation development from both the end wall and the reaction front observes Zel'dovich mechanism. Compared to hydrocarbons, hydrogen is resistant to knock onset but it is more prone to superknock development. The latter mode becomes more destructive in the presence of wall heat loss. This study isolates and identifies the role of wall heat loss on a potential mechanism for superknock development in hydrogen-fueled spark-ignition engines.     

JB-9001钝感炸药冲击Hugoniot关系测试

 利用“压力对比法”实验技术，通过锰铜压力计测试待测炸药样品和LY12铝样品在LY12铝飞片的同时撞击下的界面压力p_exp和p_Al，从冲击波关系式和正交回归直线拟合分析，确定了JB-9001钝感炸药的冲击Hugoniot关系。     

爆炸平面冲击波在金属颗粒介质中的衰减

 分析了颗粒介质在冲击载荷下的加载、卸载本构关系,应用特征线理论对平面一维爆炸冲击波在颗粒介质中的衰减进行了计算。结果表明：组成颗粒的材料、孔隙率及炸药的爆速决定了初始冲击波峰值的大小。炸药爆速越高,介质孔隙率越大,材料本身的冲击阻抗越大,初始压力越高。炸药长度、材料本身的冲击阻抗及介质的孔隙率决定了冲击波的衰减速度。炸药长度越小,材料本身的冲击阻抗越大,介质的孔隙率越高,冲击波衰减越快。