Behavior of detonation waves at low pressures

With respect to stability of gaseous detonations, unsteady behavior of galloping detonations and re-initiation process of hydrogen-oxygen mixtures are studied using a detonation tube of 14 m in length and 45 mm i.d. The arrival of the shock wave and the reaction front is detected individually by a double probe combining of a pressure and an ion probe. The experimental results show that there are two different types of the re-initiation mechanism. One is essentially the same as that of deflagration to detonation transition in the sense that a shock wave generated by flame acceleration causes a local explosion. From calculated values of ignition delay behind the shock wave decoupled from the reaction front, the other is found to be closely related with spontaneous ignition. In this case, the fundamental propagation mode shows a spinning detonation. Received 10 March 1997 / Accepted 8 June 1997     

Propagation mechanism of dust detonations

This paper summarizes the studies on dust detonations at the Stosswellenlabor of RWTH Aachen since 1987. The onset and propagation mechanism of heterogeneous dust detonations are similar to those of marginal gas phase detonations. A self-sustained dust detonation has transverse wave structures that provide the coupling between shock and reaction. Large transition distances and transverse wave spacings require large sized tubes for the propagation of self-sustained dust detonations. The Hugoniot analysis of the Chapman-Jouguet detonation predicts equilibrium detonation states being in reasonable agreement with the self-sustained dust detonations observed. Shock matching calculations at the triple point adequately determine the wave structures of those stable dust detonations.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

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.     

DDT and detonation waves in dust-air mixtures

This paper summarizes the studies of DDT and stable detonation waves in dust-air mixtures at the Stosswellenlabor of RWTH Aachen. The DDT process and propagation mechanism for stable heterogeneous dust detonations in air are essentially the same as in the oxygen environment studied previously. The dust DDT process in tubes is composed of a reaction compression stage followed by a reaction shock stage as the pre-detonation process. The transverse waves that couple the shock wave and the chemical energy release are responsible for the propagation of a stable dust-air detonation. However, the transverse wave spacing of dust-air mixtures is much larger. Therefore, DDT and propagation of a stable detonation in most industrial and agricultural, combustible dust-air mixtures require a tube that has a large diameter between 0.1 m and 1 m and a sufficient length-diameter ratio beyond 100, when an appropriately strong initiation energy is used. Two dust detonation tubes, 0.14 m and 0.3 m in diameter, were used for observation of the above-mentioned results in cornstarch, anthraquinone and aluminum dust suspended in air. Smoked-foil technique was also used to measure the cellular structure of dust detonations in the 0.3 m detonation tube. Received 11 February 2000 / Accepted 1 August 2000     

Head-on Collision of a Detonation with a Planar Shock Wave

The phenomenon that occurs when a Chapman–Jouguet (CJ) detonation collides with a shock wave is discussed. Assuming a one-dimensional steady wave configuration analogous to a planar shock–shock frontal interaction, analytical solutions of the Rankine–Hugoniot relationships for the transmitted detonation and the transmitted shock are obtained by matching the pressure and particle velocity at the contact surface. The analytical results indicate that there exist three possible regions of solutions, i.e. the transmitted detonation can have either strong, weak or CJ solution, depending on the incident detonation and shock strengths. On the other hand, if we impose the transmitted detonation to have a CJ solution followed by a rarefaction fan, the boundary conditions are also satisfied at the contact surface. The existence of these multiple solutions is verified by an experimental investigation. It is found that the experimental results agree well with those predicted by the second wave interaction model and that the transmitted detonation is a CJ detonation. Unsteady numerical simulations of the reactive Euler equations with both simple one-step Arrhenius kinetic and chain-branching kinetic models are also carried out to look at the transient phenomena and at the influence of a finite reaction thickness of a detonation wave on the problem of head-on collision with a shock. From all the computational results, a relaxation process consisting of a quasi-steady period and an overshoot for the transmitted detonation subsequent to the head-on collisions can be observed, followed by the asymptotic decay to a CJ detonation as predicted theoretically. For unstable pulsating detonations, it is found that, due to the increase in the thermodynamic state of the reactive mixture caused by the shock, the transmitted pulsating detonation can become more stable with smaller amplitude and period oscillation. These observations are in good agreement with experimental evidence obtained from smoked foils where there is a significant decrease in the detonation cell size after a region of relaxation when the detonation collides head-on with a shock wave.     

铝粉含量和粒度对CL-20含铝炸药水中爆炸反应特性的影响

为了研究CL-20基含铝炸药的爆炸反应机理,利用水中爆炸实验,测量了不同铝粉含量和粒度的CL-20炸药水中爆炸的冲击波参数、二次压力波参数,计算了冲击波能和气泡能。结果表明,水中爆炸的冲击波能和气泡能表征了爆轰和二次反应两个阶段的炸药爆炸能量分配,CL-20炸药中的铝粉主要在二次反应阶段发生反应,只有少部分的铝粉参与了早期的爆轰反应。气泡脉动形成的二次压力波能描述铝粉含量和粒度对二次反应过程的影响,铝粉含量对炸药的二次反应有显著的影响;铝粉粒度对炸药的水下爆炸的初始冲击波参数、冲击波能和气泡能的影响很小,对铝粉与爆轰产物的二次反应速率影响较大。     

Structure of the detonation wave front in a mixture of nitromethane with acetone

It is shown that the leading front of an inhomogeneous detonation wave is a shock wave in which wave structures of the type of triple shock configurations are moving. It was experimentally found that the reaction in these inhomogeneities occurs in oblique shock waves. The reaction sites at the wave front are ring-shaped. In a 75: 25 mixture of nitromethane with acetone, up to 70% of the front surface is occupied by the reaction at the sites in the wave front. Measurements of the mass velocity profile indicate that afterburning takes place in the unloading area behind the Jouguet plane. Calculations of the heat release in the reaction mixture with a decrease in the mass velocity indicate that the material that have not reacted in the inhomogeneities can be ignited in the induction zone. It is suggested that the adiabatic flashes are a mechanism that generates inhomogeneities in the detonation wave front.     

Experiments on spinning detonations with detailed analysis of the shock structure

Spinning detonations are characteristic of detonation limit phenomena in round tubes. In this work we study experimentally the structure of the transverse wave of single-headed spinning detonations. The flow field is experimentally analysed and an original approach enables us to calculate the overall shock structure. The calculations and experimental results indicate that the actual structure of the spinning detonation tries to match closely to the condition where the state parameters (pressure and temperature) reach their maximum values. This condition corresponds to a spinning head where the Mach stem is normal to the incoming flow and could be readily used as boundary condition by further investigators to determine the structure of spinning detonations. Received 1 August 1997/ Accepted 13 July 1998     

气相爆轰在T形管中传播新现象的实验研究

对2H2/O2/Ar系统爆轰波在T形管(截面为40mm×40mm)中传播现象进行了实验研究.用烟迹片记录了T形管中爆轰波的胞格结构,用压电传感器记录了分叉口附近指定点压力时间曲线,得到了爆轰波在分叉口附近的平均速度和胞格图案演变.结果表明:初压P0≥2.67kPa,在水平和垂直支管下游区域(距离分叉口约3.5—6倍方管截面边长),分叉口影响消失,爆轰波恢复稳定,且强度基本保持不变.在分叉口绕射过程中,爆轰波在膨胀区中衰减,诱导激波阵面弯曲.两个支管中发生马赫反射,三波点迹线清晰可见.该传播特性是爆轰波的诱导激波和横波共同作用的结果.分叉口附近的胞格结构先消失再恢复,在无胞格和平衡胞格之间的区域存在细密胞格的过渡区,表征了在诱导激波与化学反应阵面分离后的区域中出现二次点火.P0=2.00kPa,水平支管中稳定自持爆轰能重建,垂直支管中爆轰熄灭.P0<2.00kPa,分叉口上游已不能形成稳定爆轰.还对胞格结构中的几个特征参数进行了测量,并初步分析了P0对这些参数的影响.     

锥形长药柱水下爆炸冲击波参数计算方法

为了计算锥形长药柱水下爆炸冲击波压力,以及研究长脉宽冲击波的传输特性,基于叠加原理建立了冲击波压力-时间曲线的计算方法,通过实验验证了该方法的有效性,在此基础上分析了锥形长药柱不同方位冲击波压力的分布规律。研究结果表明:锥形长药柱产生的冲击波压力具有各向异性,在起爆端一侧形成的是具有厚波头特征的低幅值长脉宽冲击波,在装药径向形成的是接近指数衰减的高幅值冲击波,而在远离起爆端的冲击波压力幅值和脉宽则介于前两者之间。锥形长药柱与球形装药冲击波分布的差异是由于装药形状和起爆方式的改变所导致的,由于装药不同部位起爆的时间差,导致水下爆炸冲击波在不同位置的叠加效果存在明显差异,药柱周围流场中形成的冲击波压力具有方向性。利用提出的计算方法得到的计算结果与实验结果和数值模拟结果吻合较好,研究结果可为锥形长药柱水下爆炸冲击波威力场和毁伤评估提供参考和依据。     

气体-燃料液滴两相系统爆轰的数值模拟

用两相流体力学模型对气体 燃料液滴系统进行了研究。数值模拟了点火后两相系统爆轰波的发展过程，得到爆轰波的结构和参数。数值模拟结果表明气体 燃料液滴系统爆轰波有较宽的反应区，因而两相爆轰波的曲率对爆速的影响效应十分明显。进行了燃料液滴尺寸对爆轰波的结构和参数的影响的数值模拟。除了很小的液滴外，燃料液滴在爆轰波前导激波面和ＣＪ面间不能完全气化。随着液滴尺寸的增加，燃料液滴在爆轰波前导激波面和ＣＪ面间释放出的能量随之减少，爆轰参数也随之下降。     

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.     

聚奥-9C装药的传爆管殉爆

为了研究冲击波作用下引信传爆装置的响应规律,进行了以主发炸药为RDX-8701、被发装置为聚奥-9C（JO-9C）装药的传爆管（含导爆药柱）的殉爆实验。通过观测残留传爆药、壳体和见证块变形,判断传爆管的爆炸程度,分析了殉爆过程中JO-9C爆轰波的成长历程及传播规律。采用AUTODYN软件建立了殉爆实验有限元模型,计算模型中主要考虑了主发炸药产生的爆炸冲击波对传爆管的冲击响应。基于流固耦合方法,通过调整距离模拟计算得到了传爆管的临界殉爆距离和殉爆安全距离。结果表明,传爆管上端的侧角受到爆炸冲击后产生的爆轰波沿斜下方传播,使传爆药柱完全爆轰,并引起导爆药柱发生殉爆;数值模拟结果显示,JO-9C装药的传爆管临界殉爆距离为5.7 mm,殉爆安全距离为8.8 mm。     

Advances in the analytical study of the dynamics of gaseous detonation waves

Recent theoretical results on the dynamics of gaseous detonations are presented. An asymptotic analysis is performed, retaining the physical mechanisms controlling the modifications to the inner structure of the detonation. As a result, the system of hyperbolic equations for the compressible fluid mechanics coupled with a detailed chemical kinetics of heat release is reduced to a single integral equation for the propagation velocity of the combustion wave versus time. Concerning the direct initiation of spherical detonations by a blast wave, curvature effects are shown to be responsible for a critical condition of initiation. Near criticality, the role of the unsteadiness of the inner structure is pointed out. The whole complexity of the critical dynamics is reproduced and explained by the integral equation. The necessary background knowledge in gaseous detonation is recalled in the two first sections of the article in order to facilitate the reading by non-specialists.     

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。     

Strong explosion in a combustible gas mixture

Assume that a planar, cylindrical, or spherical point explosion takes place in a combustible mixture of gases. As a result of the explosion a strong shock wave develops and triggers chemical reactions with the release of heat. The solution of the problem for the case in which the thickness of the heat release zone is neglected (the infinitely thin detonation wave model) was obtained in [1–3].It was emphasized in [4] that these solutions can be considered only as asymptotic solutions for time and distance scales which are large in comparison with the scales which are characteristic for the chemical reactions, and under the assumption that as the overdriven detonation wave which is formed in the explosion is weakened by the rarefaction waves it does not degenerate into an ordinary compression shock. Here the question remains open of the possibility of obtaining such asymptotic solutions with account for finite chemical-reaction rates.In conclusion the authors wish to thank E. Bishimov for carrying out most of the computations for this study.     

Shock and detonation wave diffraction at a sudden expansion in gas–particle mixtures

Numerical modeling of the propagation of shock and detonation waves is carried out in a duct with an abrupt expansion for a heterogeneous mixture of fine particles of aluminum and oxygen. A considerable difference from corresponding flows in pure gas is found. The influence of the size and mass loading of particles on the flow and shock wave structure behind the backward-facing step is determined. As in gaseous detonations, three types of scenarios of detonation development are obtained. Specific features of the flow structure are revealed such as deformation of the combustion front due to interaction between the relaxation zone and the vortex structure. The influence of particle size and channel width on detonation propagation is analyzed. This paper is based on work that was presented at the 21th International Colloquium on the Dynamics of Explosions and Reactive Systems, Poitiers, France, July 23–27, 2007.     

气相爆轰波绕射流场显示研究

采用基于红宝石激光器（波长694.3 nm）的纹影系统,对气相爆轰波绕射进行了初步的流场显示研究。采用单色激光和合适半带宽（15 nm）的滤光片,有效地消除了爆轰波自发光对流场显示的影响。合理设置激光器同步控制系统的触发延时,得到了序列的爆轰波阵面纹影照片。结果表明:图像清晰地显示了爆轰波阵面的诱导激波、横波及化学反应区。当爆轰波在左尖点处绕射,受稀疏波作用,诱导激波与化学反应区明显分离,导致爆轰波衰减为爆燃。分离的诱导激波和折皱的化学反应区在纹影图上清晰可见。诱导激波在垂直支管右壁面反射,诱导二次起爆。畸变爆轰波在水平和垂直支管中均发生马赫反射。提高初压,爆轰波受分叉口几何属性的影响减小,畸变爆轰波在水平和垂直支管下游较易恢复为自持爆轰波。