CE/SE方法数值模拟炸药粉尘爆轰

采用CE/SE方法数值模拟悬浮在空气中的RDX炸药粉尘的两相爆轰过程.炸药颗粒在爆轰波阵面后的高温高速气流中加速并升温,释放能量支持爆轰波传播.数值模拟爆轰波管中的粉尘爆轰,得到爆轰波流场中的物理量分布,确定爆轰参数,数值结果与文献符合较好.数值模拟复杂通道中的炸药粉尘爆轰,预测了爆轰波的发展和传播过程以及爆轰波后的流场演化.数值结果表明CE/SE方法能成功模拟气体-固体两相爆轰,为粉尘爆轰的研究提供了新的数值预测手段.     

悬浮RDX炸药粉尘爆轰的数值模拟

用两相流模型对悬浮RDX炸药粉尘爆轰波进行了数值模拟。RDX炸药颗粒在爆轰波阵面后的高温高速气流中加速并升温,颗粒表面发生熔化。参考液滴在高速气流作用下剥离的效应,假设炸药熔化部分在高速气流的作用下发生剥离,破碎成极小的颗粒,瞬时发生分解反应,释放出能量支持爆轰波传播。数值模拟了在不同粒径和浓度的悬浮RDX炸药粉尘中爆轰波的发展与传播过程,得到了爆轰波流场中气-固两相的物理量分布,并确定了爆轰波参数。在较低的RDX粉尘浓度条件下,爆轰波阵面压力的峰值曲线出现振荡。当RDX粉尘浓度在80~150 g/m3时,数值模拟得到的爆轰波阵面压力峰值曲线的振荡是规则的;当RDX粉尘浓度为70 g/m3时,爆轰波阵面压力峰值曲线出现不规则振荡。     

含少量气泡流体饱和孔隙介质中的弹性波

考虑孔隙流体中含有少量气泡,且气泡在声波作用下线性振动,研究声波在这种孔隙介质中的传播特性.本文先由流体质量守恒方程和孔隙度微分与流体压力微分的关系推导出了含有气泡形式的渗流连续性方程;在处理渗流连续性方程中的气体体积分数时间导数时,应用Commander气泡线性振动理论导出气体体积分数时间导数与流体压强时间导数的关系,进而得到了修正的Biot形式的渗流连续性方程;最后结合Biot动力学方程求得了含气泡形式的位移场方程,便可得到两类纵波及一类横波的声学特性.通过对快、慢纵波的频散、衰减及两类波引起的流体位移与固体位移关系的考察,发现少量气泡的存在对快纵波和慢纵波的传播特性影响较大.     

Influence of the Fluid on the Parameters and Limits of Bubble Detonation

The compression and inflammation of reactive gas bubbles in bubble detonation waves have been studied, and the considerable influence of the fluid (liquid or vapor) on the detonation parameters has been found. It has been shown numerically that the final values of the pressure and temperature significantly decrease if the temperature dependence of the adiabatic index is taken into account at the compression stage. The parameters of reactive gas combustion products in the bubble have been calculated in terms of an equilibrium model, and the influence of the fluid that remains in the bubble in the form of microdroplets and vapor on these parameters has been investigated.

     

Bubble Velocity and Size Measurement with a Four‐Point Optical Fiber Probe

The possibility to measure the velocity and size of individual bubbles in a high‐void fraction bubbly flow is investigated by using a four‐point optical fiber probe. The air bubbles have an initial spherical equivalent diameter ranging from 4 to 10 mm and the void fraction is up to 0.3. Firstly, single bubble experiments show that intrusiveness effects, i.e. bubble deformations due to the probe, are negligible provided that the bubble approaches the probe at the axis of the central fiber. A selection criterion is utilized for multiple bubble experiments. A good compromise can be found between the required accuracy, the duration of the measurements and the number of validated bubbles required for reliable statistical averaging. In an air‐water high‐void fraction vertical bubbly pipe flow, the void fraction obtained with the instrument is found to be in good agreement with both local single‐fiber probe measurements, and with the volume average void fraction obtained from pressure gradient measurements. The area average volumetric gas flow rate, based on the bubble velocity and void fraction as measured with the four‐point probe, agree with the measured gas flow rate. Also, the liquid velocity is measured by means of a laser‐Doppler anemometer, to investigate the slip velocity. The results show that reliable and interesting measurements can be obtained by using a four‐point optical fiber probe in high void fraction flows.     

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.     

Detonation waves in bubble-drop media

Wave processes in chemically active multicomponent media: liquid — gas bubbles — liquid drops have been studied experimentally. Existence of detonation waves in multicomponent (bubble-drop) media has been proved. Structure of detonation waves in bubble-drop and bubble media is qualitatively identical: detonation waves are solitary waves with pulsation profile the pressure behind which is close in value to the one in unperturbed medium. Propagation velocity of detonation waves in bubble and bubble-drop media drops with the increase in medium gas phase concentration and with the decrease in carrier liquid viscosity. Presence of liquid drops decreases detonation wave velocity compared with bubble medium that does not contain liquid drops. Detonation wave propagation in multicomponent media causes gas bubbles fragmentation as well as fragmentation of individual liquid drops. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 04-03-33106).     

Shock waves and formation of carbon dioxide hydrate at an increased pressure in the gas-liquid medium

The processes of breaking, solution, and formation of hydrates behind a shock wave of moderate amplitude were studied experimentally in water with carbon dioxide bubbles under different initial static pressures. It is shown that an increase in the static pressure in a gas-liquid medium leads to reduction of critical relative amplitude of the shock wave, corresponding to starting development of Kelvin — Helmholtz instability and bubble splitting into small gas inclusions behind the shock wave front. It is shown that the rates of carbon dioxide solution and hydrate formation behind the shock wave front are close by the value; their dependences on medium and wave parameters are determined. Calculations by the model of gas hydration behind the shock wave are presented. The work was financially supported by the Russian Foundation for Basic Research (grants Nos. 06-01-00142 and 06-08-00657).     

凝聚炸药爆轰波在高声速材料界面上的折射现象分析

凝聚炸药爆轰在边界高声速材料约束下传播时,爆轰波会在约束材料界面上产生复杂的折射现象.本文针对凝聚炸药爆轰波在高声速材料界面上的折射现象展开理论和数值模拟分析.首先通过建立在爆轰ZND模型上的改进爆轰波极曲线理论给出爆轰波折射类型,然后发展一种求解爆轰反应流动方程的基于特征理论的二阶单元中心型Lagrange计算方法来数值模拟典型的爆轰波折射过程.从改进爆轰波极曲线理论和二阶Lagrange方法数值模拟给出的结果看出,凝聚炸药爆轰波在高声速材料界面上的折射类型有四种:反射冲击波的正规折射、带束缚前驱波的非正规折射、带双Mach反射的非正规折射、带λ波结构的非正规折射.     

Features of the dynamics of postdetonation waves

We present the results of numerical investigations of the parameters of postdetonation waves forming at a passage from the zone occupied with a bubbly liquid formed by the detonation wave to a zone filled with a liquid without bubbles. The dependence of the pressure amplitude of detonation waves and postdetonation waves on the gas volumetric content of bubbles has been studied. A possibility of the detonation transfer through the layer of a bubble-free liquid separating the regions of the bubbly liquid has been shown, the map of possible situations at the detonation transfer through the layer of this liquid has been presented.     

A simple model of ultrasound propagation in a cavitating liquid. Part I: Theory, nonlinear attenuation and traveling wave generation

The bubbles involved in sonochemistry and other applications of cavitation oscillate inertially. A correct estimation of the wave attenuation in such bubbly media requires a realistic estimation of the power dissipated by the oscillation of each bubble, by thermal diffusion in the gas and viscous friction in the liquid. Both quantities and calculated numerically for a single inertial bubble driven at 20 kHz, and are found to be several orders of magnitude larger than the linear prediction. Viscous dissipation is found to be the predominant cause of energy loss for bubbles small enough. Then, the classical nonlinear Caflish equations describing the propagation of acoustic waves in a bubbly liquid are recast and simplified conveniently. The main harmonic part of the sound field is found to fulfill a nonlinear Helmholtz equation, where the imaginary part of the squared wave number is directly correlated with the energy lost by a single bubble. For low acoustic driving, linear theory is recovered, but for larger drivings, namely above the Blake threshold, the attenuation coefficient is found to be more than 3 orders of magnitude larger then the linear prediction. A huge attenuation of the wave is thus expected in regions where inertial bubbles are present, which is confirmed by numerical simulations of the nonlinear Helmholtz equation in a 1D standing wave configuration. The expected strong attenuation is not only observed but furthermore, the examination of the phase between the pressure field and its gradient clearly demonstrates that a traveling wave appears in the medium.     

Physical and chemical characterization of shock-induced cavitation

A strong impact on a water surface induces a shock wave propagation with a significant pressure variation leading to cavitation bubble formation. A new shock induced cavitation reactor described in this work was characterized by physical and chemical techniques. Water hammer model verification with Joukowsky approach allowed to determine the wave speed propagation and gas fraction in water submitted to shock. These values were used for frequency analysis and compared with direct bubble visualization in order to estimate the influence of the experimental parameters on the shock-induced cavitation. Thereby, the shock wave contains a broad spectrum as decomposed into frequencies. This multi-frequency nature induces heterogeneous bubbles with calculated radii of 0.01 to 3.5 mm and observed radii of 0.01 to 2.8 mm depending on experimental conditions (initial pressure, impact height, gas atmosphere). For the first time, the formation of hydroxyl radicals was proven under impact-induced cavitation. The concentration of radicals increases with increasing number of successive impacts, reaching ca. 1.3 µmol.L⁻¹ after 500 impacts in the presence of 20% O₂-Ar as saturating gas. Radical generation seems to be relatively independent of the impact height but strongly depend on the type of gas saturating water, being substantially lower in the presence of air. Moreover, radical generation increases when decreasing the initial pressure and depends on the frequency at which water is impacted by the piston. Nevertheless, yield of OH radicals during shock-induced cavitation remains much lower than that produced by power ultrasound.     

声波在含气泡液体中的线性传播

为了探讨含气泡液体对声波传播的影响, 研究了声波在含气泡液体中的线性传播. 在建立含气泡液体的声学模型时引入气泡含量的影响,建立气泡模型时引用 Keller的气泡振动模型并同时考虑气泡间的声相互作用,得到了经过修正的气泡振动方程. 通过对含气泡液体的声传播方程和气泡振动方程联立并线性化求解,在满足 (ω R₀)/c << 1 的前提下,得到了描述含气泡液体对声波传播的衰减系数和传播速度. 通过数值分析发现,在驱动声场频率一定的情况下,气泡含量的增加及气泡的变小均会导致衰减系数增加和声速减小;气泡的体积分数和大小一定时, 驱动声场频率在远小于气泡谐振频率的情况下,声速会随驱动频率的增加而减小; 气泡间的声相互作用对声波传播速度及含气泡液体衰减系数的影响不明显.最终认为气泡的大小、 数量和驱动声场频率是影响声波在含气泡液体中线性传播的主要因素. 关键词： 含气泡液体 线性声波 声衰减系数 声速     

利用声衰减反演气泡群分布的方法研究

通过测量含气泡水的声衰减反演气泡群参数是获取水中气泡分布的重要方法,但是经典方法忽略了较高浓度气泡水中的强频散特性和气泡振动参数的改变,导致反演较高浓度气泡群分布时会产生巨大误差.为解决这个问题,本文基于等效媒质理论建立起了声衰减和相速度的联系,并考虑了含气泡水平均量对气泡阻尼系数和共振频率的影响.在此基础上,通过将反...     

On the mechanism of the deflagration-to-detonation transition in a hydrogen-oxygen mixture

The flame acceleration and the physical mechanism underlying the deflagration-to-detonation transition (DDT) have been studied experimentally, theoretically, and using a two-dimensional gasdynamic model for a hydrogen-oxygen gas mixture by taking into account the chain chemical reaction kinetics for eight components. A flame accelerating in a tube is shown to generate shock waves that are formed directly at the flame front just before DDT occurred, producing a layer of compressed gas adjacent to the flame front. A mixture with a density higher than that of the initial gas enters the flame front, is heated, and enters into reaction. As a result, a high-amplitude pressure peak is formed at the flame front. An increase in pressure and density at the leading edge of the flame front accelerates the chemical reaction, causing amplification of the compression wave and an exponentially rapid growth of the pressure peak, which “drags” the flame behind. A high-amplitude compression wave produces a strong shock immediately ahead of the reaction zone, generating a detonation wave. The theory and numerical simulations of the flame acceleration and the new physical mechanism of DDT are in complete agreement with the experimentally observed flame acceleration, shock formation, and DDT in a hydrogen-oxygen gas mixture.     

离心泵叶轮内气液两相三维流动数值研究

本文采用欧拉模型对离心泵叶轮内气液两相泡状流动进行了数值模拟。分析了叶轮内压力、速度等的分布规律,发现在靠近轮盖侧吸力面入口附近压力较低,气液两相速度较大,气泡容易凝聚而导致含气率较高;靠近轮盘处含气率较低。分析了不同进口截面含气率对叶轮内部两相流动的影响机理,建立了进口截面含气率对内部相态分离及泵外特性的影响关系。     

气相爆轰波在分叉管中传播现象的数值研究

数值研究气相爆轰波在分叉管中的传播现象.用二阶附加半隐龙格-库塔法和5阶WENO格式求解二维欧拉方程,用基元反应描述爆轰化学反应过程,得到了密度、压力、温度、典型组元质量分数场及数值胞格结构和爆轰波平均速度.结果表明:气相爆轰波在分叉管中传播,分叉口左尖点的稀疏波导致诱导激波后压力、温度急剧下降,诱导激波和化学反应区分离,爆轰波衰减为爆燃波(即爆轰熄灭).分离后的诱导激波在垂直支管右壁面反射,并导致二次起爆.畸变的诱导激波在水平和垂直支管中均发生马赫反射.分叉口上游均匀胞格区和分叉口附近大胞格区的边界不是直线,其起点通常位于分叉口左尖点上游或恰在左尖点.水平支管中马赫反射三波点迹线始于右尖点下游.分叉口左尖点附近的流场中出现了复杂的旋涡结构、未反应区及激波与旋涡作用.旋涡加速了未反应区的化学反应速率.反射激波与旋涡作用并使旋涡破碎.反射激波与未反应区作用,加速其反应消耗,并形成一个内嵌的射流.数值计算得到的波系演变和胞格结构与实验定性一致.     

Numerical analysis of the effect of bubble distribution on multiple-bubble behavior

Understanding multiple-bubble behavior in a megasonic field is essential for efficient megasonic nanodevice cleaning without pattern damage. In this study, we numerically studied the effects of equilibrium radius and initial void fraction on multiple-bubble behavior and induced pressure. We analyzed the nonspherical collapse, coalescence, and breakup of bubbles in a megasonic field using a compressible, locally homogeneous model of a gas-liquid two-phase medium. Bubbles were simulated with a uniform equilibrium radius or with a bubble size distribution. Our results indicate that the bubble behavior and induced pressure depended mainly on the initial void fraction. For the case of bubbles with uniform equilibrium radius, small bubbles generated high wall pressure at large initial void fractions. When there was a size distribution, bubbles with small equilibrium radii contributed little to the wall pressure because of the damping effect of the oscillation of larger bubbles. Furthermore, the addition of a large bubble suppressed the resonant behavior of the bubbles that induced high wall pressure.     

垂直圆管内向上稀疏层流泡状流相分布的实验研究

采用三维照相法对垂直圆管内稀疏层流泡状流充分发展段的相分布进行了实验研究。得到了８个流动工况下均匀尺寸气泡形成的泡状流的空泡率分布以及６个流动工况下非均匀尺寸气泡形成的泡状流的总体和大、小气泡组各自的空泡率分布．实验结果表明当气泡组的平均直径小于约３．５ｍｍ时,其空泡率分布在管壁附近出现尖峰;当气泡组的平均直径大于约 ３．５ ｍｍ时,其空泡率分布的尖峰移向管中心;气泡尺寸对泡状流的相分布有重要影响．