沉积粉尘的激波点火

采用瞬态阴影技术及红外光电传感器技术实验研究了沉积玉米粉的激波点火过程，并对此进行了理论分析。实验与理论分析结果表明，激波掠射沉积粉尘床后，粉尘颗粒先上扬到一定高度后才点火，颗粒的点火延迟时间与激波波前马赫数、气相氧气含量等因素有关。另外，沉积粉尘的激波点火延迟时间比相同条件下的悬浮粉尘激波点火延迟时间长。     

抽吸激波管的性能分析与计算

为了减小由于反射激波和透射激波分叉引起的反射型激波风洞试验气体提前受到污染的现象,本文研究了一种新的具有抽吸的激波管流动,分析了抽吸缝的作用,给出了这种抽吸激波管性能参数的计算方法,同时还给出了反射激波与边界层相互作用引起的激波分叉的形状随抽吸量变化的计算公式。实验证实了边界层抽吸可以有效地减小激波与壁面边界层相互作用所产生的分离现象。计算与测量结果是一致的。     

低稀释度条件下乙烯点火特性的激波管研究简

在激波管中利用反射激波后高温环境加热燃料,以燃料点火过程中氢氧自由基特征发射光谱强度的急剧变化作为点火发生的标志,在温度范围800 1 650K,压力0.2MPa, 0.7MPa, 1.2MPa,化学当量比为0.5, 1, 2,O₂ 浓度为空气含量20% 的条件下,进行了C₂H₄/O₂/Ar 混合气在低稀释度条件下点火特性的实验研究. 获得了乙烯点火延时随温度、压力、化学当量比、燃料以及氧化剂浓度等参数变化的拟合关系式. 对乙烯点火转爆轰现象进行了初步观察,考察了初始温度对乙烯点火特性以及点火转爆轰的影响.     

低稀释度条件下乙烯点火特性的激波管研究

在激波管中利用反射激波后高温环境加热燃料,以燃料点火过程中氢氧自由基特征发射光谱强度的急剧变化作为点火发生的标志,在温度范围800 1 650K,压力0.2MPa, 0.7MPa, 1.2MPa,化学当量比为0.5, 1, 2,O₂ 浓度为空气含量20% 的条件下,进行了C₂H₄/O₂/Ar 混合气在低稀释度条件下点火特性的实验研究. 获得了乙烯点火延时随温度、压力、化学当量比、燃料以及氧化剂浓度等参数变化的拟合关系式. 对乙烯点火转爆轰现象进行了初步观察,考察了初始温度对乙烯点火特性以及点火转爆轰的影响.      

Interaction of a high-speed combustion front with a closely packed bed of spheres

The head-on collision of a combustion front with a closely packed bed of ceramic-oxide spheres was investigated in a vertical 76.2 mm diameter tube containing a nitrogen diluted stoichiometric ethylene–oxygen mixture. A layer of spherical beads in the diameter range of 3–12.7 mm was placed at the bottom of the tube and a flame was ignited at the top endplate. Four orifice plates spaced at one tube diameter were placed at the ignition end of the tube in order to accelerate the flame to either a “fast-flame” or a detonation wave before the bead layer face. The mixture reactivity was adjusted by varying the initial mixture pressure between 10 and 100 kPa absolute. The pressure before and within the bead layer was measured by flush wall-mounted pressure transducers. For initial pressures where a fast-flame interacts with the bead layer peak pressures recorded at the bead layer face were as high as five times the reflected Chapman–Jouget detonation pressure. The explosion resulting from the interaction developed by two distinct mechanisms; one due to the shock reflection off the bead layer face, and the other due to shock transmission and mixing of burned and unburned gas inside the bead layer. The measured explosion delay time (time after shock reflection from the bead layer face) was found to be independent of the incident shock velocity. As a result, the explosion initiation is not the direct result of the shock reflection process but instead is more likely due to the interaction of the reflected shock wave and the trailing flame. The bead layer was found to be very effective in attenuating the explosion front transmitted through the bead layer and thus isolating the tube endplate. 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.     

Near-wall imaging using toluene-based planar laser-induced fluorescence in shock tube flow

A quantitative thermometry technique, based on planar laser-induced fluorescence (PLIF), was applied to image temperature fields immediately next to walls in shock tube flows. Two types of near-wall flows were considered: the side wall thermal boundary layer behind an incident shock wave, and the end wall thermal layer behind a reflected shock wave. These thin layers are imaged with high spatial resolution (15μm/pixel) in conjunction with fused silica walls and near-UV bandpass filters to accurately measure fluorescence signal levels with minimal interferences from scatter and reflection at the wall surface. Nitrogen, hydrogen or argon gas were premixed with 1–12% toluene, the LIF tracer, and tested under various shock flow conditions. The measured pressures and temperatures ranged between 0.01 and 0.8 bar and 293 and 600 K, respectively. Temperature field measurements were found to be in good agreement with theoretical values calculated using 2-D laminar boundary layer and 1-D heat diffusion equations, respectively. In addition, PLIF images were taken at various time delays behind incident and reflected shock waves to observe the development of the side wall and end wall layers, respectively. The demonstrated diagnostic strategy can be used to accurately measure temperature to about 60 μm from the wall.     

Mechanism of deflagration-to-detonation transitions above repeated obstacles

Experiments are carried out to investigate the mechanism of the deflagration-to-detonation transition (DDT). Because, this mechanism has relevance to safety issues in industries, where combustible premixed gases are in general use. A stoichiometric gas of oxygen and hydrogen (oxy-hydrogen) is ignited in a tube, repeated obstacles are installed, and the DDT behaviours are visualized using a high-speed video camera. The pitch and height of the repeated obstacles and the initial pressure of the oxy-hydrogen premixed gas are varied in an attempt to obtain the optimum conditions that cause DDT a short distance from the ignition source. The experiments identified DDT as being essentially caused by one of the following mechanisms: (1) A deflagration wave is accelerated in terms of a vortex, which is generated behind the obstacle, and the flame acceleration induces a secondary shock wave. Eventually, the shock–flame interaction ahead of the obstacle causes DDT via a very strong local explosion. (2) Each shock wave generated by relatively weak local explosions between the obstacles is not sufficient to cause DDT directly, but DDT results from an accumulation of shock waves. The detonation induction distance is also examined, taking into account the physical and chemical parameters of the obstacles and the oxy-hydrogen premixed gas.     

硅烷对JP10和煤油点火特性影响的激波管研究

在JP10和煤油点火特性激波管实验的基础上,实验研究了硅烷对这两种典型高碳数碳氢燃 料点火特性的影响. 在预加热到70 C的激波管上,采用缝合运行条件获得了近7ms 的实验时间,将实验延伸至低温区. 采用气相色谱分析和高精度真空仪直接测定压力相结合 的方法,确定了燃料气相浓度,解决了高碳数碳氢燃料点火激波管实验时由于管壁吸附影响 燃料气相浓度确定的困难. 实验记录了点火过程中OH自由基发射强度变化,并作为判断点 火发生的标志. 实验温度范围880~1800K, 压力范 围0.16~0.53\,MPa. 当硅烷加入量约为燃料的10%~15%(摩尔比), 质量比为2%~3%, 观测到明显的点火促进作用. 该研究对超燃研究中发动机设计、 燃料选择等方面具有直接的工程意义,也可用于检验燃烧化学动力学模型的合理性.     

On critical conditions for detonation initiation by shock reflection from obstacles

A series of experiments supported by numerical simulations are reported on the interaction between a planar incident shock and a single obstacle. The test mixtures used were stoichiometric hydrogen and oxygen diluted with either argon or nitrogen at sub-atmospheric pressures. The main aim of the study was to determine the conditions under which a reflected detonation was generated. Observed critical conditions are compared with a simple predictive criterion based on the ratio of auto-ignition delay time behind an ideal reflected shock to the acoustic transit time across the face of the obstacle. Received 7 December 2001 / Accepted 3 March 2002 Published online 8 July 2002     

多障碍物通道中激波诱导气相爆轰的数值研究

应用数值模拟方法,研究了直通道中激波经过多块矩形障碍物时诱导 H2/O2混合气体起爆的物理机制. 研究表明：在前导激波强度不足以诱导波后气 体直接起爆的条件下,经过激波压缩的可燃气体也可能在远离激波的障碍物之间的凹槽部位 起爆;障碍物表面产生的压缩波、膨胀波和气流滑移面对可燃气体的起爆、爆轰波的形成和 传播过程有重要的影响;添加不同稀释比的氮气可以影响爆轰波后流场的温度分布;增加障 碍物的间距可以改变可燃气起爆位置.     

A comparison between constant volume induction times and results from spatially resolved simulation of ignition behind reflected shocks: implications for shock tube experiments

The induction time measured in shock tube experiments is typically converted into kinetic data assuming that the reaction takes place in a constant volume process, thus neglecting spatial gradients. The actual process of shock ignition is, however, both time- and space-dependent; ignition takes place at a well-defined location, and subsequently a front travels, which may couple with the pressure wave that it created and forms a detonation wave behind the shock that reflects off the wall. To assess how different the actual processes are compared with the constant volume assumption, a numerical study was performed using a simplified three step chain-branching kinetic scheme. To overcome the difficulties that arise when simulating shock-induced ignition due to the initial absence of a domain filled with shocked reactive mixture, the problem is solved in a transformed frame of reference. Furthermore, initial conditions are derived from short-time asymptotics, which resolves the initial singularity. The induction times obtained using the full unsteady formulation with those of the homogeneous explosion are compared for various values of the heat release. Results for the spatially dependent formulation show that the evolution of the post-shock flow is complex, and that it leads to a gradient in induction times, after the passage of the reflected shock. For all cases simulated, thermal explosion initially occurs very close to the wall, and the corresponding induction time is found to be larger than that predicted under the constant volume assumption. As the measurement is made further away however, the actual time interval between passage of the reflected shock, and the specified pressure increase denoting ignition, decreases to a value close to zero, corresponding to that obtained along a Rayleigh line matching that of a steady ZND process (assuming a long enough tube). In situations where the constant volume assumption is expected to be weak, more accurate kinetic data will be obtained using a spatially resolved computation such as the one used in the current comparison.     

可燃气体中激波聚焦的点火特性

数值模拟了二维平面激波从抛物面上反射在可燃气体中聚焦的过程,研究了形 成爆轰波的点火特性. 对理想化学当量比氢气/空气混合气体,在初始压强20kPa的条件下, 马赫数2.6-2.8的激波聚焦能产生两个点火区：第1个点火区是反射激波会聚引起的,第 2个点火区是由入射激波在抛物面上发生马赫反射引起的. 这种条件下流场中会出现爆燃转 爆轰,起爆点分别分布在管道壁面、抛物反射面和第2点火区附近. 起爆机理分别为激波管 道壁面反射、点火诱导激波的抛物面反射和点火诱导的激波与第2点火区产生的爆燃波的相 互作用. 不同的点火和起爆过程导致了不同的流场波系结构,同时影响了爆轰波传播的波动 力学过程.     

STUDY ON THE FUEL AIR MIXING INDUCED BY A SHOCK WAVE PROPAGATING INTO A H_2-AIR INTERFACE

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Viscous solution of the triple-shock reflection problem

The reflection of a triple-shock configuration was studied numerically in two dimensions using the Navier–Stokes equations. The flow field was initialized using three shock theory, and the reflection of the triple point on a plane of symmetry was studied. The conditions simulated a stoichiometric methane-oxygen detonation cell at low pressure on time scales preceding ignition when the gas was assumed to be inert. Viscosity was found to play an important role on some shock reflection mechanisms believed to accelerate reaction rates in detonations when time scales are small. A small wall jet was present in the double Mach reflection and increased in size with Reynolds number, eventually forming a small vortex. Kelvin–Helmholtz instabilities were absent, and there was no Mach stem bifurcation at Reynolds numbers corresponding to when the Mach stem had travelled distances on the scale of the induction length. Kelvin–Helmholtz instabilities are found to not likely be a source of rapid reactions in detonations at time scales commensurate with the ignition delay behind the Mach stem.     

The interaction of a shock wave with the boundary layer in a reflected shock tunnel

The influence of a nontotal reflection on the interaction of a reflected shock wave with the boundary layer in a reflected shock tunnel has been investigated. The calculating method of the velocity, the temperature and the Mach number profiles in the boundary layer in reflected shock fixed coordinates has been obtained. To account for equilibrium real gas effects of nitrogen, the numerical results show that the minimum Mach number in the boundary layer has been moved from the wall into the boundary layer with the increasing of the incident shock Mach number. The minimum Mach number, the shock angle in the bifurcated foot and the jet velocity along the wall to the end plate are reduced owing to the increasing of the area of nozzle throat. The numerical results are in good agreement with measurements.     

Shock tube study of n-decane ignition at low pressures

Ignition delay times for n-decane/O 2 /Ar mixtures were measured behind reflected shock waves using endwall pressure and CH* emission measurements in a heated shock tube. The initial postshock conditions cover pressures of 0.09-0.26 MPa, temperatures of 1 227-1 536 K, and oxygen mole fractions of 3.9%-20.7% with an equivalence ratio of 1.0. The correlation formula of ignition delay dependence on pressure, temperature, and oxygen mole fraction was obtained. The current data are in good agreement with available low-pressure experimental data, and they are then compared with the prediction of a kinetic mechanism. The current measurements extend the kinetic modeling targets for the n-decane combustion at low pressures.     

Shock wave ignition in N2O-CO(N2)-He mixtures

The behavior of the pressure behind shock waves in N₂O-CO-He mixtures is investigated. The pressure can be relatively easily measured and, at the same time, reflects the general influence of the experimental conditions on flow formation in the shock tube. Shock-wave mixture ignition effects and, moreover, hydrogen combustion in the contact zone are analyzed from the standpoint of their influence on the stagnation parameters and the optical properties of the flow in the shock tube nozzle. The correctness of modeling the conditions behind the reflected wave by means of mixtures similar in composition to the equilibrium products of the N₂O-CO reaction [1] is discussed in the same context.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 164–170, September–October, 1989.The authors wish to thank G. D. Smekhov for calculating the equilibrium compositions of the mixtures behind the shock waves and A. P. Zuev for stimulating discussions.     

Shock-tube experiments on the stability of regular reflection in the dual-solution domain

Regular reflection (RR) and Mach reflection (MR) are theoretically both possible in the dual-solution domain of oblique shock reflection. The physical difference between the two types of reflection is the pressure behind the reflected shock wave: that of MR is lower than that of RR for strong shock reflection. The magnitude relation of these pressures is inverted for weak shock reflection. In the present paper, we performed two kinds of experiment, depending on whether the oblique shock reflection is weak or strong. For strong shock reflection, we decreased the pressure behind the reflected wave of RR using a convex double-wedge. For weak shock reflection, we increased this pressure using a concave double-wedge. Thus, we investigated the stability of RR against pressure disturbances. The results indicate that RR in a shock tube is stable, in the dual-solution domain, for both weak and strong incident shocks.     

Theoretical investigation of ignition and detonation of coal-particle gas mixtures

A mathematical model of ignition of a coal-particle gas suspension is developed within the framework of mechanics of reacting heterogeneous media. Some qualitative features are studied, which reveal different typical variants of heat dynamics of the mixture: heterogeneous ignition caused by the coke residue ignition reaction, homogeneous ignition induced by volatile oxidation in the gas phase, and hybrid ignition due to the simultaneous effect of surface and volume reactions. Verification of the model is performed using available experimental data on ignition delay times for coal-particle suspensions in air and oxygen behind reflected shock waves. The model is extended to detonation processes in the suspension, verified by experimental data on the relation between the propagation velocity and initial particle concentration. Stationary detonation regimes are studied, and specific features of detonation structures are discussed.This paper was presented at the 18th International Colloquium on the Dynamics of Explosions and Reactive System, Seattle, Washington, USA, August, 2001     

Diffraction and re-initiation of detonations behind a backward-facing step

Diffraction phenomena of gaseous detonation waves behind a backward-facing step in a tube are observed by using high-speed schlieren photography and soot-track records as well as by pressure measurements on the sidewall. Mixtures are stoichiometric oxyhydrogen and those diluted by argon at sub-atmospheric pressures. Three types of phenomena are observed, that is, continuous propagation of detonation, re-initiation after a temporal extinction of detonation and complete extinction of detonation. The continuous propagation means that the diffracted wave does not affect the main propagation although reflected shock waves from the bottom surface of the tube may affect it. The re-initiation occurs at a wall surface of the tube behind a reflected shock wave after the main detonation wave has been extinguished. Positions and conditions of the re-initiation are discussed. The complete extinction is defined as disappearance of detonation cells behind the step within a certain length of the tube. Cases exist where an ignition occurs after several reflections off the bottom and top surface of the tube.