A high-speed photographic study of the transition from deflagration to detonation wave

Experiments were conducted to investigate the DDT process of the oxyhydrogen gas in the rectangular detonation tube of 3 m long. The repeated obstacle was installed near the ignition plug and the effects of the obstacle on the DDT process were investigated. The behaviour of the combustion and detonation wave were visualized utilizing Imacon high-speed camera with the aid of Schlieren optics. As a result, DDT process was visualized, i.e. (i) multiple shock waves were induced by the expanding combustion wave, because the combustion flame played a role as a piston and compressed the unburned gases. (ii) The acceleration of the combustion wave was occurred and the distance between the shock wave and the combustion flame became shorter. (iii) Eventually, the local explosion was occurred and cause overdriven detonation wave to propagate at the velocity of about 3 kms⁻¹. 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     

氢燃料缓燃向爆震转捩过程中波与火焰的匹配特性

为了解氢燃料爆震过程中压力波与火焰之间相互匹配的特性,在60mm60mm2000mm 方 爆震管内,用氢气和空气混合物进行了单爆震性能研究。在爆震转捩区内布置压力传感器与离子探针,用来 监控压力波和火焰的信号,同时利用高速摄影仪集中拍摄转捩区域。根据压力波和火焰面在爆震管不同时刻 的强度特性、速度特性及位置特性来分析爆震过程中波与火焰匹配的规律。结果表明:压力波和火焰的强度 呈现为相互正反馈匹配性质;缓燃向爆震转捩(DDT)过程中,压力波和火焰的速度表现为相互交替的变化过 程,且缓燃阶段中火焰速度的增幅大于压力波速度的增幅;当火焰面追赶上激波时,产生过爆,火焰面会临时 位于激波前面;在过爆衰减为正常爆震波的过程中,激波在火焰前面。     

Transition from fast deflagration to detonation under the influence of periodic longitudinal perturbations

A computational and experimental investigation have been carried out to study deflagration to detonation transition under the influence of periodic longitudinal perturbations. The fast deflagration prior to transition is obtained by suppressing the oscillatory structure of a detonation first. In the one-dimensional computational study, the subsequent re-transition is stimulated using periodic density perturbations. The optimal perturbation period to induce transition is of the order of the rea ction time of the detonation structure. In the experiments, re-transition is stimulated using periodic wire screens placed across the channel. The gas mixture used is 2C₂H₂+5O₂+75%Ar. Laser stroboscopic photography showed the generation of longitudinal pressure waves as the leading shock passes through the wire screens. The interactions of the pressure waves with the leading shock and the reaction zone establish coupling for transition to occur. The computational and experimental studies showed that the time required for transition to take place is dependent on the frequency of the applied perturbation. The present results indicate the need to examine deflagration to detonation transition from the point of view of the formation of an organized oscillatory system.     

Hydrogen detonation and fast deflagration triggered by a turbulent jet of combustion products

Initiation of detonation by a turbulent jet of combustion products has been studied in a detonation tube of 141 mm inner diameter. Jet formation techniques based on either a perforated plate or bursting membrane subjected to the impact of a stable detonation wave were utilized. Critical conditions of detonation initiation in hydrogen–air and hydrogen–oxygen–nitrogen mixtures have been found to depend on both the mixture sensitivity and the geometrical parameters of the arrangement. PACS 47.70.Fw; 82.33.Vx; 82.40.Fp This paper was based on work that was presented at the 19th Inter-national Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003     

Study of detonation initiation in hydrogen/air flow

While extensive studies have been conducted concerning the formation of detonation waves in various combustible gaseous mixtures under static conditions since the 1950s, there is very little experimental work on simple flowing systems. In this study, experiments on the deflagration to detonation transition (DDT) of a hydrogen–air flow system were carried out to see the effects of tube diameter, equivalence ratio, and flow types in a premixed and non-premixed flow. Tube diameters used were 25, 50, and 100 mm. The premixed experiments show that the larger tube diameter provides a wider range in run-up distance, reduction of L _DDT/D (ratio of the run-up distance, L _DDT to tube diameter), and expansion of the detonable concentration limit by spreading the cell width. The result of the non-premixed experiments show that similar values of the run-up distance to the premixed experiments are obtained at an equivalence ratio of about 1.0, however, fluctuations of DDT occur near the DDT concentration limit. Under laminar flow conditions at a Reynolds number of less than 2,300, the difference between the two systems could not be observed. However, when the Reynolds number increases towards turbulent conditions, the DDT run-up distance decreases compared to that of static flow conditions. This paper is based on work that was presented at the 21st International Colloquium on the Dynamics of Explosions and Reactive Systems, Poitiers, France, July 23–27, 2007.     

爆轰波透射孔栅形成的高速爆燃波的结构和行为

以实验为主,研究光滑直管中乙炔氧气爆轰波透射孔栅形成的高速爆燃波结构和行为。实验以基于纹影平台的高速转鼓摄影记录孔栅下游近场内初始爆燃波的结构和发展,并以压力传感器跟踪其后继走向。研究发现,高速爆燃波是前驱激波和火焰的组合结构,随着初始压力的提升,它分别表现为紧随于前驱激波的层流和湍流燃烧火焰。入射爆轰胞格尺度小于或与孔栅扰动尺度相当时,湍流燃烧在下游近场迅速形成;层流结构的爆燃波通常无法抵制背景稀疏波而走向衰弱,而湍流燃烧结构可发生加速和向爆轰的转捩;他们之间存在一个不稳定的临界状态,高速爆燃波得以以0.5~0.6倍CJ爆轰速度传播较长距离,这一状态对应于双间断Rankine-Hugoniot关系的等容燃烧解。     

Global modelling of heat release during initiation and propagation of detonation and deflagration waves in methane-air-particle systems

In the present paper the direct initiation of a self supporting detonation and propagation of a low-speed combustion in methane-air-coal particles mixtures are solved. For particles, a heterogeneous regime of combustion is used, for methane one overall chemical reaction is taken into account: CH + 2O = CO + 2HO. The heat release rate is assumed to be defined as a delay time based on the well-known thermal theory of Frank-Kamenetsky (1967). The proposed model allows one to investigate the influence inert particles or coal dust on the explosion limits of methane-air mixtures. It is shown that the addition of a limited quantity of particles leads to detonation stability. In low speed combustion problems this method allows one to get a good correlation between theoretical and experimental velocities of steady flame propagation in carbon-hydrogen gaseous mixtures. Coal dust influence on gasdynamics of a methane-air mixture combustion is investigated in an unsteady problem by using of the global modelling. It is shown that limited coal dust concentration increases the flame wave intensity in lean methane-air mixtures in contrast to inert particles. In stoichiometric gas mixtures, sand and coal dusts decrease a flame velocity. Far from the ignition point flame, the velocity is largely defined by the dust mass concentration and not by the size of particles. Received 5 July 1997 / Accepted 13 July 1998     

Experimental and numerical studies on transmission of gaseous detonation to a less sensitive mixture

A phenomenon of detonation transmission from one gaseous mixture (donor) to another of lower sensitivity (acceptor) was studied experimentally and numerically. The objective was to study effects of a donor mixture length and acceptor mixture sensitivity on the possibility of detonation transmission. Experiments were carried out in detonation tube 9.5–12 m long and 174 mm id. Three types of donor mixtures were used in the driver: stoichiometric acetylene/air, stoichiometric hydrogen/air, and 20% of hydrogen/air. Air mixtures with 14–29.6% of hydrogen were used as acceptors. Driver length varied from 0.17 to 5.6 m. Detonation transmission was studied for an abrupt opening of interface between two mixtures. Series of 1D and 2D calculations were made to simulate the problem numerically. Both, results of experiments and calculations revealed a set of parameters that effect transmission process. Critical conditions were determined as minimum driver length expressed in terms of characteristic chemical reaction length scales of acceptor mixture. They were shown to depend on differences in reaction rates and energy contents of donor and acceptor mixture. Received 6 January 1997 / Accepted 20 March 1997     

Thermal degradation of two liquid fuels and detonation tests for pulse detonation engine studies

The use of liquid fuels such as kerosene is of interest for the pulse detonation engine (PDE). Within this context, the aim of this work, which is a preliminary study, was to show the feasibility to initiate a detonation in air with liquid-fuel pyrolysis products, using energies and dimensions of test facility similars to those of PDEs. Therefore, two liquids fuels have been compared, JP10, which is a synthesis fuel generally used in the field of missile applications, and decane, which is one of the major components of standard kerosenes (F-34, Jet A1, ...). The thermal degradation of these fuels was studied with two pyrolysis processes, a batch reactor and a flow reactor. The temperatures varied from 600°C to 1,000°C and residence times for the batch reactor and the flow reactor were, respectively, between 10–30 s and 0.1–2 s. Subsequently, the detonability of synthetic gaseous mixtures, which was a schematisation of the decomposition state after the pyrolysis process, has been studied. The detonability study, regarding nitrogen dilution and equivalence ratio, was investigated in a 50 mm-diameter, 2.5 m-long detonation tube. These dimensions are compatible with applications in the aircraft industry and, more particularly, in PDEs. Therefore, JP10 and decane were compared to choose the best candidate for liquid-fuel PDE studies. This paper was based on work that was presented at the 20th International Colloquium on the Dynamics of Explosions and Reactive Systems, Montreal, Canada, July 31 – August 5, 2005.     

Effect of obstacle size and spacing on the initial stage of flame acceleration in a rough tube

Experiments were conducted to study flame acceleration in an orifice plate laden detonation tube. Orifice plate area blockage and spacing were varied to determine their affect on flame acceleration. The tube used in the study was 3.05 m long with an inner diameter of 14.0 cm. Experiments were primarily carried out with stoichiometric propane-air, however the affect of mixture reactivity was also investigated by varying the mixture equivalence ratio. The distance required for the flame to achieve a velocity equal to the speed of sound in the unburned gas mixture was measured. This run-up distance is used to characterize the early stage of the flame acceleration process. It was found that in all cases, the flame run-up distance decreased with increased blockage ratio and with increased mixture reactivity. It was found that for higher blockage ratios plates flame acceleration was greatest for a plate spacing of one tube diameter, but for lower blockage ratio plates the results obtained for one-half, one, and one and one-half tube diameter plate spacing were very similar. The most rapid flame acceleration was observed when the ratio of the orifice plate spacing and the orifice plate height (half of the difference between the tube and orifice plate diameter) is on the order of 5. It is proposed that this optimum acceleration corresponds to the condition where the plate spacing is roughly equal to the length of the unburned gas re-circulation zone downstream from the orifice plate. PACS 47.40.-x; 47.70.Fw This paper was based on work that was presented at the 19th Interna-tional Colloquium on the Dynamics of Explosions and Reactive Sys-tems, Hakone, Japan, July 27 - August 1, 2003     

快速热点火熔奥梯铝炸药燃烧转爆轰实验研究

以熔铸型含铝混合炸药熔奥梯铝为对象,研究铸装含铝混合炸药快速热点火后的燃烧转爆轰特性。建立了快速热点火燃烧转爆轰实验平台,由实验装置(加热装置、约束钢管、炸药)、压力测试系统、光纤测速系统组成;加热装置加热15 mm厚45钢钢板,峰值温度大于1 100℃,温升速率为85～95℃/s。开展了快速热点火带壳熔奥梯铝炸药燃烧转爆轰实验,由加热装置加热约束钢管内熔奥梯铝炸药,炸药化学反应阵面压力和传播速度分别由压电性高压压力传感器和光纤探针测定;实测阵面压力约1 GPa,传播速度最大约2 600 m/s。由光纤数据获得炸药化学反应阵面传播轨迹,通过特征线方法获得冲击形成点,半定量给出冲击形成距离大于850 mm;并比较了管体破片质量实测值与炸药完全爆轰时破片平均质量计算值,实测值远小于计算值。综合实测化学反应阵面传播速度和压力、冲击形成距离分析、破片质量比较,可确定熔奥梯铝炸药没有发生完全爆轰,其化学反应状态为爆燃。另外,采用Adams和Pack模型、CJ燃烧模型,都能够半定量的预估冲击形成距离和燃烧波后压力,为实验设计提供依据,但CJ燃烧模型的计算结果更接近于实测值。     

Ignition and detonation of solid explosives: a micromechanical burn model

This paper describes a new computational framework for modeling splid explosives and proof-of-concept calculations. Our goal is to expand predictive model capability through the inclusion of various micro-mechanical burn processes. We propose a model which is complicated enough to represent underlying physics, but simple enough for engineering scale computations. Key components of the model include energy localization, the growth of hot spots, micro-mechanics in/around hot spots, and a phase-averaged mixture equation of state. The nucleation and growth of locally heated regions is treated by a statistical model based on an exponential size distribution. Proof-of-concept calculations are limited to shock loading, but show the capability of simulating Pop-plots, initial temperature effect, detonation waves in 2D, detonation shock confinement test, and multi-dimensional effects in a unified fashion based on micro-physics.

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Detonation and deflagration initiation at the focusing of shock waves in combustible gaseous mixture

Abstract. Detonation and deflagration initiation under focusing conditions in a lean hydrogen-air mixture was experimentally investigated. The experiments were carried out in a shock tube equipped with the laser schlieren system and pressure transducers. Two-dimensional wedges (53° and 90°), semi-cylinder and parabola, were used as the focusing elements. The peculiarities of mild and strong ignition inside the reflector cavity were visualized. A hydrogen-nitrogen mixture was taken for comparison between reactive and inert mixture. It was found that mild ignition inside the reflector cavity can lead to detonation initiation outside the cavity. Schlieren pictures of the process were obtained and the dependence of the distance of detonation initiation on Mach number of the incident shock wave was established. Received 30 August 1999 / Accepted 23 February 2000     

Some observations of the controlled generation and onset of detonation

Detailed observation of deflagration to detonation transition (DDT) is inherently difficult. This is primarily due to the stochastic nature of flame acceleration and shock formation processes that in most practical situations give rise to the conditions required for detonation to emerge. The present paper describes how shock tube techniques have been used to control the conditions required for the onset of detonation. The paper first outlines some initial experiments involving turbulent flame acceleration before concentrating on experiments in a reflected shock mode. To aid interpretation of the observations the paper also presents a simple gasdynamic analysis of particle trajectories and considers the various physical and chemical processes that could lead to the onset of detonation. Received 27 November 2001 / Accepted 28 January 2002     

考虑微观结构特征长度演化的内变量黏塑性本构模型

在金属晶体材料高应变率大应变变形过程中,存在强烈的位错胞尺寸等微观结构特征长度细化现象,势必对材料加工硬化、宏观塑性流动应力产生重要影响。基于宏观塑性流动应力与位错胞尺寸成反比关系,提出了一种新型的BCJ本构模型。利用位错胞尺寸参数,修正了BCJ模型的流动法则、内变量演化方程,引入了考虑应变率和温度相关性的位错胞尺寸演化方程,建立了综合考虑微观结构特征长度演化、位错累积与湮灭的内变量黏塑性本构模型。应用本文模型,对OFHC铜应变率在10-4~103 s-1、温度在298~542 K、应变在0~1的实验应力-应变数据进行了预测。结果表明:在较宽应变率、温度和应变范围内,本文模型的预测数据与实验吻合很好;与BCJ模型相比,对不同加载条件下实验数据的预测精度均有较大程度的提高,最大平均相对误差从9.939%减小为5.525%。     

爆轰加载下柱壳断裂时刻微小差异测量方法

对于断裂时刻差异较小的对比实验,提出一种判读方法:同时采用高速摄影和干涉测速,利用高速摄影判读壳体断裂时刻应变,利用干涉测速获取壳体位移及应变曲线,两者结合得出较为精确的壳体断裂时刻差异。利用该方法得出45钢柱壳在JO-9159和JOB-9003两种炸药加载下断裂时间相差0.45 μs,钨合金柱壳在两种炸药加载下的断裂时间相差0.39 μs。同时该方法可以推广应用于单发壳体膨胀断裂实验中,更精确测定壳体的断裂时刻。     

Detonability of simple and representative components of pyrolysis products of kerosene: pulsed detonation engine application

The use of liquid fuels such as kerosene is of interest for the Pulse Detonation Engine (PDE). In this context, a representative gaseous mixture of the lighter products resulting from the decomposition of a kerosene of type JP-10 was studied. The detonability limits of simple components (hydrogen, ethylene, propylene) and mixtures of these components were tested in a 50 mm diameter and 2.5 m long detonation tube. This dimension is compatible with the applications of the aircraft industry and more particularly the PDE. The influences of the nitrogen dilution, geometry of the DDT device (Schelkin spiral), ignition energy and initial pressure were investigated. This paper was based on work that was presented at the 19th International Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003. Communicated by J.E. Shepherd     

The influence of detonation cell size and regularity on the propagation of gaseous detonations in granular materials

This paper presents results from an experimental study of transmission of gaseous detonation waves through various granular filters. Spherical glass beads of 4 and 8 mm diameter and crushed rock of 7.5 mm volume averaged diameter were used as filter material. Varying the initial pressure of the detonating gas mixture controlled the detonation cell size. Dilution with argon was used to vary the detonation cell regularity. The complete range from almost no detonation velocity deficit to complete extinction of the combustion wave was observed. The existing correlation for gaseous detonation velocity deficit where is the critical diameter for the gaseous detonation and is the pore size, is found to be applicable for both smooth spherical particles and irregular crushed rock when considering irregular detonation structures. Soot films and pressure measurements show that as the detonation cell size is increased, reinitiation of a reanular filter until it finally no longer occurs at . Complete extinction of the combustion wave occurs at . These two limits appear to be about the same for irregular and regular detonation cell structures. For irregular structures without argon dilution, can be found for detonation wave failure, and can be found for complete extinction of the combustion wave. For argon dilution these limits are changed to and , respectively. The data are a bit scarce as a basis for proposing a new correlation for regular structures, but as a first approximation log is suggested for regular structures. The detonation or combustion wave is found to approach a constant velocity in the granular filter if not extinguished. Received 31 October 2001 / Accepted 15 July 2002 Published online 4 November 2002 Correspondence to: T. Slungaard (e-mail: slung@maskin.ntnu.no) An abridged version of this paper was presented at the 18th Int. Colloquium on the Dynamics of Explosions and Reactive Systems at Seattle, USA, from July 29 to August 3, 2001     

Natural gas ignition delay times behind reflected shock waves: Application to modelling and safety

Ignition delay times have been measured behind reflected shock waves using infrared and ultraviolet emission spectroscopies in the wide ranges of temperature (1485-1900 K), pressure (0.3-1.3 MPa), equivalence ratio (0.5-2) and dilution with argon (95-99%). For a mixture composed of several alkanes, it has been shown that the ignition delay can be evaluated from a combination of individual expressions obtained for each alkane (methane, ethane and propane). This method is applied to ignition delay time calculations for mixtures of several natural gases and validated with a particular Algerian natural gas. The measured ignition delay time are compared with those computed using a detailed kinetic mechanism. Agreement is generally satisfactory but the deviation between measurements and calculations increases when the ethane relative fraction increases in natural gas. Dynamic parameters of detonation in air are evaluated from these correlations. The predicted kinetic parameters of detonation (cell width) with a natural gas from Abu Dhabi, especially rich in ethane, are about half as those estimated with a Russian natural gas.Received: 16 April 2002, Accepted: 4 October 2002, Published online: 8 May 2003     

Effect of an inhibitor on high-speed turbulent flames and the transition to detonation

The influence of an inhibitor (CF₃Br or Halon 1301) on the propagation of high-speed turbulent flames, quasi-detonations and the transition to detonation has been investigated for methane-air, propane-air and acetylene-air mixtures. The experiments are carried out in a 13 m tube (15 cm diameter) filled with regularly spaced orifice plates (blockage ratio of 0.39) to ensure rapid flame acceleration. In all cases, the addition of the inhibitor reduces the turbulent flame velocity and extinguishes the flame with sufficient inhibitor concentration (2.7% and 7.5% for methane-air and propane-air, respectively). For acetylene-air mixtures, the quasi-detonation speed is progressively reduced with increasing inhibitor concentration and eventually causes the failure of the quasi-detonation and transition back to a fast turbulent flame. The inhibitor also narrows the propagation limits in all cases. To elucidate the inhibition mechanism, detailed modelling of both the turbulent flame structure as well as the chemical kinetics are required.