Experimental observation of the onset of detonation downstream of a perforated plate

In this study, the onset of detonation downstream of a perforated plate subsequent to the reflection of a Chapman–Jouguet detonation upstream is investigated. The experiments were performed with C₃H₈ + 5O₂ and C₂H₂+2.5O₂+70%Ar. The former has a much more irregular transverse wave pattern whereas the latter is known to have a piecewise laminar structure with a regular cellular structure. The onset of detonation phenomenon was found to be significantly different for the two mixtures. For the high argon diluted mixtures, the onset of detonation occurs in the vicinity downstream of the perforated plate. However, if the onset of detonation does not occur close to the plate, the precursor shock decouples from the reaction zone and a deflagration results. For the propane–oxygen mixtures, the onset of detonation is found to occur relatively far from the perforated plate at critical conditions. The major difference between these two mixtures is that the metastable turbulent reaction front can be maintained for relatively long distances for the propane–oxygen mixture. This turbulent metastable regime is also observed to be able to maintain a relatively constant propagation velocity for many channel widths prior to the onset of detonation. For the propane–oxygen mixtures, the onset is caused by a strong local explosion within the turbulent reaction zone.     

A study on jet initiation of detonation using multiple tubes

A detonator consisting of a dense bundle of small-diameter tubes (4.4–19 mm) is tested experimentally using stoichiometric mixtures of hydrogen–oxygen and hydrogen–air. Tests are conducted in a 5,200-mm long detonation tube fitted with a schlieren photograph section and smoked foil to record the deflagration to detonation (DDT) transition. It is confirmed that the flame jet emanating from the tube assembly causes detonation initiation immediately downstream of the detonator, with little dependence on the size of the detonation tube. For the fuel–air mixture, the insertion of Shchelkin spirals into each of the smaller tubes enhances the development of the turbulent flame jet, leading to a shorter DDT distance. Multi-point spark ignition is also shown to provide a further reduction in the DDT distance compared to single-point ignition. PACS 47.40.-x; 47.40.Nm; 47.70.Fw; 82.40.-g; 82.40.Fp     

Measurement and chemical kinetic model predictions of detonation cell size in methanol–oxygen mixtures

In this study, detonation cell sizes of methanol–oxygen mixtures are experimentally measured at different initial pressures and compositions. Good agreement is found between the experiment data and predictions based on the chemical length scales obtained from a detailed chemical kinetic model. To assess the detonation sensitivity in methanol–oxygen mixtures, the results are compared with those of hydrogen–oxygen and methane–oxygen mixtures. Based on the cell size comparison, it is shown that methanol–oxygen is more detonation sensitive than methane–oxygen but less sensitive than hydrogen–oxygen.     

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     

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。     

A Numerical Study Promoting Algebraic Models for the Lewis Number Effect in Atmospheric Turbulent Premixed Bunsen Flames

This numerical investigation carried out on turbulent lean premixed flames accounts for two algebraic – the Lindstedt–Vaos (LV) and the classic Bray–Moss–Libby (BML) – reaction rate models. Computed data from these two models is compared with the experimental data of Kobayashi et al. on 40 different methane, ethylene and propane Bunsen flames at 1 bar, where the mean flame cone angle is used for comparison. Both models gave reasonable qualitative trend for the whole set of data, in overall. In order to characterize quantitatively, firstly, corrections are made by tuning the model parameters fitting to the experimental methane–air (of Le = 1.0) flame data. In case of the LV model, results obtained by adjusting the pre-constant, i.e., reaction rate parameter, C_R, from its original value 2.6 to 4.0, has proven to be in good agreement with the experiments. Similarly, for the BML model, with the tuning of the exponent n, in the wrinkling length scale, L_y = C_l⋅ l_x(s_L/u′)ⁿ from value unity to 1.2, the outcome is in accordance with the measured data. The deviation between the measured and calculated data sharply rises from methane to propane, i.e., with increasing Lewis number. It is deduced from the trends that the effect of Lewis number (for ethylene–air mixtures of Le = 1.2 and propane–air mixtures of Le = 1.62) is missing in both the models. The Lewis number of the fuel–air mixture is related to the laminar flame instabilities. Second, in order to quantify for its influence, the Lewis number effect is induced into both the models. It is found that by setting global reaction rate inversely proportional to the Lewis number in both the cases leads to a much better numerical prediction to this set of experimental flame data. Thus, by imparting an important phenomenon (the Lewis number effect) into the reaction rates, the generality of the two models is enhanced. However, functionality of the two models differs in predicting flame brush thickness, giving scope for further analysis.     

Limits and mechanism of detonation re-initiation behind a multi-orifice plate

An experimental investigation was performed to establish the dependence of concentration limits of detonation re-initiation behind a multi-orifice plate on mixture composition and initial pressure for hydrogen–air mixtures. The experiments were carried out in detonation tubes of diameter 106 and 141 mm, separated by a multi-orifice plate into two sections. The tubes were equipped with pressure gauges and a semi-cylindrical smoked plate. It is shown that initial pressure has strong influence on the value of concentration limit, especially for lean hydrogen–air mixtures. On the basis of soot records it can be inferred that re-initiation occurs due to two different mechanisms that depend on the mixture sensitivity and properties of the multi-orifice plate.     

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.     

Comparison of critical conditions for DDT in regular and irregular cellular detonation systems

Abstract. The results of an experimental study of DDT in mixtures with regular and irregular detonation cellular structures are presented. Experiments were carried out in a tube 174 mm i. d. with obstacles (blockage ratios were 0.1, 0.3, and 0.6). Mixtures used were hydrogen–air and stoichiometric hydrogen–oxygen diluted with , Ar, and He. The critical conditions for DDT are shown to depend on the regularity of the cellular structure of test mixtures. The critical values of the cell sizes in Ar- and He-diluted mixtures are shown to be significantly smaller than those in -diluted mixtures. This means that systems with a highly regular detonation cellular structure have far less capacity for undergoing DDT compared to irregular ones with the same values of detonation cell sizes. Received 18 November 1999 / Accepted 15 May 2000     

两种不同气体中的高速爆燃波及其向爆轰的转变

对2C2H2 5O2及2C2H2 5O2 80%Ar两种可燃混合气体中的高速爆燃波及其向爆轰的转变过程进行实验研究.高速爆燃波由孔栅干涉爆轰波的方法直接生成,观测手段则以高速转鼓摄影获取孔栅近场流场x-t纹影图,以传感器追踪波面的后继发展.研究发现,两种气体中的爆燃波具有迥异的特性.前者燃烧波面在较低初压条件下为层流结构,而较高初压下为湍流结构,向爆轰转变点可以延伸至下游较长距离;后者在不同初压条件下燃烧波面无明显差异,爆轰的再次形成只能在孔栅下游近场内建立.两种气体中高速爆燃波的维持和爆轰转变过程均非纯粹激波压缩所致,湍流输运在其中起着必不可少的作用.分析显示,激波压缩效应对纯氧炔气体的高速爆燃和DDT贡献较小,湍流输运占主导地位;而氩气稀释气体较为稳定,缺乏自行衍生剧烈湍流燃烧的能力,因而激波压缩和外界扰动对其高速爆燃传播和爆轰转变起十分重要的作用.     

Detonability of THDCPD-exo–air mixtures

In the frame of industrial risk and propulsive application, the detonability study of JP10–air mixtures was performed. The simulation and measurements of detonation parameters were performed for THDCPD-exo/air mixtures at various initial pressure (1 bar < P ₀ < 3 bar) and equivalence ratio (0.8 < Φ < 1.6) in a heated tube (T ₀ ~ 375 K). Numerical simulations of the detonation were performed with the STANJAN code and a detailed kinetic scheme of the combustion of THDCPD. The experimental study deals with the measurements of detonation velocity and cell size λ. The measured velocity is in a good agreement with the calculated theoretical values. The cell size measurements show a minimum value for Φ ~ 1.2 at every level of initial pressure studied and the calculated induction length L _i corresponds to cell size value with a coefficient k = λ/L _i = 24 at P ₀ = 1 bar. Based on the comparison between the results obtained during this study and those available in the literature on the critical initiation energy E _c, critical tube diameter d _c and deflagration to detonation transition length L _DDT, we can conclude that the detonability of THDCPD–air mixtures corresponds to that of hydrocarbon–air mixtures.

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This paper is based on the work presented at the 33rd International Pyrotechnics Seminar, IPS 2006, Fort Collins, July 16–21, 2006.     

DDT in a smooth tube filled with a hydrogen–oxygen mixture

Results of experimental study on DDT in a smooth tube filled with sensitive mixtures having detonation cell size from 1 to 3 orders of magnitude smaller than the tube diameter are presented. Stoichiometric hydrogen–oxygen mixtures were used in the tests with initial pressure ranging from 0.2 to 8 bar. A dependence of the run-up distance to DDT on the initial pressure is studied. This dependence is found to be close to the inverse proportionality. It is suggested that the flow ahead of the flame results in formation of the turbulent boundary layer. This boundary layer controls the scale of turbulent motions in the flow. A simple model to estimate the maximum scale of the turbulent pulsations (boundary layer thickness) at flame positions along the tube is presented. The largest scale of the turbulent motions at the location of the onset of detonation is shown to be 1 order of magnitude greater than the detonation cell widths, λ, in all the tests. It is suggested that the onset of detonation is triggered during flame acceleration as soon as the maximum scale of the turbulent pulsations increases up to about 10 λ. The model to estimate the maximum size of turbulent motions, δ, and the correlation δ≈ 10λ, give a basis for estimations of the run-up distances to DDT in tubes with internal diameter D > 20λ. PACS 47.40.-x; 47.27.Nz 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     

Detonation initiation developing from the Richtmyer–Meshkov instability

Detonation initiation resulting from the Richtmyer–Meshkov instability is investigated numerically in the configuration of the shock/spark-induced-deflagration interaction in a combustive gas mixture. Two-dimensional multi-species Navier–Stokes equations implemented with the detailed chemical reaction model are solved with the dispersion-controlled dissipative scheme. Numerical results show that the spark can create a blast wave and ignite deflagrations. Then, the deflagration waves are enhanced due to the Richtmyer–Meshkov instability, which provides detonation initiations with local environment conditions. By examining the deflagration fronts, two kinds of the initiation mechanisms are identified. One is referred to as the deflagration front acceleration with the help of the weak shock wave, occurring on the convex surfaces, and the other is the hot spot explosion deriving from the deflagration front focusing, occurring on the concave surfaces. The project supported by the National Natural Science Foundation of China (90205027 and 10632090).     

DDT in fuel–air mixtures at elevated temperatures and pressures

An experimental study was carried out to investigate flame acceleration and deflagration-to-detonation transition (DDT) in fuel–air mixtures at initial temperatures up to 573 K and pressures up to 2 atm. The fuels investigated include hydrogen, ethylene, acetylene and JP-10 aviation fuel. The experiments were performed in a 3.1-m long, 10-cm inner-diameter heated detonation tube equipped with equally spaced orifice plates. Ionization probes were used to measure the flame time-of-arrival from which the average flame velocity versus propagation distance could be obtained. The DDT composition limits and the distance required for the flame to transition to detonation were obtained from this flame velocity data. The correlation developed by Veser et al. (run-up distance to supersonic flames in obstacle-laden tubes. In the proceedings of the 4th International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions, France (2002)) for the flame choking distance proved to work very well for correlating the detonation run-up distance measured in the present study. The only exception was for the hydrogen–air data at elevated initial temperatures which tended to fall outside the scatter of the hydrocarbon mixture data. The DDT limits obtained at room temperature were found to follow the classical d/λ = 1 correlation, where d is the orifice plate diameter and λ is the detonation cell size. Deviations found for the high-temperature data could be attributed to the one-dimensional ZND detonation structure model used to predict the detonation cell size for the DDT limit mixtures. This simple model was used in place of actual experimental data not currently available. 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     

Aluminum Particles–air Detonation at Elevated Pressures

The effect of initial pressure on aluminum particles–air detonation was experimentally investigated in a 13 m long, 80 mm diameter tube for 100 nm and 2 μm spherical particles. While the 100 nm Al–air detonation propagates at 1 atm initial pressure in the tube, transition to the 2 μm aluminum–air detonation occurs only when the initial pressure is increased to 2.5 atm. The detonation wave manifests itself in a spinning wave structure. An increase in initial pressure increases the detonation sensitivity and reduces the detonation transition distance. Global analysis suggests that the tube diameter for single-head spinning detonation or characteristic detonation cell size would be proportional to (d ₀: aluminum particle size, p ₀: initial pressure). Its application to the experimental data results in m ~ O(1) and n ~ O(1) for 1 to 2 μm aluminum–air detonation, thus indicating a strong dependence on initial pressure and gas-phase kinetics for the aluminum reaction mechanism in detonation. Hence, combustion models based on the fuel droplet diffusion theory may not be adequate in describing micrometric aluminum–air detonation initiation, transition and propagation. For 2 μm aluminum–air mixtures at 2 atm initial pressure and below, experiments show a transition to a “dust quasi-detonation” that propagates quasi-steadily with a shock velocity deficit nearly 40% with respect to the theoretical C–J detonation value. The dust quasi- detonation wave can propagate in a tube with a diameter less than 0.4–0.5 times the diameter required for a spinning detonation wave.     

气/固/液三相混合物燃烧转爆轰过程实验研究

利用多相燃烧爆炸实验系统,通过高压喷粉/喷雾以及高能点火等过程,对化学当量比条件下3种 典型燃料空气炸药,即硝基甲烷/铝粉/空气、硝酸异丙酯/铝粉/空气、乙醚/铝粉/空气三相混合物的燃烧转爆 轰过程进行了实验研究,同时根据实验结果对比了3种三相混合物的燃爆性能。得到了三相悬浮混合物燃烧 转爆轰过程的宏观规律以及三相混合物燃爆性能随质量浓度变化的规律。     

Investigation of detonation initiation in aluminium suspensions

Detonation initiation is investigated in aluminium/oxygen and aluminium/air mixtures. Critical conditions for initiation of spherical detonations are examined in analogy with the criteria defined for gaseous mixtures, which correlate critical parameters of detonation initiation to the characteristic size of the cellular structure. However, experimental data on the detonation cell size in these two-phase mixtures are very scarce, on account of the difficulty to perform large-scale experiments. Therefore, 2D numerical simulations of the detonation cellular structure have been undertaken, with the same combustion model for Al/air and Al/O₂ mixtures. The cell size is found to be λ = 37.5 cm for a rich (r = 1.61) aluminium–air mixture, and λ = 7.5 cm for a stoichiometric aluminium-oxygen mixture, which is in reasonable agreement with available experimental data. Calculations performed in large-scale configurations (up to 25 m in length and 1.5 m in lateral direction) suggest that the critical initiation energy and predetonation radius for direct initiation of the unconfined detonation in the aluminium–air mixture are, respectively, 10 kg of TNT and 8 m. Moreover, numerical simulations reveal that the structure of the detonation wave behind the leading front is even more complicated than in pure gaseous mixtures, due to two-phase flow effects. 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.     

Detonation propagation in hydrogen–air mixtures with transverse concentration gradients

The influence of transverse concentration gradients on detonation propagation in \(\hbox {H}_2\)–air mixtures is investigated experimentally in a wide parameter range. Detonation fronts are characterized by means of high-speed shadowgraphy, OH* imaging, pressure measurements, and soot foils. Steep concentration gradients at low average \(\hbox {H}_2\) concentrations lead to single-headed detonations. A maximum velocity deficit compared to the Chapman–Jouguet velocity of 9 % is observed. Significant amounts of mixture seem to be consumed by turbulent deflagration behind the leading detonation. Wall pressure measurements show high local pressure peaks due to strong transverse waves caused by the concentration gradients. Higher average \(\hbox {H}_2\) concentrations or weaker gradients allow for multi-headed detonation propagation.     

大型水平爆轰管中悬浮铝粉爆炸过程的实验研究

铝粉的燃烧与爆轰性能是粉尘爆炸领域研究的热点.利用长29.6m,内径199mm,配有40套喷粉扬尘系统的水平爆轰管,在40J电火花点火条件下,实现了悬浮铝粉-空气混和物火焰加速、爆燃、爆轰及其转捩过程,测得了爆炸波传播过程中的压力信号,并且观察到了爆轰波的稳定传播现象.实验结果表明,当铝粉浓度为300 g/m3时,在距离点火端10.15m(长径比L/D=51)处发生了DDT,测得的爆轰波传播过程中管内的最大爆速为1840m/s,最大峰值超压为10.5MPa.铝粉尘爆炸波在爆轰管内的传播过程可分为爆燃段、爆燃转爆轰(DDT)、爆轰增强以及稳态爆轰四个阶段.     

爆震室壁面条件对脉冲爆震发动机爆震性能的影响

在爆震室内快速形成稳定传播的爆轰波是脉冲爆震发动机的关键．本文利用有限速率化学反应模型,考虑粘性、热对流,基于N-S方程对氢气与空气/氧气为反应混合物的爆震发动机爆震室内流场进行计算．从流场压力、速度、涡量、湍流动能等方面研究爆震室壁面条件对燃烧爆轰性能的影响,分析流场爆轰波压力与流场湍动能的关系,讨论可燃气体燃烧转爆轰的机理．结果表明：爆震室内燃烧爆轰机理受到化学反应能量释放、壁面摩擦效应、壁面与外界热交换的影响．在文中讨论的范围内,相比于半圆形和三角形的爆震室装置,矩形的爆震室增强装置能在更短的时间内得到较高的爆轰波压力和湍动能峰值．壁面粗糙层高度（粗糙度）影响爆震室的燃烧爆轰性质．当壁面粗糙度为0.15mm时,粗糙度对爆轰的激励作用大于抑制作用,能较快形成稳定的爆轰波,且推力为35.5N;随着壁面对流换热系数的增大,爆震室壁面的散热加剧．当壁面对流换热系数大于临界值2.6W/(m2·K)时,爆震室内不能形成稳定的爆震波．