Initiation and propagation of detonation waves in combustible high speed flows

The initiation and propagation of detonation waves in combustible high speed flows were studied experimentally. A planar detonation wave traveling in an initiation tube was transmitted into a test section where a combustible high speed flow was induced by an incident shock wave generated in a shock tube. In this study, the flow Mach numbers were obtained as 0.9 and 1.2. The experimental results show that depending on the flow velocity, the apparent propagation velocity of a detonation wave is higher in the upstream and lower in the downstream direction than the CJ velocity. Smoked plate records reveal cellular patterns deformed in the flow direction, and the calculated aspect ratios of the cell were found to agree well with the experimental ones on the basis of the assumption that the velocity of the transverse wave is not affected by the flowing mixture. By analyzing the shock-wave diffraction at the position where there is an abrupt change in the area, on the basis of Whitham’s theory, it was deduced that in the present experimental set-up, the detonation was initiated by the reflection of the diffracted shock waves on the sidewalls of the test section. The agreement between the experimental and calculated results regarding the position of the cellular patterns on the smoked plate record indicated that the position of detonation initiation in high speed flows is shifted downstream due to the flow velocity.     

An experimental investigation of the onset of detonation

An experimental configuration is devised in the present investigation whereby the condition at the final phase of the deflagration to detonation transition (DDT) process can be generated reproducibly by reflecting a CJ detonation from a perforated plate. The detonation products are transmitted downstream through the plate, generating a turbulent reaction front that mixes with the unburned mixture and that drives a precursor shock ahead of it at a strength of about M = 3. The gasdynamic condition that is generated downstream of the perforated plate closely corresponds to that just prior to the onset of detonation in the DDT process. The turbulence parameters can be controlled by varying the geometry of the perforated plate; thus, the condition leading to the onset of detonation can be experimentally investigated. A one-dimensional theoretical analysis of the steady wave processes was first performed, and the experimental results show good agreement, indicating that the present experimental condition can be theoretically described. Two different detonation tube geometries (one with a square cross-section of 300 mm by 300 mm and the other with a circular cross-section of 150 mm) are used to demonstrate the independence of the tube diameter at the critical condition for DDT. Perforated plates with different hole diameters (d = 8, 15, and 25 mm) were tested, and the hole spacing to hole diameter ratio was maintained at 0.5. Different hydrogen–air mixtures were tested at normal temperature and pressure. For the plate with 8 mm holes, the onset of detonation is never observed. For the plate with 15 mm holes, successful initiation of a detonation is achieved for 0.8 < < 1.75 in both detonation tubes. For the plate with 25 mm holes, detonation initiation is observed for 0.7 < < 2.1 in the square detonation tube and for 0.8 < < 1.6 in the smaller circular detonation tube.     

Experimental characterization of detonation initiation modes in a confined chamber flush-mounted with small-diameter ignition tubes

This work reports the experimental characterization of detonation initiation modes in a confined chamber in respect to the different types of reacting waves generated in various small-diameter ignition tubes. Depending on the length of the tube and mixtures composition, four types of reacting waves can be generated and utilized to initiate detonation in the main chamber, namely the over-driven detonation ignition wave, CJ detonation ignition wave, high-speed deflagration ignition wave and deflagration ignition wave. Based on the mechanisms of detonation initiation in the main chamber, four initiation modes can be observed: the direct initiation, the local explosion initiation, and the fast and slow deflagration-to-detonation transition (DDT) initiation. By comparing the detonation initiation positions and flame-tip velocities, the first two modes show appreciably shorter initiation distances compared to the DDT modes. The over-driven detonation ignition wave is shown to yield a high probability of direct initiation, while contrary to expectation, the high-speed deflagration ignition wave exhibits superior initiation performance compared to the CJ detonation ignition wave. It is illustrated that the energy decay through diffraction and the effect of precursor shock wave reflection on the wall of the rectangular chamber are viable factors responsible for this observation. The deflagration ignition wave is also shown to be able to rapidly initiate the detonation near the inlet of the chamber, albeit with a lower success rate.     

Direct blast initiation of spherical gaseous detonations in highly argon diluted mixtures

In this study, direct initiation of spherical detonations in highly argon diluted mixtures is investigated. Direct initiation is achieved via a high voltage capacitor spark discharge and the critical energy is estimated from the analysis of the current output. Stoichiometric acetylene–oxygen mixtures highly diluted with 70% argon is used in the experiment. Previous investigations have suggested that detonations in mixtures that are highly diluted with argon have been shown to be “stable” in that the reaction zone is at least piecewise laminar described by the ZND model and cellular instabilities play a minor role on the detonation propagation. For the acetylene–oxygen mixture that is highly diluted with argon, the experimental results show that the critical energy where the detonation is “stable” is in good agreement with the Zel’dovich criterion of the cubic dependence on the ZND reaction length, which can be readily determined using the chemical kinetic data of the reaction. The experimental results are also compared with those estimated using Lee’s surface energy model where empirical data on detonation cell sizes are required. Good agreement is found between the experimental measurement and theoretical model prediction, where the breakdown of the 13λ relationship for critical tube diameter – and hence a different propagation and initiation mechanism – is elucidated in highly argon diluted mixtures and this appears to indicate that cellular instabilities do not have a prominent effect on the initiation process of a stable detonation.     

激波管内燃料两相云雾爆轰的实验研究

 在立式激波管中采用压力传感器和烟迹技术实测了6种碳氢燃料（环氧丙烷、硝酸异丙酯、C₅~C₆、己烷、庚烷和癸烷）与空气混合物的临界起爆能和爆轰波胞格尺寸。结果表明，燃料气液两相云雾爆轰的临界起爆能与当量比呈“U”形曲线关系，并且最敏感的起爆点在富燃料一侧，这与气相爆轰结论是一致的；由三波点运动的烟迹记录结果分析得出，雾滴的碎解、蒸发汽化过程以及燃烧区前导是控制气液两相云雾爆轰的作用机制。     

Propagation of shock and detonation waves in channels with U-shaped bends of limiting curvature

Systematic experimental and theoretical studies of the propagation of shock and detonation waves in cylindrical tubes and planar channels with two U-shaped bends of limiting curvature were performed. It was demonstrated that U-shaped bends substantially facilitate detonation initiation in gases. The minimum shock wave velocity required to initiate the detonation of a stoichiometric propane-air mixture under normal conditions in a near-critical diameter tube with two U-shaped bends of limiting curvature was found to be ～800 m/s.     

非均匀炸药冲击起爆和起爆后的行为

 介绍并分析了Campbell 等人及其他作者研究非均匀炸药冲击起爆和起爆后行为所获得的实验结果,但不涉及其冲击起爆条件。足够强的冲击波进入非均匀炸药后,爆轰将瞬时（指不经过感应时间）且直接（指不经过其他过程,如爆燃）被引发;非均匀炸药起爆后,其中传播的自始至终是一个不断增长的爆轰波,直至发展为正常爆轰,整个过程都是爆轰的增长（新定义）过程。不存在由反应冲击波不断增长并转变为爆轰波的所谓向爆轰的增长。所谓向爆轰的增长,实际上是爆轰的增长（按新定义）的初期;Craig原定义的爆轰的增长,实际上是爆轰的增长（按新定义）的后期;而所谓反应冲击波,实际上是增长中的初期爆轰波。爆轰的增长（按新定义）是所有猛炸药的特性,炸药反应不充分并逐渐趋于充分是爆轰的增长的化学机制。     

Performance enhancement of a pulse detonation rocket engine

Utilizing liquid kerosene as the fuel, oxygen as oxidizer and nitrogen as purge gas, a series of multi-cycle detonation experiments was conducted to improve the performance of pulse detonation rocket engine (PDRE). In order to improve the performance of the engine, it is crucial to develop an effective DDT enhancement device with less flow loss and higher survival in hostile detonation tube; therefore, three spiraling internal grooves were tested. The three spiraling internal grooves were semicircle, square and inversed-triangle grooves, respectively. The results showed that the spiraling internal groove can effectively enhance DDT and prolong the operation time of PDRE. The effect of groove shape on thrust enhancement of PDRE and the optimum length of spiraling groove were then investigated. To improve the detonability of liquid kerosene and prolong the durability of PDRE, experiments on the kerosene preheating based on active cooling were conducted. The results demonstrated that with the aid of fuel preheating, the detonation initiation time for liquid kerosene was noticeably reduced and a fully-developed detonation wave was achieved in the position away from igniter 4.67 times the diameter of the detonation tube. By adding the additive to liquid kerosene, the detonation initiation time from 0.75 ms decreased to 0.34 ms and the detonability of fuel was dramatically improved. Finally, experiments were conducted to investigate the effects of the operating frequency on the detonation parameters, the fill fraction and PDRE performance. The results indicated that detonation pressure and temperature vary with the operating frequency of PDRE, and the fill fraction has a significant influence on the specific impulse of PDRE. With the strategy of partial filling in detonation tube, the specific impulse can be remarkably enhanced.     

Combustion wave propagation and detonation initiation in the vicinity of closed-tube end walls

There are not many studies on DDT with no obstacles and the initiation of DDT near the end of a closed tube. Therefore in the present study we experimentally investigate the mechanism of the combustion wave transition to a detonation wave when there are no obstacles. In particular, we show that a local explosion near the tube wall is necessary for the initiation of a detonation. Parameters that we varied are the wall configuration, distance between the ignition point and the wall, and initial filling pressure. The combustion waves and the compression waves are visualized using the Schlieren optical system. From the results, we found it is necessary for the combustion wave to reach four walls so that the detonation could be initiated by the local explosion. In the conditions of the present experiment, we exhibited that the local explosion did not occur in the vicinity of a single wall and four orthogonal walls; instead, the local explosion occurred in a situation with five orthogonal walls. The time of the local explosion and the detonation initiation is 2.6 ± 1.1 and 2.0 ± 0.1 times the characteristic time for the combustion wave to propagate hemispherically from an ignitor and reach the four walls.     

均匀炸药冲击起爆和起爆后的行为

 介绍并分析了Campbell等人研究均匀炸药冲击起爆和起爆后行为所获得的实验结果,但不涉及其冲击起爆条件。Campbell等人的实验表明,足够强的冲击波进入硝基甲烷后,经过若干微秒的感应时间,爆轰发生在隔板与炸药间的界面处。这就是说,在均匀炸药中,足够强的冲击虽非瞬时但直接（指不经过其它过程,如爆燃）引发了爆轰。重新处理后的实验数据表明：硝基甲烷起爆后,爆轰波的净爆速小于正常爆速;当进入硝基甲烷的初始冲击波的有效压力p_eff由8.82 GPa升至12.14 GPa时,感应时间t_ind的实验值由3.06 μs降至0.705 μs。以两相的排平（A,m）物态方程描述爆轰产物,较为严格地重新推导了基于热起爆理论的估算感应时间t_ind的公式。在上述p_eff的变化范围内,t_ind的理论值则由248 μs降至0.99 μs,明显地高于实验值。这表明,热起爆理论不适于描述硝基甲烷的冲击起爆行为。从本质上讲,热起爆理论对均匀炸药的冲击起爆行为的描述,不符合物质运动的微观图像,因此,它不适于描述均匀炸药的上述行为。     

脉冲爆轰发动机热射流起爆机理数值分析

应用频散可控耗散差分格式,求解具有化学反应项的Euler方程,探讨了热射流起爆可燃混合气缩短DDT过程的物理机制.数值研究模拟了不同条件下的起爆过程,从氢氧链式反应出发详细分析了氢氧爆轰直接起爆的SWACER(能量释放而形成激波或压缩波的相干放大)机制的建立条件,讨论了热射流起爆存在超临界、临界和亚临界三种直接起爆机制.     

多管PDE爆轰噪声传播过程物理特性实验研究

为研究多管脉冲爆轰发动机爆轰噪声传播过程及声波物理特性,对单管至四管脉冲爆轰发动机爆轰噪声物理特性开展实验研究,获得了多管脉冲爆轰发动机爆轰噪声的波形、声压衰减规律、辐射特性、持续时间和频谱特性等物理参数.结果表明,在管口区域,爆轰噪声峰值衰减较快,在远离管口区域,衰减速率逐渐放缓.随着爆轰管数量的增加,爆轰峰值噪声在...     

Effect of actuating frequency on plasma assisted detonation initiation

Aiming at studying the influence of actuating frequency on plasma assisted detonation initiation by alternating current dielectric barrier discharge, a loosely coupled method is used to simulate the detonation initiation process of a hydrogenoxygen mixture in a detonation tube at different actuating frequencies. Both the discharge products and the detonation forming process which is assisted by the plasma are analyzed. It is found that the patterns of the temporal and spatial distributions of discharge products in one cycle are not changed by the actuating frequency. However, the concentration of every species decreases as the actuating frequency rises, and atom O is the most sensitive to this variation, which is related to the decrease of discharge power. With respect to the reaction flow of the detonation tube, the deflagration-todetonation transition(DDT) time and distance both increase as the actuating frequency rises, but the degree of effect on DDT development during flow field evolution is erratic. Generally, the actuating frequency affects none of the amplitude value of the pressure, temperature, species concentration of the flow field, and the combustion degree within the reaction zone.     

Effect of the spatial configuration of the corona discharge on the initiation of detonation in gas mixtures

The effect of the spatial configuration of corona discharge plasma on the initiation of detonation in the shock tube was examined. The dependences of the combustion wave velocity for propane-butane-oxygen-nitrogen mixtures on the mixture stoichiometry and nitrogen content were obtained. The optimal configuration of the electrodes was determined. A model of the process was proposed, and its predictions were compared to the experimental results. It was established that the effect of a pulsed corona discharge could be explained by the formation of local overheated areas.     

Experimental study of the influence of annular nozzle on acoustic characteristics of detonation sound wave generated by pulse detonation engine

Acoustic characteristics of the detonation sound wave generated by a pulse detonation engine with an annular nozzle, including peak sound pressure, directivity, and A duration, are experimentally investigated while utilizing gasoline as fuel and oxygen-enriched air as oxidizer. Three annular nozzle geometries are evaluated by varying the ratio of inner cone diameter to detonation tube exit diameter from 0.36 to 0.68. The experimental results show that the annular nozzles have a significant effect on the acoustic characteristics of the detonation sound wave. The annular nozzles can amplify the peak sound pressure of the detonation sound wave at 90° while reducing it at 0° and 30°. The directivity angle of the detonation sound wave is changed by annular nozzles from 30° to 90°. The A duration of the detonation sound wave at 90° is also increased by the annular nozzles. These changes indicate that the annular nozzles have an important influence on the acoustic energy distribution of the detonation sound wave, which amplify the acoustic energy in a direction perpendicular to the tube axis and weaken it along the direction of the tube axis.     

碳氢混合燃料（C5~C6）气云爆轰胞格结构实验研究

 改进烟迹技术之后，在初始为常温常压的条件下，进行了碳氢混合燃料（C₅~C₆）的气云爆轰胞格结构实验研究，得到了清晰的胞格烟迹记录，同时讨论了燃料气云的当量比和起爆能对胞格结构的影响。实验表明：C₅~C₆混合燃料的爆轰波胞格宽度和胞格长度均随当量比的增加而线性增大；随着起爆能的不断增大，混合燃料的胞格宽度和胞格长度均是先增大后减小，且当起爆能足够高时，在胞格结构内可观察到精细结构的存在。     

铝粉-空气混合物爆燃转爆轰过程及爆轰波结构

 在长为32.4 m、内径为0.199 m的大型长直水平管道中,对铝粉-空气两相流的燃烧转爆轰（DDT）过程及爆轰波结构进行了实验研究。对铝粉-空气混合物弱点火条件下DDT过程不同阶段的特征进行了分析,实验结果显示混合物经历了缓慢反应压缩阶段、压缩波加速冲击波形成阶段、冲击反应过渡阶段、冲击反应向过压爆轰过渡阶段和爆轰阶段,得到了混合物各阶段的DDT参数,由此进一步分析了DDT浓度的上、下限。在1.4 m爆轰测试段的4个截面的环向上各均匀安装8个传感器,对爆轰波结果进行测试,并对铝粉-空气混合物爆轰波的单头结构进行了分析。     

乙炔-氧气混合气体强爆轰参数的理论估算与实验研究

 针对非定常的气相强爆轰过程,建立了气相爆轰的理论计算模型,结合C-J理论和多方气体物态方程,对乙炔-氧气混合气体的强爆轰参数进行了理论估算,并在激波管中开展了化学计量比的乙炔-氧气混合气体的强爆轰实验。对比研究表明：爆速的理论估算值与实验值符合较好,证实了采用C-J理论估算气相强爆轰参数的可行性,计算数据具有一定的参考价值。     

Experimental observations of detonation in ammonium-nitrate-fuel-oil (ANFO) surrounded by a high-sound-speed, shockless, aluminum confiner

Detonations in explosive mixtures of ammonium-nitrate-fuel-oil (ANFO) confined by aluminum allow for transport of detonation energy ahead of the detonation front due to the aluminum sound-speed exceeding the detonation velocity. The net effect of this energy transport on the detonation is unclear. It could enhance the detonation by precompressing the explosive near the wall. Alternatively, it could decrease the explosive performance by crushing porosity required for initiation by shock compression or destroying confinement ahead of the detonation. At present, these phenomena are not well understood. But with slowly detonating, non-ideal high explosive (NIHE) systems becoming increasing prevalent, proper understanding and prediction of the performance of these metal-confined NIHE systems is desirable. Experiments are discussed that measured the effect of ANFO detonation energy transported upstream of the front by a 76-mm-inner-diameter aluminum confining tube. Detonation velocity, detonation front-shape, and aluminum response are recorded as a function of confiner wall thickness and length. Detonation shape profiles display little curvature near the confining surface, which is attributed to energy transported upstream modifying the flow. Average detonation velocities were seen to increase with increasing confiner thickness, while wavefront curvature decreased due to the stiffer, subsonic confinement. Significant radial sidewall tube motion was observed immediately ahead of the detonation. Axial motion was also detected, which interfered with the front-shape measurements in some cases. It was concluded that the confiner was able to transport energy ahead of the detonation and that this transport has a definite effect on the detonation by modifying its characteristic shape.     

氢氧爆轰波在激波管中的成长机制研究

 针对气相爆轰波成长机制研究,采用压力传感器和高速摄影技术,测试了氢氧混合气体在点火后的火焰波、前驱冲击波以及爆轰波的成长变化过程,计算了冲击波过程参数和气体状态参数,分析了火焰加速机制。实验结果表明,APX-RS型高速摄影系统可用于拍摄气相爆轰波的成长历程;氢氧爆轰波的产生是由于湍流火焰和冲击波的相互正反馈作用,导致反应区内多处发生局部爆炸,爆炸波与冲击波相互耦合,最终成长为定常爆轰波。