Photochemical initiation of gaseous detonations

Direct initiation of detonations in gaseous mixtures of C₂H₂-O₂, H₂-O₂ and H₂-Cl₂ in the pressure range of 10–150 torr using flash photolysis was studied. Similar to blast initiation using a concentrated powerful energy source, it was found that for photochemical initiation, there exists a certain threshold of flash intensity and energy for each mixture at any given initial pressure and composition below which a deflagration is formed. At the critical threshold, however, a fully developed detonation is rapidly formed in the immediate vicinity of the window of incident UV radiation. However, at super critical flash energies, the amplitude of the detonation formed decreases and combustion of the entire irradiated volume approaches a constant volume explosion. It was found that photo-chemical initiation requires both a certain minimum peak value of the free radical concentration generated by the photo-dissociation as well as an appropriate gradient of this free radical distribution. The minimum peak radical concentration permits rapid reaction rates for the generation of strong pressure waves, while the gradient is necessary for the amplification of the shock waves to a detonation. If the gradient is absent and the free radicals are uniformly distributed in the mixture, then the entire volume simply explodes as in a constant volume process. The present study reveals that the mechanism of photochemical initiation is one of proper temporal synchronization of the chemical energy release to the shock wave as it propagates through the mixture. In analogy to the LASER, the term SWACER is introduced to represent this mechanism of Shock Wave Amplication by Coherent Energy Release. There are strong indications that this SWACER mechanism is universal and plays the main role in the formation of detonations whenever a powerful concentrated external source is not used to generate a strong shock wave in the explosive.     

爆震波点火器在氢氧塞式喷管的工程应用

建立了氢氧爆震波点火器试验系统,并根据试验塞式喷管发动机工作状态要求设计了爆震波点火器。在高空条件下(0.005￣0.002MPa),爆震波点火器供气压力0.3MPa、混合比3左右,对爆震波点火器的点火性能进行了试验,成功实现了高空条件下爆震波点火火炬。在同样高空条件下对爆震波点火器点燃单元塞式喷管试验发动机成功进行了点火试验。试验结果表明,氢氧爆震波点火器能以较低的供气压力实现可靠点火。爆震波点火器在气氢气氧单元塞式喷管试验发动机点火的成功应用,为下一阶段应用于多管塞式喷管发动机的实际点火试验提供了技术基础。     

A numerical study of the charge diameter effect on unsteady detonation

A numerical study for the unsteady detonation of an unconfined tetryl charge of small diameter, which is assumed to be homogeneous, was performed by using the two-dimensional Lagrangian hydrodynamic computer code, 2 DL, with the first order Arrhenius equation of reaction rate. Becker-Kistiakowsky-Wilson (BKW) and Kihara-Hikita (KH) equations of state have been applied to the detonation products.In the case of BKW, it is shown that the rarefaction waves propagating inward from the lateral surface make the reaction rate slow and give a curvature to the front. Then after an induction time, a strong initiation occurs in the reaction zone near the lateral surface and higher pressure zone moves to the axis. This higher pressure accelerates the detonation propagation near the lateral surface and the curvature of detonation front is reduced. Then, the reaction at the lateral surface again begins to decay by the rarefaction waves. Such a sequence of process is repeated periodically.The possibility of the occurrence of the strong initiation depends on the pressure and temperature in the shocked zone near the surface. In a small diameter charge, the delayed explosion becomes weaker near the surface, while its frequency increases. No shock interaction occurs because the direction of the particle flow is always divergent.In the case of KH equation of state, the temperature of detonation is higher than that obtained by BKW and the behaviour of instability becomes rather different from the previous result, i.e. in the axis the pressure oscillates repeating the overdriven and underdriven detonation similar with the case of BKW.     

Hydrodynamic instabilities and transverse waves in propagation mechanism of gaseous detonations

The present study examines the role of transverse waves and hydrodynamic instabilities mainly, Richtmyer–Meshkov instability (RMI) and Kelvin–Helmholtz instability (KHI) in detonation structure using two-dimensional high-resolution numerical simulations of Euler equations. To compare the numerical results with those of experiments, Navier–Stokes simulations are also performed by utilizing the effect of diffusion in highly irregular detonations. Results for both moderate and low activation energy mixtures reveal that upon collision of two triple points a pair of forward and backward facing jets is formed. As the jets spread, they undergo Richtmyer–Meshkov instability. The drastic growth of the forward jet found to have profound role in re-acceleration of the detonation wave at the end of a detonation cell cycle. For irregular detonations, the transverse waves found to have substantial role in propagation mechanism of such detonations. In regular detonations, the lead shock ignites all the gases passing through it, hence, the transverse waves and hydrodynamic instabilities do not play crucial role in propagation mechanism of such regular detonations. In comparison with previous numerical simulations present simulation using single-step kinetics shows a distinct keystone-shaped region at the end of the detonation cell.     

Cylindrical heterogeneous detonation waves

Further experimental studies of blast wave initiated cylindrical heterogeneous (liquid fuel drops, gas oxidizer) detonation waves are described. A pie-shaped shock tube, used for these studies, was altered in certain ways so as to improve the modeling of cylindrical waves. These modifications, along with some operational aspects, are briefly discussed. The breech of the facility, where the blast wave is generated by an explosive, became distorted with usage. Results are presented which show that lower detonation velocities are realized with the damaged breech (other conditions being the same). A photographic and pressure switch wave time of arrival study was made to ascertain the wave shape. Photographs are shown which show that the waves, blast as well as detonation, are close to cylindrical. However, in some cases there is appreciable distortion of the wave front by debris ahead of the wave. Presumably this debris comes from the blasting cap used to ignite the condensed explosive. A series of experiments was conducted using kerosene drops of 388 μm diameter dispersed in air through use of a large number of hypodermic needles. Radial fuel void regions were established by cutting off the fuel flow to a number of needles. Preliminary results relating to the effect of the size of the cloud gap on detonation velocity, quenching, and the initiator energy levels required for detonation are discussed.     

Pulse detonation engines: Technical approaches

The paper contains analysis of the problems preventing from wide use of pulse detonation engines (PDE), and provides suggestions to overcome those problems. In particular, the results of theoretical investigations of basic operating cycle in PDE—deflagration-to-detonation transition (DDT) processes in combustible gaseous mixtures and transmission of detonation into large chambers—are presented. The paper investigates the effect of implosion shock waves on the onset of detonation in gases, and suggests the scheme of detonation transmission from a narrow gap into a wide chamber, which makes it possible to reduce the predetonation length thus shortening the necessary length of the engine.     

煤油温度对于爆震波形成影响的实验研究

在内径为30mm的脉冲爆震发动机模型上，以煤油为燃料，以空气为氧化剂，成功地进行了两相爆震实验，获得了充分发展的脉冲爆震波。测试了在化学恰当比，不同爆震频率及燃油温度下的爆震波压力，并对其变化进行了分析。通过分析实验结果发现，在化学恰当比下，爆震频率不变时，煤油温度的升高明显促进了爆震的形成，在内径小于混合物胞格尺寸的爆震管内，可以形成充分发展的两相脉冲爆震波。     

Some aspects of blast from fuel-air explosives

Numerical solutions of the blast wave flow from a spherical explosive charge were obtained using the artificial viscosity technique as employed by Oppenheim. The flow is treated as adiabatic and inviscid and ideal equations of state are used for reactants, products and the surrounding air environment. Differences are noted in the peak pressure, static impulse and dynamic impulse resulting from three representative types of idealized initiation: (1) centrally initiated, self-similar Chapman-Jouguet detonation, (2) edge initiated spherical implosion and (3) constant volume energy release followed by sudden venting to the environment. These are compared to the ideal point blast with counterpressure of the same total energy release. In addition, numerical solutions are presented for centrally initiated, stoichiometric hydrogen-oxygen mixtures surrounded by air for detonation and for deflagration according to an empirically determined, non-steady flame velocity. The greater energy transfer to the environment in the case of detonation is demonstrated.     

Shock waves in water foams

Propagation of shock waves in tubes filled with water foams is studied using pressure gauges. Low amplitude shock waves consist of a precursor which propagates at a velocity slightly less than the acoustic velocity in the gas, and of a main compression wave which propagates slower than the precursor. Stronger shock waves have a single front. Maximum pressure rise in the incident and reflected shock waves cannot be calculated using one-dimensional conservation equations at the shock front. It is suggested that the flow of the liquid in foam cells has to be taken into account in order to predict the behavior of shock waves in foams. The nature of the gas which fills the cells is shown to have a strong effect on the quenching of blast waves in foams.     

Initiation of spherical detonation in hydrocarbon/air mixtures

Spherical detonations have been initiated by solid explosive (Tetryl) charges in well-mixed stoicheiometric air mixtures with each of the hydrocarbons, ethane, propane, n-butane, isobutane and ethylene at atmospheric pressure. Prior to initiation, the gases were contained in plastic bags; total gas volume and available path length were up to 1.6 m³ and 2 m, respectively. The detonations were shown to be self-sustained by continuous measurement of detonation velocity using X-band microwave interferometry. Measured detonation velocities were in all cases close to calculated C-J values.In a few experiments close to the limits of detonability, velocity and blast pressure/time records indicated that the propagating wave system is sometimes irregular. The irregularity that occurs just after initiation is characterised by a reaction front velocity very much lower than the constant detonation velocity, but subsequently attaining the latter by an acceleration process. These observations indicate the existence of a dissociated phase in which shock and reaction fronts may no longer be coupled.Because similar experimental conditions were used throughout, it was possible to establish the relative susceptibilities of the various fuel gases to detonation. Comparison is made with the Zeldovich criterion and a detonation kernel theory of Lee.     

Recent experiments in laser supported absorption waves

Recent experimental studies with laser supported absorption waves, arising from intense laser radiation incident upon various surface materials, are discussed. The propagation characteristics of both subsonic (laser supported combustion), intensity about 10⁵ W/cm², and supersonic (laser supported detonation), intensity greater than 10⁶ W/cm², waves are described, including the dependence of wave speed on laser intensity, beam diameter, gas density and laser wave length. Measurements of the plasma properties in such waves are described, and the analysis of these data allows conclusions identifying the dominant transport mechanism in such waves. It is also concluded that in the LSC (subsonic) case, radial flow ahead of and within the wave are dominant features of the wave structure. The equipment and laser facilities used in the experiments are also discussed.     

Effects of a hot jet on detonation initiation and propagation in supersonic combustible mixtures

The effects of a hot jet on detonation initiation and propagation in supersonic combustible mixtures has been studied with two-dimensional numerical simulations with the open-source program AMROC that uses a block-structured adaptive mesh refinement method. Results indicate that the hot jet could ignite the detonation effectively in supersonic combustible mixtures like a pneumatic ramp. After the realization of the detonation initiation, the hot jet can still play an important role on the detonation propagation during its continuous ejection. For a hot jet with certain diameter, it can result in an overdriven detonation with almost constant overdrive degree. After the shutdown of the hot jet, the stable CJ detonation combustion was realized finally in the supersonic combustible mixtures. With the re-ejection of the hot jet, the failed detonation could be reinitiated quickly. Through the control of the re-ejection of the hot jet, it plays a key role not only in the initiation process, but also in the subsequent continuous detonation combustion period.     

Detonation engine fed by acetylene–oxygen mixture

The advantages of a constant volume combustion cycle as compared to constant pressure combustion in terms of thermodynamic efficiency has focused the search for advanced propulsion on detonation engines. Detonation of acetylene mixed with oxygen in various proportions is studied using mathematical modeling. Simplified kinetics of acetylene burning includes 11 reactions with 9 components. Deflagration to detonation transition (DDT) is obtained in a cylindrical tube with a section of obstacles modeling a Shchelkin spiral; the DDT takes place in this section for a wide range of initial mixture compositions. A modified ka-omega turbulence model is used to simulate flame acceleration in the Shchelkin spiral section of the system. The results of numerical simulations were compared with experiments, which had been performed in the same size detonation chamber and turbulent spiral ring section, and with theoretical data on the Chapman–Jouguet detonation parameters.     

A study of detonation in methane/air clouds

In the present paper, we study the problem of detonation in unconfined, gaseous mixtures of methane/oxygen/nitrogen. A numerical simulation approach is employed in which we use a one-dimensional (spherical symmetry), time-dependent computer model to simulate the coupled compressible fluid dynamics-chemical kinetics processes which occur upon direct initiation of detonation. We establish the magnitude of explosive yield of tetryl required to initiate detonation in mixtures of CH₄ + 2O₂ with varying degrees of nitrogen dilution, up to and including stoichiometric . The numerical simulations illustrate the features of direct initiation observed in many experimental investigations, e.g. shock-wave breakaway followed by detonation reestablishment via a quasi-steady, oscillatory flow regime which occurs before the establishment of a steadily propagating spherical detonation. Our results compare well with recent experimental data obtained by Bull et al. (1976) over the range of tetryl masses studied by them. We find that tetryl explosive masses in excess of 10⁷ grams would be required to initiate detonation in an unconfined, stoichiometric mixture.     

Shock compression of liquid hydrogen in various experimental geometries

In connection with the use of cryogenic liquids in high-speed gas dynamics and high-pressure physics, shock-wave processes in liquid hydrogen were investigated under plane, cylindrical and hemispherical loading.The plane loading of liquid hydrogen consisted of a multicyclic, nearly isentropic compression. A transducer employing a contact electrical effect was used to record this multicyclic compression process between a rigid wall and a flyer, resulting in a sequence of shock steps of decreasing amplitude, whose integrated action is equivalent to the isentropic compression of liquid hydrogen up to 500 kbar.The cylindrical loading was generated by detonating a high-explosive charge enclosing a cylindrical cavity along its axis that was filled with liquid hydrogen. Under these conditions shock velocities up to 13.7 km/sec were recorded, and pressure in the shock-compressed hydrogen reached 90 kbar. The formation of a boundary layer and expansion of the cylindrical cavity limited further pressure increases in the column of compressed liquid and lead to a decrease in the flow velocity. The observed increase in detonation velocity is associated with the influence of the channel wave on the detonation regime in the neighboring explosive layers.Under hemispherical loading, an increase in the converging shock velocity from 6 to 20 km/sec was recorded. The final pressure reached 210 kbar, and the total specific energy exceeded 200 kJ/g. During the release of the shock-compressed hydrogen into air at 0.1 torr, shock waves with velocities exceeding 50 km/sec were obtained.     

Detonation in two-layer systems

Detonation in heterogeneous systems involving parallel layers of two different substances was investigated. It was assumed that the denser medium can undergo fast chemical reaction without mixing with the other medium, and the detonation propagates along the two layers. The process of initiation is associated with a shock wave advancing through the medium of lesser density. An idealized gasdynamic model of the phenomenon is proposed in order to evaluate detonation parameters of the two-layer system on the basis of the characteristic features of its constituents. As calculation shows, in the absence of mixing between the two layers, the detonation is capable of propagating at a higher velocity than in the case when the constituents forming the layers are mixed.     

氢氧火箭发动机组件研制阶段可靠性技术综述

氢氧火箭发动机是火箭的"心脏",必须具备极高的可靠性,才能够保证发射成功.国内外针对氢氧火箭发动机开展了大量的可靠性工作.在发动机的研制阶段,通常是按照自下而上的思路,从组件的失效机理出发,开展稳健设计和试验验证,来保证氢氧火箭发动机的高可靠性.组件的可靠性工作主要是基于失效机理开展分析、监测、试验验证和设计改进;同时...     

连续爆轰发动机的研究进展

连续爆轰发动机是一种基于爆轰波将推进剂的化学能转化成热能的新概念发动机,近年来受到世界各主要国家的高度关注。现已成功获得多种燃料长时间稳定的连续爆轰,较深入地认识了连续爆轰流场结构,初步测得推力和比冲,验证了连续爆轰发动机的性能优势并在火箭模态、冲压模态以及涡轮模态下都实现了稳定连续爆轰。对连续爆轰发动机的工作原理,以及近年来世界各主要国家在连续爆轰发动机的基础研究和应用研究方面取得的代表性成果进行了综述,并给出尚待解决的问题,为其进一步工程化应用提供参考。     

Space-time structure of ion-cyclotron waves in the topside ionosphere as observed onboard the <Emphasis Type="Italic">ST</Emphasis>-<Emphasis Type="Italic">5</Emphasis> satellites

Results of observations of ion-cyclotron (IC) waves onboard the ST-5 satellites in the topside ionosphere (heights from a few hundred up to thousands of km) are presented. In this project, three identical micro-satellites were located during three months in 2006 in almost identical orbits with distances between them from first thousands to hundreds of km. All ion-cyclotron wave packets detected by two-three probes were observed at crossing one and the same latitude, which manifests their narrow localization in latitude with a characteristic scale from the first tens to 100 km. In no event IC waves were recorded with comparable amplitudes by all three satellites. At the same time, in the case of ST-5 flight near the ground-based induction magnetometer, a long emission in the same frequency range on the ground corresponded to a burst of IC waves in the topside ionosphere. This can indicate to the fact that an IC instability develops not continuously, but in the pulsing regime with a characteristic time of up to ∼10 min. A change in the rotation direction when a satellite crosses the wave structure is a characteristic feature of the polarization structure of registered transverse waves. The detected effects are discussed from the point of view of the existing models of generation and waveguide propagation of IC waves.     

不同压力条件下水平平板的表面换热实验研究

为了解不同压力下水平平板的气体对流换热变化情况,搭建了一个提供不同气压和环境温度的实验舱,开展了在不同压力(0.1 Pa、0.1 k Pa、0.2 k Pa、0.5 k Pa、1 k Pa、10 k Pa、50 k Pa和常压)与几种加热量(75、150、300 W/m~2)组合条件下的水平平板换热实验研究。通过对辐射换热和自然对流换热的比较,得到不同压力下气体的对流换热系数。结果表明:对流换热系数在环境气体压力小于1 k Pa时非常小,而在1 k Pa以上时才较大;在大于1 k Pa时,对流换热系数随压力的升高呈二次方增加。