Detonation Initiation Research Articles

Numerical investigation of the effect of jet intensity from internal jet tube on detonation initiation characteristics

Detonation initiation is one of the key problems in the application of pulse detonation engine (PDE). In recent years, the experimental research and numerical simulation results of multi-point jet initiation show that multi-point jet initiation can expand ignition energy and accelerate Deflagration-to-Detonation Transition (DDT) process. However, it will bring some difficulties to the actual implementation process due to the increase in the number of jet tubes. In order to reduce the number of ignition sources and keep multiple jets, this paper proposed a new structure of jet tube, in which the jet tube was inserted into the detonation tube, and multiple jets were generated by opening holes at the jet tube wall. The effect of the jet intensity propagated from the jet tube through the jet hole on the detonation initiation characteristics was studied by changing the initial pressure of the ignition zone. Stoichiometric hydrogen-air mixture was used. The simulation results indicated that the jet intensity had a great influence on the characteristics of detonation wave. When the initial pressure was 0 or 0.2 MPa, the stable detonation wave could not be initiated due to the jet intensity was weak. When the initial pressure of ignition zone reached 0.4 MPa, the intensity of the jet propagating from the jet tube was obviously strengthened. Then, a local detonation point was generated near the center axis of the jet tube at about x = 20 mm and developed into detonation wave. The detonation wave attenuated into quasi-Chapman-Jouguet (C-J) detonation wave, and the coupling of pressure and temperature gradually declined through the transversal jet hole. Finally, high pressure points were formed near the wall of the detonation tube, interacted at the center of the detonation tube, and a plane detonation wave was generated through reflection from the wall of the detonation tube. As the initial pressure of ignition zone continued to increase to 0.6 MPa, the development process of detonation wave was similar to that of 0.4 MPa, but the jet intensity was further improved, which made the distance and time of DDT significantly shortened. In addition, multi-stage jets could obviously reduce DDT distance and time compared with the transverse jet.

Numerical Research on Effect of Sudden Cross-Section Expansion on Detonation Initiation

Numerical Research on Effect of Sudden Cross-Section Expansion on Detonation Initiation

Asymptotic Solutions of Two Fundamental Problems in Gaseous Detonations

ABSTRACT Two recent analytical results concerning i) the direct initiation of gaseous detonations in free space and ii) the deflagration to detonation transition in tubes are revisited in this article. The first problem is treated by an asymptotic analysis in the limit of small heat release, enlightening the mechanisms controlling the direct initiation process of real detonations. The second one is limited to a one-dimensional mechanism of transition at work on the tip of elongated flames in tubes. Particular attention is paid to the physical mechanisms more than to the technical details of the analyses. However, the mathematical formulation as well as the different steps of the analysis are clearly presented in a synthetic manner.*

Hypergolic Continuous Detonation with Space-Storable Propellants and Additively Manufactured Injector Design

Initiation and sustained detonation of hypergolic liquid rocket propellants have been assessed in an annular combustor. An established impinging injector design was modified for additive manufacturing to improve hydraulic characteristics. Computational fluid dynamics analysis was used to verify decreased flow resistance and increased relative backflow resistance afforded by tapered propellant passageways enabled by additive manufacturing. Two injectors were manufactured and a honing process was employed on one of the injectors to reduce surface roughness associated with additive manufacturing. Cold-flow testing confirmed decreased hydraulic resistance due to tapering of the injector passageways and due to the honing process. The detonability of hypergolic space-storable propellants in an annular combustor with the injectors was assessed experimentally with dinitrogen tetroxide as the oxidizer and monomethylhydrazine as the fuel. The presence of rotating detonation waves was verified via multiple instrumentation methods, including a high-speed camera. The performance of the combustor was seen to increase with the total number of detonation waves, the total propellant flow rate, and the presence of a downstream flow constriction. The work demonstrates continuous detonability of hypergolic space-storable propellants under various operation conditions relevant to in-space rocket propulsion concepts, as well as several benefits of utilizing additive manufacturing for injection-system design.

Effect of solid obstacle distribution on flame acceleration and DDT in obstructed channels filled with hydrogen-air mixture

Ensuring hydrogen safety has become of great significance nowadays, whose leakage can possible result in the deflagration to detonation (DDT). The numerical study aims to explore the effect of solid obstacle distribution on the DDT in a homogeneous hydrogen-air mixture. Results show that the detonation initiation process can be classified into two types: i) local spherical detonation caused by the coupling of flame surface and high-pressure region; ii) detonation triggered by the interaction between the upper wall and multiple compression waves in front of the flame. Also, this study finds that the flame acceleration (FA) experiences two periodic “acceleration-deceleration” processes before the detonation initiation, and the initiation distance and time are the shortest when the obstacles are symmetrically distributed. Further, the higher the unilateral blockage ratio, the more unfavorable DDT occurs. The present results highlight the effect of different solid obstacle distribution patterns on the FA and DDT process.

Flame acceleration and deflagration-to-detonation transition in narrow channels filled with stoichiometric hydrogen-air mixture

Understanding flame acceleration (FA) and deflagration-to-detonation transition (DDT) is important for combustion applications. However, DDT is a high-speed, nonlinear and complex process and the exact underlying mechanisms remain unclear, especially for narrow channels with a width smaller than detonation cell size. In this paper, numerical simulations were carried out to study the FA and DDT in a stoichiometric hydrogen-air mixture in narrow channels with width on the order of magnitude comparable to laminar flame thickness. The fully-compressible reactive Navier-Stokes equations were solved by using a high-order numerical method. Hydrogen-air combustion was taken into account using a calibrated simplified chemical-diffusion model. Four different channel widths, i.e., d = 2.0 mm (∼6 δl), 1.0 mm (∼3 δl), 0.5 mm (∼1.5 δl), 0.25 mm (∼0.7 δl), were considered in the simulations to explore the size effect, where δl is the laminar flame thickness. The results show that the FA is mainly determined by boundary layer effects for all the channels considered. In the early stages, the flame keeps accelerating due to the stretching effect of boundary layer that leads to a significant increase in flame surface area. In the later stages of acceleration, an ultra-fast flame develops in the boundary layer ahead of the flame front due to the viscous heating effect associated with wall friction. It was found that the DDT mechanism is essentially consistent in all the channels. Detonation arises from the contact of two neighboring flame fronts in the shocked region that is vulnerable to a mild temperature gradient or fluctuation. The contact of the two high-temperature reaction fronts instantly causes a very high concentration of heat flux and consequently a detonation wave emerges from the contact spot. The detonation propagation after DDT, however, depends on the channel width. When d ≥ 1.5 δl, a quasi-steady or a single-head detonation forms and sustains until the end of the combustion. For the case of d < 1.5 δl, the detonation decays into a high-speed flame. When the channel width is sufficiently small, e.g., d ∼0.7 δl, detonation initiation can occur occasionally, but soon decouples into a shock and a flame.

Detonation Initiation upon Interaction of a Shock Wave with a Combustible Gas Bubble

Detonation Initiation upon Interaction of a Shock Wave with a Combustible Gas Bubble

The Potential Energy Hotspot: Effects of Impact Velocity, Defect Geometry, and Crystallographic Orientation

In energetic materials, the localization of energy into when a shock wave interacts with the material's microstructure is known to dictate the initiation of chemical reactions and detonation. Recent results have shown that, following the shock-induced collapse of pores with circular cross-sections, more energy is localized as internal potential energy (PE) than can be inferred from the kinetic energy (KE) distribution. This leads to a complex thermo-mechanical state that is typically overlooked. The mechanisms associated with pore collapse and hotspot formation and the resulting energy localization are known to be highly dependent on material properties, especially its ability to deform plastically and alleviate strain energy, as well as the size and shape of the pore. Therefore, we use molecular dynamics simulations to characterize shock-induced pore collapse and the subsequent formation of hotspots in TATB, a highly anisotropic molecular crystal, for various defect shapes, shock strengths and crystallographic orientations. We find that the the localization of energy as PE is consistently higher and its extent larger than as localized as KE. A detailed analysis of the MD trajectories reveal the underlying molecular process that govern the effect of orientation and pore shape on the resulting hotspots. We find that the regions of highest PE for a given KE relate to not the impact front of the collapse, but the areas of maximum plastic deformation, while KE is maximized at the point of impact. A comparison with previous results in HMX reveal less energy localization in TATB which could be a contributing factor to its insensitivity.

Research on Dynamic Response of Slopes With Weak Interlayers Under Mining Blasting Vibration

As blasting technology starts to be used in a wide range of areas, blast loading has led to an increasing number of geological disasters such as slope deformation, collapses, and soil slippage. Slopes with weak interlayers are more likely to be deformed and damaged under the influence of blast loading. It is of great importance to study the evolution for the deformation of slopes with weak interlayers during blasting excavation. This study constructed a slope model with a weak interlayer to investigate the influence of different factors of blasting, including explosive charge, blast radius, blast origin, and multi-hole blasting, on the internal dynamic response. The deformation mechanism of slopes with weak interlayers under the influence of blast loading was analyzed. Test results show that each layer of the model had a different displacement response (uncoordinated dynamic response) to blasting with various factors. Explosive energy and the pattern of dynamic response of each layer varied depending on different settings of blasting factors such as explosive charge, blast radius, blast origin, and detonation initiation method. When the explosive energy produced under the influence of various factors was small, the change in the uncoordinated dynamic response between layers was significant, and the change gradually became less significant as the explosive energy increased. Therefore, this study has proposed the concept of critical explosive energy, and it is speculated that when the explosive energy produced with various factors is less than critical explosive energy, the dynamic response is mainly affected by the internal structure of the slope (property difference induced geologic layers). In other words, the uncoordinated motion of material’s particles in each layer is caused by different limitations and the degree of movement of the particles, which leads to the uncoordinated dynamic response and uncoordinated deformation of each layer. If the explosive energy is greater than the critical value, the dynamic response of each layer is mainly affected by the explosive energy. The differences in the internal structure of the slope are negligible, and the incoordination of dynamic responses between layers gradually weakens and tends to synchronize.

Onset of Oblique Detonation Waves for a Cavity-Based Wedge

To shorten the ignition length of the wedge-induced oblique detonation wave, a geometric setup in which the leading and aft wedges are connected by a cavity is proposed. High-resolution viscous computations to solve the reactive Reynolds-averaged Navier–Stokes equations have been performed, which examine the onset and evolution of combustion and flow configuration with premixed mixtures of inflow Mach numbers ranging from 4.0 to 6.0. The unreactive flowfield of the cavity-based wedge is analogous to that of a sole cavity with the exclusion of shock/shock interaction downstream. For and 5.0, no sustainable oblique detonation wave is achieved, and the flame holds in the cavity and near-wall area of the aft wedge; however, a short-lived detonation wave triggered by the shock focuses is observed during the combustion procedure with . For , nearly direct detonation initiation of postshock occurs, and an oblique detonation wave is established above the fore edge of the cavity. Unexpectedly, a delayed ignition downstream contributes to the development of a recirculation zone, which results in a detached oblique detonation wave for .

Initiation of Detonation in Interaction of a Shock Wave with Combustible Gas Bubble of Various Densities

Initiation of Detonation in Interaction of a Shock Wave with Combustible Gas Bubble of Various Densities

Initiation of Detonation of an Emulsion Explosive by Impact of a Thin Plate

Initiation of Detonation of an Emulsion Explosive by Impact of a Thin Plate

Spontaneous runaway of fast turbulent flames for turbulence-induced deflagration-to-detonation transition

One of the fundamental mechanisms for detonation initiation is deflagration-to-detonation transition (DDT). This research experimentally explores the runaway condition for highly turbulent fast flames before DDT, which are characterized by extremely high turbulent flame speeds. Such fast turbulent flames experience increased effects of compressibility and may develop a runaway acceleration combined with a pressure buildup that leads to a turbulence-induced DDT (tDDT) mechanism that has been recently reported. The flame dynamics and the associated reacting flow field are characterized using simultaneous high-speed particle image velocimetry, OH* chemiluminescence, pressure measurements, and schlieren imaging. We study the flow-field conditions for runaway acceleration of fast turbulent flames and effects of compressibility on the evolution of these flames. The locally measured turbulent flame speed is found to be greater than that of a Chapman–Jouguet deflagration speed, which places the flame in the runaway transition regime that would eventually lead to a detonation.

Effects of obstacle layout and blockage ratio on flame acceleration and DDT in hydrogen-air mixture in a channel with an array of obstacles

Numerical simulations were performed to study flame acceleration and deflagration-to-detonation transition (DDT) in hydrogen-air mixture in a channel with a two-dimensional array of cylindrical obstacles. A high-order numerical algorithm with adaptive mesh refinement was applied to solve reactive Navier-Stokes equations. The effect of obstacle layout was examined by considering three layouts at a fixed blockage ratio of 0.5: staggered, inline-concentrated, and inline-scattered. Three blockage ratios, 0.33, 0.5, and 0.67, were used for the case of staggered obstacles to explore the influence of blockage ratio. The results show that both obstacle layout and blockage ratio have significant effects on flame acceleration and DDT occurrence, although the basic mechanism of detonation initiation is consistent for all the cases involving shock focusing. In the staggered case, the head-on collisions of flame and pressure waves with obstacles greatly promote the growth of flame surface area and thus lead to the fastest flame acceleration and shortest detonation onset time. In the inline-concentrated case, flame propagates slower than that in the staggered case due to smaller flame surface area. However, compared to the zigzag path in the staggered case, the straight passages parallel to flame propagation direction in the inline-concentrated case are more conducive to producing strong shock focusing and thus result in the shortest DDT distance. In the inline-scattered case, the straight passage along the centerline of channel facilitates the early acceleration of flame, but it has the slowest flame propagation in lateral directions and thereby the longest DDT time and distance. For the staggered obstacles at different blockage ratios, flame acceleration rate increases with increasing blockage ratio. The occurrence of DDT is hindered by the most congested obstacles, because the shock focusing is insufficiently strong to initiate detonation after passing through the excessively narrow gaps.

Recruiting Perovskites to Degrade Toxic Trinitrotoluene.

Everybody knows TNT, the most widely used explosive material and a universal measure of the destructiveness of explosions. A long history of use and extensive manufacture of toxic TNT leads to the accumulation of these materials in soil and groundwater, which is a significant concern for environmental safety and sustainability. Reliable and cost-efficient technologies for removing or detoxifying TNT from the environment are lacking. Despite the extreme urgency, this remains an outstanding challenge that often goes unnoticed. We report here that highly controlled energy release from explosive molecules can be accomplished rather easily by preparing TNT–perovskite mixtures with a tailored perovskite surface morphology at ambient conditions. These results offer new insight into understanding the sensitivity of high explosives to detonation initiation and enable many novel applications, such as new concepts in harvesting and converting chemical energy, the design of new, improved energetics with tunable characteristics, the development of powerful fuels and miniaturized detonators, and new ways for eliminating toxins from land and water.

Numerical investigation of free oblique detonation wave induced by non-intrusive energy deposition

To validate the concept of free oblique detonation waves (ODWs) induced by non-intrusive energy deposition applied in ODW engines (ODWEs), numerical simulations are performed by modeling the laser spark as a high-temperature and high-pressure zone embedding in the high-speed incoming flow and by solving the non-dimensional reactive Euler equations with one-step chemistry. The initiation process, flow structures, and self-adapted characteristics of such kinds of free ODWs in an open space are discussed with varying dimensionless source energy and inflow Mach number. The results show that free ODWs can be initiated successfully by local non-intrusive energy deposition. In particular, four ignition regimes, namely, the subcritical regime, the critical I regime, the critical II regime, and the supercritical regime, which are analogous to the one-dimensional direct initiation of detonation, are distinguished by varying the dimensionless source energy. However, the typical energy required by each ignition regime is higher than that in the one-dimensional case due to multi-dimensional energy transmission and instabilities. Moreover, the dimensionless source energy should be moderate to obtain a stable free ODW flow field because extremely low source energy leads to ignition failure, while extremely high source energy causes premature ignition. When the inflow Mach number varies, it is found that free ODWs can adjust automatically to a nearly Chapman–Jouguet state, indicating the self-sustaining nature of free detonation waves. Due to this feature, ODWE performance will be relatively stable despite the change in inflow Mach numbers if free ODWs are utilized in the combustor.

Numerical investigation of mass flow rate effects on multiplicity of detonation waves within a H2/Air rotating detonation combustor

This paper presents results from numerical simulations of a non-premixed hydrogen-air rotating detonation combustor with radial injection. The fuel and air mass flow rate are varied in order to hold a unity global equivalence ratio. The calculations show that multiple detonation waves co-exist when the mass flow rate is increased. Conditional statistics of the detonation structure and combustion processes suggest similarities across co-existing waves. Quantification of the injection response to the rotation of a detonation indicates that at higher flow rate the refill time is short enough to allow for a quick and well mixed composition prior to the new front passage. Details of the combustion characteristics are analyzed. The results elucidate the correlation between initial injection conditions and detonation multiplicity on the overall physics within the combustor.

Experimental and theoretical analysis of detonation products state on bubble dynamics and energy distribution in underwater explosion

The initial states and pressure of detonation products in a bubble have a great impact on bubble pulsation in underwater explosions; particularly, the initial accelerated expansion of a bubble can determine the energy distribution. The energy output and distribution of explosives were obtained on the basis of the underwater explosion experiment in this paper. To study the process of bubble pulsation and energy output, we proposed a gas equation of state (EOS) combining the pvk form, Jones–Wilkins–Lee (JWL) EOS, and the initial states of a bubble to take the effects of the initial bubble pressure and detonation products state transformation into account; furthermore, the bubble radius, velocity, and acceleration vs time were obtained through the Rayleigh–Plesset equation under our experimental condition. The differences of bubble behaviors were compared by adopting the JWL EOS and a polytropic EOS with k = 3. The results showed that the initial bubble pressure and detonation products state transformation influence the accelerating expansion and the subsequent bubble oscillation, respectively. Subsequently, comparisons of the energy output and distribution for different gas EOSs showed that the initial shock wave energy for the JWL EOS was underestimated in accelerating expansion, and the bubble energy was overestimated using the polytropic EOS for k = 3; the obtained energy output and distribution had a better agreement with experimental data when adopting the improved gas EOS. In addition, the energy distribution was determined before the detonation products turned to the explosion gas state in initial expansion based on the relationship of the accelerating expansion characteristics and the initial shock wave energy generation. The research has a great significance to reveal the mechanism of bubble pulsation in underwater explosions.

The Improvement of Picric Acid Synthesis Methodology

Explosives have the greatest importance in human practical activities, not only at time of war, but at tranquility as well. Nowadays, huge amount different type of explosives is synthesized, and fabricated for military and civilian applications. Nevertheless, this fact doesn’t exclude necessity of synthesis of new explosives for optimization of their characteristics, such as prime cost, power, safety during production, storage, transfer and etc. Picric acid is a fairly strong and energetic explosive, at the same time, beside to its explosive properties, it is characterized with antibacterial nature and it is the best yellow dye, especially for dyeing animal and plant tissues. The synthesis of picric acid structural analogues, is the main purpose of the research. One of the reasons of synthesis picric acid and further preparation of its structural analogues is safe nature of substances of this group, which makes them safe to various manipulations. On the other hand, it is well known that the synthesis and production of explosives is classified as high risk and costly technology. Therefore, even a small increase of their production productivity, is interesting from the economic effect point of view. During an investigation, changing of reaction conditions (temperature, concentrations and dosage of reagents) the improved method for syntheses of picric acid was developed. As result the significant increase of practical yield of picric acid, from 46% to 86% was achieved. Synthesized picric acid was placed in steel tube and tested on initiation of detonation in explosive camera. The description of modified method, comparisons to conventional technology, as well as explosion testing results is described in the paper.

Detonation Initiation of Strong Shock Waves to Study the Radiation Characteristics of High-Temperature Gases

Detonation Initiation of Strong Shock Waves to Study the Radiation Characteristics of High-Temperature Gases