Detonation Characteristics Research Articles

Effects of content of titanium fiber on detonation characteristics and mechanical properties of RDX explosive

ABSTRACT In this study, a novel type of composite explosive was prepared by adding titanium fiber into the RDX explosive. The effects of the content of titanium fiber on the performance of the RDX explosive were systematically studied, and the optimal concentration was obtained by comparing its performances with the RDX explosive. Underwater explosion experiments showed the optimal concentration was 15 mass%, in which the value of specific bubble energy, as well as total energy was acquired the maximum. Especially, the energy density of composite explosive with 10 mass% titanium fiber was 10.64 kJ·cm−3, compared with aluminum fiber explosive, which was increased by 17.96%. Meanwhile, results of the MTS809 machine indicated that the optimal concentration was 19.16 volume%, in which the ultimate stress increased by 175%, and critical strain increased by 393%. Additionally, better mechanical properties ensured process safety in the course of transportation and storage of explosives. Therefore, titanium fiber could be a potential energetic additive and played double roles in improving the properties of the RDX explosive significantly including detonation performances and anti-pressure ability.

Effects of nano-sized aluminum on detonation characteristics and metal acceleration for RDX-based aluminized explosive

Nano-sized aluminum (Nano-Al) powders hold promise in enhancing the total energy of explosives and the metal acceleration ability at the same time. However, the near-detonation zone effects of reaction between Nano-Al with detonation products remain unclear. In this study, the overall reaction process of 170 nm Al with RDX explosive and its effect on detonation characteristics, detonation reaction zone, and the metal acceleration ability were comprehensively investigated through a variety of experiments such as the detonation velocity test, detonation pressure test, explosive/window interface velocity test and confined plate push test using high-resolution laser interferometry. Lithium fluoride (LiF), which has an inert behavior during the explosion, was used as a control to compare the contribution of the reaction of aluminum. A thermochemical approach that took into account the reactivity of aluminum and ensuing detonation products was adopted to calculate the additional energy release by afterburn. Combining the numerical simulations based on the calculated afterburn energy and experimental results, the parameters in the detonation equation of state describing the Nano-Al reaction characteristics were calibrated. This study found that when the 170 nm Al content is from 0% to 15%, every 5% increase of aluminum resulted in about a 1.3% decrease in detonation velocity. Manganin pressure gauge measurement showed no significant enhancement in detonation pressure. The detonation reaction time and reaction zone length of RDX/Al/wax/80/15/5 explosive is 64 ns and 0.47 mm, which is respectively 14% and 8% higher than that of RDX/wax/95/5 explosive (57 ns and 0.39 mm). Explosive/window interface velocity curves show that 170 nm Al mainly reacted with the RDX detonation products after the detonation front. For the recording time of about 10 μs throughout the plate push test duration, the maximum plate velocity and plate acceleration time accelerated by RDX/Al/wax/80/15/5 explosive is 12% and 2.9 μs higher than that of RDX/LiF/wax/80/15/5, respectively, indicating that the aluminum reaction energy significantly increased the metal acceleration time and ability of the explosive. Numerical simulations with JWLM explosive equation of state show that when the detonation products expanded to 2 times the initial volume, over 80% of the aluminum had reacted, implying very high reactivity. These results are significant in attaining a clear understanding of the reaction mechanism of Nano-Al in the development of aluminized explosives.

Detonation of an Explosive Containing Carbon Nanotubes

RDX containing a small amount of single-walled carbon nanotubes is obtained by the method of co-precipitation out of a solution. The detonation of this composition is studied by an electromagnetic method of mass velocity measurement and by a high-resolution electroconductivity method. A clearly expressed chemical spike is observed. Preliminary indications of reaction acceleration in the presence of nanotubes are obtained. The electroconductivity measurements are complicated because of the noticeable conductivity of the original material; if this factor is taken into account, the electrical conductivity profiles behind the detonation front are similar to those observed for pure RDX. Possible reasons for the influence of nanotubes on detonation characteristics are discussed.

Geomechanical behavior of jointed rock mass in the large-scale blast impact zone

The review of literature sources has allowed determining the mechanism of seismic blasts waves in jointed rock mass. The author presents the theoretical formulas for calculating stresses, relative strains and displacement velocities induced in rock mass by large-scale blasts. The formulas take into account the detonation characteristics of explosives, the borehole diameter, the length of explosive charges, the number of simultaneously blasted charges per groups and the factor of explosion energy redistribution to rock throw. Moreover, the formulas include the properties of rocks between the blasting point and a guarded object, and on the exposed surface of a guarded object. The compressive stresses, relative strains and displacement velocities in rocks are calculated from the formulas. The theoretically found change in the displacement velocity versus distance agrees with the field measurement data obtained in an open pit mine of Polyus company. The formulas of stresses and relative strains were used to define seismic impact exerted by large-scale blasts on the barrier of dry dock No. 1. Based on the calculated results, the recommendations are developed on reduction of seismic impact of blasts during dredging in the dock. The research findings are applicable to developing blasting regulations toward reduction in seismic impact of large-scale explosions on exposed surfaces in rock mass, as well as on the ground surface structures and buildings.

Experimental investigation on the propagation characteristics of detonations in a semi-confined straight channel

In a rotating detonation engine, detonation waves propagate in the combustor chamber with one side unconfined which will introduce lateral relief and, therefore, results in deficits of the propagation velocity and the peak pressure. To study the critical mixture height for detonation propagation, a simple combustor without considering the curvature has been used, which provides a planar semi-confined channel with different channel heights, e.g., 10 mm, 20 mm, and 30 mm. Effects of the nitrogen dilution ratio, the equivalence ratio, and the channel height on the propagation modes, the velocity deficit, and the cellular structure have been investigated when ethylene was utilized. The non-dimensional channel height (h/λa) was also adopted to evaluate the impacts of lateral relief. Experimental results indicated that three different propagation modes existed in the semi-confined channel. Velocity deficits resulted from the lateral relief mechanism were observed and the maximum velocity deficit is around 10–15%. Besides, the cell width increased and became more irregular when reducing the channel height. Successful detonation propagation can be obtained with a minimum normalized channel height of h/λa > 2, which will provide information for practical detonation operations.

Velocity fluctuation and cellular structure of near-limit detonations in rough tubes

Detonation limits are characterized by a decrease in the propagation velocity, cellular structures to lower unstable modes and an increase in the velocity fluctuation of the detonation. The increase in the average velocity deficit as the limits are approached is not a sensitive change since the failure of the detonation can occur at a relatively small velocity deficit of the order of 20%. A more sensitive indication of the onset of detonation limits is the lowering of the unstable mode (i.e., towards single-headed spin) and the large longitudinal fluctuation of the detonation velocity. In this paper, recent results are reported for the aforementioned near-limit detonation characteristics for a number of detonable mixtures and tube diameters for both smooth and rough tubes. Mixtures include H2, C2H2, C3H8, CH4 fuels with both O2 or N2O as oxidizers. Tube diameters were 25.4 mm, 38.1 mm, 50.8 mm and 76.2 mm. To investigate the effect of wall roughness on the limits phenomena in tubes, wire spirals with different diameters were inserted into the different diameter test tubes. Regularly spaced photodiodes (IF-950C) along the tube were used for velocity measurements and smoked mylar foils were inserted into the tube for the measurement of the cellular structure. Results confirm that the cellular structure evolution towards the lower unstable modes follows well the observed increase in velocity fluctuation; the subsequent detonation failure defined by the absence of cells occurs also at high-velocity fluctuation and an abrupt increase in the average velocity deficit.

Cis‐1,3,4,6‐Tetranitrooctahydroimidazo‐[4,5‐d]Imidazole (BCHMX) as a Part of Low Sensitive Compositions based on DATB or HNAB

AbstractA new focus in the fields of energetic materials is on preserving high performance while enhancing manipulation safety. In this work, a special type of plastic‐bonded explosive (PBX) mixture has begun to be developed by combining a new nitramine, cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole (BCHMX), with the thermally stable explosives 1,3‐diamino‐2,4,6‐trinitrobenzene (DATB) and 2,2’,4,4’,6,6’‐hexanitroazobenzene (HNAB) bonded by a styrene‐butadiene rubber (SBR) binder. The studied PBXs prepared are insensitive explosives. They have been compared with Czech plastic explosives (SEMTEX 10 and SEMTEX 1H), a new Egyptian plastic explosive (EPX‐1), a Swedish plastic explosive (Sprängdeg m/46) and with analogous PBXs, filled by 1,3,5‐trinitro‐1,3,5‐triazinane (RDX), β‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (β‐HMX), and cis‐1,3,4,6‐tetranitrooctahydroimidazo‐[4,5‐d]imidazole (BCHMX). The individual explosives, PETN, BCHMX, β‐HMX and RDX, have been studied as well. Thermal behavior of the pure active components has been investigated. The impact and friction sensitivities have been obtained along with the measured detonation velocities. The detonation characteristics have also been calculated using the EXPLO5 code. The relationship between performance and sensitivity to mechanical impulses has been investigated. The results have shown that DATB has a significant influence on the reduction of the sensitivity of BCHMX to a level close to TNT and much lower than all the other studied plastic explosives. In addition, the new PBX based on BCHMX and DATB has higher detonation characteristics than the commercial PBXs and TNT. The influence of DATB on BCHMX is a promising topic requiring more research for its application.

On models of blast overpressure effects to the thorax

Shock waves from explosions can cause lethal injuries to humans. Current state-of the-art models for pressure induced lung injuries were typically empirically derived and are only valid for detonations in free-field conditions. In built-up environments, though, pressure–time histories differ significantly from this idealization and not all explosions exhibit detonation characteristics. Hence, those approaches cannot be deployed. However, the actual correlation between dynamic shock wave characteristics and gradual degree of injury have yet to be fully described. In an attempt to characterize the physical response of the human body to complex shock-wave effects, viscoelastic models were developed in the past (Axelsson and Yelverton, in J Trauma Acute Care Surg 40, 31S–37S, 1996; Stuhmiller et al., in J Biomech. https://doi.org/10.1016/0021-9290(95)00039-9, 1996). We discuss those existing modeling approaches especially in view of their viscoelastic behavior and point out drawbacks regarding their response to standard stimuli. Further, we suggest to fully acknowledge the experimentally anticipated viscoelastic behavior of the effective thorax models by using a newly formulated standard model for viscoelastic solids instead of damped harmonic oscillators. Concerning injury assessment, we discuss the individual injury criteria proposed along with existing models pointing out desirable improvements with respect to complex blast situations, e.g. the necessity to account for repeated exposure (criteria with time-memory), and further adaption with respect to nonlinear gas dynamics inside the lung. Finally, we present an improved modeling approach for complex blast overpressure effects to the thorax with few parameters that is more suitable for the characteristics of complex blast wave propagation than other current models.

Theoretical Study of Thermal Stability and Detonation Characteristics of Derivatives of Uracil

A series of derivatives of uracil were designed through substituting the hydrogen atoms by nitro groups to look for high-energy-density compounds. To explore the thermal stabilities, the heats of formation (HOF) are calculated at the G3MP2 using the isodesmic reaction accompanied with the atomization reaction. Furthermore, the determinate factors of the stability are also discussed in detail. The bond dissociation energy (BDE) were calculated at the same level through homolytic cleavage reaction accompanied with the energy gaps (Eg). Based on our calculations, excellent stabilities are confirmed not only thermally but also kinetically. To confirm the possibility as high-energy-density compounds, the detonation characters including detonation pressure, detonation velocity, molecular density, and explosive heats are predicted by using the Kamlet-Jacobs equation. Finally, two molecules (C2 and C3) are evaluated as the candidates of high-energy-density materials. It is expected that our works can benefit the further research about title molecules.

Methane–oxygen detonation characteristics at elevated pre-detonation pressures

The detonation characteristics of methane–oxygen mixtures at pre-detonation pressures of 101–1,013 kPa were investigated in a detonation tube. Both pure methane–oxygen mixtures and mixtures with argon dilution were explored. Measurements made include cell sizes via soot foil, wave speed via high speed ion probes / pressure transducers, and temperature / H2O molar concentration profiles via 100 kHz absorption spectroscopy. Measured cell widths agreed with predicted cell widths based on a ZND length correlation. In addition, the power law fit of cell width with pre-detonation pressure agreed with previous data at less than 101 kPa. Measured detonation wave speeds agreed within 3% of Chapmen-Jouguet for all cases. H2O molar density and temperature were successfully captured up to 507 kPa. However, above 507 kPa pre-detonation pressure, low signal to noise ratio and poor spectral fits at the extreme conditions of the von Neumann spike resulted in unacceptable uncertainty. These results provide a unique dataset to validate kinetics models and high-fidelity computation fluid dynamics codes for methane-oxygen detonations at elevated pre-detonation pressures relevant to rotating detonation rocket engines.

One-step rapid fabrication of high-purity onion-like carbons as efficient lubrication additives

Onion-like carbons (OLCs), as a dazzling star of carbon nanomaterials, have been applied in various fields, such as lubrication, biomedicine, catalysis and energy storage. However, it is still a challenge to prepare OLCs using a solvent-free, time-saving and efficient strategy. Herein, the high-purity OLCs in high reproducibility without any post-treatment are prepared within 3–5 ms by a one-step gaseous detonation approach using untreated benzoic acid as the precursor, as well as acetylene (C2H2) and oxygen (O2) as explosion sources. The as-prepared OLCs with an average diameter of ca. 50.5 nm possess near-spherical and multi-layered concentric structure, which is highly beneficial for lubrication. Based on the characteristics of gaseous detonation and the morphology of OLCs, the formation mechanism of OLCs is speculated. Moreover, using a four-ball tester, the tribological properties of OLCs as lubricant additives for 150SN mineral oil are investigated. Strikingly, compared with those of neat 150SN mineral oil, the mean friction coefficient (f), wear scar diameter (d), wear scar depth (D) and wear rate (δ) of lower balls of OLCs/150SN suspensions with the concentration of 0.8 wt% under 392 N are reduced by 47.2%, 30.7%, 56.7% and 77.0%, respectively, indicating that OLCs can serve as excellent lubrication additives. This research not only provides a one-pot, rapid and facile gaseous detonation route to fabricate high-purity and high-reproducibility OLCs on a large scale, but also demonstrates the potential application of OLCs in lubrication.

Computation of the mean hydrodynamic structure of gaseous detonations with losses

In this paper, computations of gaseous detonations in several configurations are performed and discussed. The objective is to investigate the detonation characteristics, specially the influence of losses, through a quantitative analysis of the mean hydrodynamic structure. The results are divided into two parts: The first one relates to the propagation of ideal detonations without losses and to their characterization for a specific set of thermodynamic parameters, and the second part investigates the influence of losses on the averaged hydrodynamic structure of detonations. This is achieved by means of a specific and canonical configuration, in which a detonation wave propagates in a reactive layer bounded by an inert gas. This topology is investigated through a comparison of the instantaneous flow-field, cellular structure, and averaged quantities such as the mean curvature and the hydrodynamic thickness. This configuration may be regarded as non-ideal, as the detonation experiences losses due to the expansion of the detonation products toward the inert layer. The prediction of the detonation characteristics, as well as the conditions at which quenching occurs, remains a challenge when losses are involved. The main objective is to assess to what extent the hydrodynamic thickness can be used as a relevant length scale in the analysis of the resulting database. The results also highlight the key features of this configuration, which isolates a specific issue related to the rotating detonation engines. The temperature of the inert layer strongly affects the structure of the detonation–shock combined wave. For a high-temperature inert gas confinement, a detached shock appears in the upper layer and a jet of fresh mixture develops downstream of the front. For this condition, we observed a reduction in the critical dimensions by a factor of two. The degree of regularity of the detonation structure, from weakly unstable to mildly unstable cases, through the variation of the reduced activation energy, was investigated. The hydrodynamic thickness appears to be a useful characteristic length scale for the detonation, as it allows a better analysis of the results and a scaling of data. Moreover, the detonation velocity deficit can be globally expressed as a function of the ratio of the hydrodynamic thickness to the mean radius of curvature at the bottom of the wall.

Experimental optimisation comparison of detonation characteristics between leaded aviation gasoline low lead and its possible unleaded alternatives

Federal Aviation Administration’s twenty years of research with two-hundred unleaded blends has not found a “drop-in” unleaded replacement for aviation gasoline 100 low lead, a specifically formulated fuel for usage in naturally aspirated aircraft engines and contains tetraethyl lead additive to assist on detonation. Tetraethyl lead emission from piston aircrafts can cause serious health impacts and is a human carcinogenic substance. This research optimised the detonation efficiency of 14 aviation/motor fuels/blends. Type of fuels/blends and engine speed were chosen as input parameters. Relative detonation index was the output parameter. The engine speed was differed from 2000 to 2700 and the fuels were changed at fourteen levels. The design matrix consisted of 98 runs and was based on one factor response surface methodology. Analysis of variance was done on the design model to authenticate the significance. Engine speed and type of fuel were set to be in range while relative detonation index was minimised. Confirmation experiment was performed to check and confirm the actual and predicted outcomes. Results showed that at 2300 engine speed, Research Octane Number 98 fuel exhibited the best desirability of 0.759 among all fuels/blends with 57.153% of relative detonation index. The outcome revealed that tetraethyl lead addition into the aviation gasoline low lead can be eliminated.

Numerical simulation of hydrogen-air flame acceleration and detonation initiation in tubes equipped with arc obstacles of different chord lengths

Numerical simulation of hydrogen-air detonation characteristics in tubes equipped with arc obstacles of different chord lengths is carried out by using OpenFoam package. The results show the optimal chord length is half the diameter, in which the detonation wave propagates at a high velocity while maintaining small deflagration-to-detonation transition distance. Three different regimes were found in the flame acceleration and detonation initiation for different chord length: (1) For the small chord length, the flame experienced the transition from fingertip to tulip flame, then recovered to fingertip flame again until initiation. The detonation is caused by the hot spot formed by the spontaneous combustion of unburned premixed gas. (2) For the intermediate chord length, the tulip flame quenched by reverse flow will turn into fingertip flame under the jet effect of obstacles. Then, the fingertip flame is transformed into tulip flame again under the action of flow instability. The detonation is initiated by the fusion of two local hot spots produced by the coupling of tulip flame and diffraction wave. (3) For the large chord length, the flame propagates as a fingertip flame. The detonation is initiated by the explosion of the flame surface compressed by reflected wave and shock wave.

High-Pressure Structural and Electronic Properties of Potassium-Based Green Primary Explosives

Recently synthesized green primary explosives potassium 4,4′-bis(dinitromethyl)-3,3′-azofurazanate (K2BDAF) and potassium 1,1′-dinitramino-5,5′-bistetrazolate (K2DNABT) offer fast, powerful initiation capacity and high-performance detonation characteristics to replace the long-standing toxic primary explosives. In the present study, we report the structural and electronic properties of the emerging green primary explosives K2BDAF and K2DNABT under hydrostatic pressure up to 10 GPa. We observed that dispersion correction methods are important for capturing weak intermolecular interactions in order to accurately describe the geometries of the primary explosives. The computed ground state structural properties using optB86b-vdW method are in good agreement with the experimental results. The pressure-dependent lattice constants and bond parameters show anisotropic nature and also a sharp discontinuity around 4–5 GPa. The obtained equilibrium bulk modulus shows that K2BDAF is softer than K2DNABT. Bonding analysis revealed that the C-NO2 energetic functional groups are more sensitive than the stable ring structure under hydrostatic pressure. The calculated electronic structure of K2BDAF using the Tran–Blaha-modified Becke–Johnson (TB-mBJ) potential shows a direct-to-indirect band gap transition around 5 GPa, which is consistent with the aforementioned discontinuity of the structure, while K2DNABT is found to be a direct-band-gap insulator in the studied pressure range of 1–10 GPa. The abnormal trends in the structural and electronic properties suggest that K2BDAF may undergo a structural transition/distortion around 4–5 GPa of pressure.

Effects of content and particle size of cenospheres on the detonation characteristics of emulsion explosive

ABSTARCT Cenospheres are the main component of industrial waste of fly ash and taking full advantage of them has good economic and environmental values. In this paper, cenospheres sensitized emulsion explosive was prepared by adding cenospheres into the emulsion matrix as the sensitizers. The effects of content and particle size of cenospheres on detonation performance of emulsion explosives were studied, and the optimum formula was determined by comparing its detonation performance with glass microspheres (GMs) sensitized emulsion explosive. The optimum content and particle size of cenospheres were 12 mass% and 83 μm, and the brisance and detonation velocity were 18.34 mm and 4970 m/s, respectively, which were close to the maximum values of the traditional GMs sensitized emulsion explosive. Air blast experiments indicated that the influence of particle size of cenospheres on the work capability of emulsion explosive was consistent with its explosion power. Shelf-life test and optical microscope results showed that cenospheres sensitized emulsion explosives had good storage stability. Therefore, cenospheres could be a promising physical sensitizer for emulsion explosives.

Experimental study of using biogas in Pulse Detonation Engine with hydrogen enrichment

Pulse Detonation Engine (PDE) is one of the pressure gain engines that provide prominent features over conventional engines such as higher thermodynamic efficiency, higher thrust, and less design complexity. Biogas is one of the promising alternative energy sources. The aim of this paper is to study the influence of hydrogen addition in biogas on detonation characteristics performance of PDE. The ideal detonation combustion characteristics of biogas are estimated using NASA Chemical Equilibrium with Applications (CEA). The equivalence ratio range is varied from 0.7 to 1.4. It was found that the higher the percentage of methane concentration in biogas increased the detonation characteristics of biogas such as pressure, temperature, and Mach number. However, a reduction in the flame speed, temperature, and pressure of biogas compared to pure methane was stipulated to be due to the presence of CO2 that acts as a dilutant in biogas. It was found also that, the optimal percentage of hydrogen addition into the biogas fuel mixture is 15%. At this percentage, the detonation pressure improved by 23%. The performance of PDE fuelled by biogas is presented experimentally at a frequency of 10 Hz. The results show a successful and stable detonation initiation in PDE by using biogas with 15% and 20% addition of hydrogen.

Discovery of a High-Octane Environmental Gasoline Based on the Gasoline Fischer–Tropsch Process

Gasoline produced from fischer-tropsch technology has low detonation characteristics and do not have any aromatic hydrocarbons, thus they should be maximized by blending with oxygenated compounds, ...

Comparison of detonation characteristics for typical binary blended fuel

For optimum utilization of explosive energy sources, it is significant to investigate detonation initiation and propagation characteristics of typical blended fuel containing aluminum particles with respectively JP-10, PO (propylene oxide) or EE (ethyl ether). A numerical model to examine the critical detonation initiation of dust/gas binary blended fuel was first proposed. The critical energy of detonation initiation of 50%Al:50%JP-10, 50%Al:50%PO and 50%Al:50%EE at overall concentrations of 280, 240, stoichiometric, 110, 80 and 50 g/m3 was numerically obtained. A compression wave was generated by the ignition source and expansion of combustion products. The interaction of flame with shock increases the surface area of the flame and creates strong vorticity, then local multiple explosions are established and amplified to direct initiation of detonation. Results reveal that the critical energy of detonation initiation reaches the lowest at stoichiometric concentration, and appears a typical ‘U’ shape behavior versus concentration. Specifically, the critical energy of detonation initiation is 500 J (0.1768 MJ/m2), 450 J (0.1592 MJ/m2) and 500 J (0.1768 MJ/m2) for respective stoichiometric concentration. 50%Al:50%EE has the lowest critical energy of detonation initiation at fuel-lean side, and 50%Al:50%JP-10 has the lowest at fuel-rich side, whereas 50%Al:50%PO has the lowest nearly at stoichiometry. In terms of detonation parameters, 50%Al:50%JP-10 achieves the highest overpressure at far fuel-lean side, and 50%Al:50%EE achieves the highest at far fuel-rich side, whereas 50%Al:50%PO achieves the highest nearly at stoichiometry. The onset process of stable detonation also reveals the detonation sensitivity of binary blended fuel.

Effect of arc obstacles blockage ratio on detonation characteristics of hydrogen-air

In this paper, the influence of maximum blockage ratio of arc obstacles on hydrogen-air detonation characteristics is simulated and analyzed by using OpenFoam package. The results show that the arc obstacles can effectively reduce the deflagration-to-detonation transition distance and the optimal maximum blockage ratio is 0.7. As the maximum blockage ratio increases, the flame acceleration and initiation of detonation wave show 3 different regimes: (1) For small maximum blockage ratio, the flame is affected by Squish flow, resulting the velocity differences between the flame near the wall and in the center of the tube, and the tulip flame formation. The tulip flame will generate two local explosion points and the explosion points develop and merge to initiate detonation. (2) For intermediate maximum blockage ratio, under the interaction of the reflected shock wave and the obstacles, the flame is a narrow fingertip shape. The initiation of detonation wave is engendered by the development and fusion of multiple hot spots. (3) For large maximum blockage ratio, the flame is also a narrow fingertip-shaped. The detonation wave is initiated by the coupling of the flame front with the leading shock wave at the throat of the obstacle.