Mechanism Of Detonation Initiation Research Articles

Effects of staggered opposed hot jets on the initiation and propagation of gaseous detonation in a supersonic combustible inflow

In this paper, the detonation initiation mechanism of a supersonic combustible mixture triggered by a staggered opposing combined hot jets was performed. Two-dimensional reactive Navier–Stokes equations with a one-step Arrhenius chemistry model were solved using a structured adaptive mesh refinement framework. The results show that a high temperature and pressure region triggers a rapid detonation initiation after the jet-induced bow shock focusing. Further analysis showed that there is a large baroclinic torque behind the local detonation wave induced by the staggered hot jet, which leads to a large Richtmyer–Meshkov instability at the end of the unburned jet, and the generated periodic shedding vortex structure thereby enhances the diffusion effect in the unburned region. However, the released heat cannot support the propagation of the detonation wave. In addition, different jet intensity distribution schemes and jet spacing will change the ignition point position. It is worth noting that the distance of detonation initiation can be significantly shortened by reducing the front jet intensity while maintaining the total jet energy. Increasing the jet spacing will significantly slow down the detonation initiation process.

Different Mechanisms of Detonation Initiation: Academician Fortov’s “Evergreen Theme”

Different Mechanisms of Detonation Initiation: Academician Fortov’s “Evergreen Theme”

Flame acceleration and transition to detonation in a pre-/main-chamber combustion system

Numerical simulations are performed to study the mechanism of deflagration to detonation transition (DDT) in a pre-/main-chamber combustion system with a stoichiometric ethylene–oxygen mixture. A Godunov algorithm, fifth-order in space, and third-order in time, is used to solve the fully compressible Navier–Stokes equations on a dynamically adapting mesh. A single-step, calibrated chemical diffusive model described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The two-dimensional simulation shows that a laminar flame grows in the pre-chamber and then develops into a jet flame as it passes through the orifice. A strong shock forms immediately ahead of the flame, reflecting off the walls and interacting with the flame front. The shock–flame interactions are crucial for the development of flame instabilities, which trigger the subsequent flame development. The DDT arises due to a shock-focusing mechanism, where multiple shocks collide at the flame front. A chemical explosive mode analysis (CEMA) criterion is developed to study the DDT ignition mode. Preliminary one-dimensional computations for a laminar propagating flame, a fast flame deflagration, and a Chapman–Jouguet detonation are conducted to demonstrate the validity of CEMA on the chemical-diffusive model, as well as to determine the proper conditioning value for CEMA diagnostic. The two-dimensional analysis with CEMA indicates that the DDT initiated by the shock-focusing mechanism can form a strong thermal expansion region at the flame front that features large positive eigenvalues for the chemical explosive mode and dominance of the local autoignition mode. Thus, the CEMA criterion proposed in this study provides a robust diagnostic for identifying autoignition-supported DDT, of which the emergence of excessive local autoignition mode is found to be a precursor. The effect of grid size, initial temperature, and orifice size are then evaluated, and results show that although the close-chamber DDT is highly stochastic, the detonation initiation mechanism remains robust.

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.

Effect of ignition parameters on detonation initiation using toroidal shock wave focusing

For fast detonation initiation, a novel compact initiator based on shock wave focusing has been proposed in our work. Two-dimensional numerical investigation was performed to understand the fast detonation initiation mechanism in hydrogen-air mixtures. Detailed evolution processes, namely, flame acceleration, shock wave focusing and detonation initiation were analyzed. Moreover, effects of ignition energy and energy release time on detonation initiation have been discussed thoroughly. The numerical results indicate that flame acceleration process can be divided into three stages: a) curling deformation of flame with low speed, b) tensile deformation of flame with medium speed, c) steady propagation of flame with high speed. After shock focusing, a highly compressed and heated region triggers the rapid detonation initiation. Further analysis indicated that detonation initiation time increases with the decrease of ignition energy. And the reason is mainly due to initial ignition delay differences controlled by ignition energy. In particular, once ignition energy is smaller than 12.5 mJ, mixtures will be no longer ignited. Besides, with the increase of energy release time, both the run-up time and distance of detonation initiation increase. The rapid release of energy can not only make mixtures ignited earlier, but also enhance initial flame velocity. As a result, when energy release time is reduced from 200 μs to 50 μs, the run-up time and distance are decreased by 50% and 32.5%.

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.

Mechanisms of detonation initiation in multi-focusing systems

The work is dedicated to the experimental and numerical study of the mechanisms of gaseous detonation initiation in a stoichiometric hydrogen–oxygen mixture due to the reflection of a shock wave from a complex-shaped end wall. Several elliptic surfaces of different geometries, including distributed ones, were considered. We refer to such reflectors with multiple elliptical surfaces as multi-focusing systems. The experiments were carried out in a shock tube. Visualization of the process was carried out with a high-speed schlieren system. In the experiments, the ignition delay times and the critical incident shock wave Mach number for detonation initiation were measured. Two-dimensional Euler simulations, on a fully unstructured computational grid, were carried out to determine the mechanism of detonation initiation. The numerical approach was verified using the experimentally measured ignition delay times. Reasonable agreement between the simulations and experiments for the critical Mach number of detonation initiation and for the efficiency of various multi-focusing systems for detonation initiation was achieved. Different regimes of detonation initiation depending on the incident shock wave Mach number were observed.

Numerical Research on the Toroidal Shock Wave Focusing Detonation Initiation

Abstract Pressure gain combustion (PGC) is considered to be a potential technology to increase the cycle efficiency of gas turbine. As one viable candidate for PGC, rotating detonation engine (RDE) draws more attention due to its significant advances in continuous mode of operation. In practical, one of the basic challenges for RDE application is to reliably initiate detonation wave. For this purpose, both detonation initiation mechanism and enhancement approach are urgently needed to be understood. In this work, a toroidal shock wave focusing detonation initiator is presented. On this basis, the two-dimensional numerical simulations are carried out to investigate the detonation initiation characteristics by using the toroidal shock wave focusing. All of the flame acceleration, shock wave focusing, detonation wave forming, and propagation are analyzed in detail. The numerical results show that the toroidal shock wave focusing initiator developed in this study can rapidly realize the detonation initiation over a short distance and performs significantly better than the traditional smooth or obstructed tube based imitators under different operating conditions. Under the same operating condition, the novel developed initiator decreases time of 59.2% and distance of 84.7% for the smooth tube based initiator, and time of 52% and distance of 78.9% for the obstructed one. Besides, the multifield analysis indicates that both the local explosion induced by shock wave focusing in concave cavity and the entrainment vortex generated by shock wave and jet flame in front of diaphragm are important mechanisms to initiate detonation wave. This study is expected to enhance the understanding of the physical mechanism of shock wave focusing detonation initiation and contribute to the development of detonation propulsion technology.

Numerical investigation of the effect of obstacle shape on deflagration to detonation transition in a hydrogen–air mixture

The potential for deflagration to detonation transition (DDT) in an uncontained failure poses extreme risk to nearby personnel. This study performs numerical simulations with detailed chemistry models of confined stoichiometric hydrogen–air mixtures interacting with flow obstructions to better understand the mechanisms of detonation initiation which will inform future risk assessments. Unique obstacle geometries, including both rectangular and curved obstacles, are considered in an effort to isolate important contributors to DDT. Contributors are shown to be pressure wave interactions in unburned fuel and flow features, such as vortical structures, which encourage flame acceleration. In this study, detonation was only observed in cases with sharp-edged obstacles and not in smooth-walled cases. The sharp edges introduced vortex shedding which contributed to flame distortion and resulted in acceleration. In addition, detonation was observed where strong pressure waves and reflections interacted in unburned fuel. The variations in geometry within the sharp-edged obstacles had some effect on vortex shedding and the reflections of generated shocks resulting in small changes in detonation location, however, the mechanism of DDT appeared the same, and the changes were small in comparison to the smooth-walled cases which did not detonate.

Carbon Detonation Initiation in Turbulent Electron-degenerate Matter

Abstract SNe Ia play a critical role in astrophysics, yet their origin remains mysterious. A crucial physical mechanism in any SN Ia model is the initiation of the detonation front that ultimately unbinds the white dwarf progenitor and leads to the SN Ia. We demonstrate, for the first time, how a carbon detonation may arise in a realistic three-dimensional turbulent electron-degenerate flow, in a new mechanism we refer to as turbulently driven detonation. Using both analytic estimates and three-dimensional numerical simulations, we show that strong turbulence in the distributed burning regime gives rise to intermittent turbulent dissipation that locally enhances the nuclear burning rate by orders of magnitude above the mean. This turbulent enhancement to the nuclear burning rate leads in turn to supersonic burning and a detonation front. As a result, turbulence plays a key role in preconditioning the carbon–oxygen fuel for a detonation. The turbulently driven detonation initiation mechanism leads to a wider range of conditions for the onset of carbon detonation than previously thought possible, with important ramifications for SNe Ia models.

Detonation initiation from shock and material interface interactions in hydrogen-air mixtures

A numerical study was conducted to explore the mechanisms of detonation initiation in a stoichiometric hydrogen-air mixture resulting from the interaction between a Mach 2.8 shock and a perturbed material interface. The simulations used a high-order compressible numerical method for fluid dynamics with both detailed and simplified chemical-diffusive models. Three material interfaces were considered: no interface, a perturbed planar flame, and a perturbed helium interface. The case with no interface did not evolve into a detonation. The case with the flame produced a series of additional shock-flame and shock-shock interactions. The shock-shock interactions produced a series of contact surfaces and sliplines with increasing temperature. Hot spots eventually formed along these sliplines and a detonation was initiated shortly thereafter through a reactivity gradient mechanism. The overall process of detonation initiation was similar for both detailed and simplified chemical-diffusive models. Only the fine details, such as the precise time and location of the hot spots, were different. This indicates that simplified chemical-diffusive models are adequate to describe the initiation of detonations in the present configuration. The processes that ignited the detonation were also similar in the case where the flame was replaced with a helium interface. Helium has a similar acoustic impedance to the products and produced similar wave refraction patterns. Thus, the primary effect of the flame is to facilitate the shock-shock interactions that produce hot spots and initiate the detonation. The chemical energy released by the flame has a secondary influence.

Longitudinal pulsed detonation instability in a rotating detonation combustor

The peculiar phenomenon of longitudinal pulsed detonation (LPD) in a rotating detonation combustor (RDC) is studied using hydrogen–air mixtures, by utilizing: (i) two air injection schemes having different inlet areas, and (ii) a convergent nozzle assembly with different spacers that affixes to the RDC exit. By varying the air injection pressure ratio and the backpressure, the regime of occurrence and the mechanism of this pulsed detonation instability are investigated. Immense evidence to suggest that LPD is caused by a peculiar detonation initiation mechanism enabled by a reflected shock wave from the RDC exit is discovered through an ensemble axial pressure profile analysis. Distance–time plots show that a single cycle of the pulsed detonation has two components: a fast-moving axial forward decaying detonation wave (with 75% of the ideal detonation speed) and a slower reflected detached shock wave (with 30% of the ideal speed). When the weak reflected wave comes in contact with the fresh reactants at the RDC headwall, another strong axial detonation is produced, thereby continuing the cycle without external ignition. LPD is also found to have three diverse facets, namely, inception, sustenance and operating frequency. For similar backpressures, the two air injection schemes have completely different operating regimes, leading to the inference that while backpressure is necessary for the onset of the pulsed detonation instability, by virtue of enabling reflected shock waves from the exit, lower air injection pressure ratio dictates the sustenance of the instability. A narrow band of injection pressure ratios, between 1.4 and 1.85, under back-pressurized RDC operation has high proclivity to produce sustained periodic longitudinal pulsed detonations in the combustor. Above this range, stable rotating detonation is preferred, and below this range, the operation is distinguished by the mixed presence of both rotating and pulsed detonations for a given test point, finally breaking down into chaotic instability for lower pressure ratios. The frequency of the pulsed detonation operation is found to depend on the initial combustor pressure and equivalence ratio, with higher frequency observed with an increase in backpressure and equivalence ratio.

Inelastic neutron scattering and ab initio studies of cyclotrimethylenetrinitramine

As an important secondary explosive, cyclotrimethylenetrinitramine (RDX, C3H6O6N6) is extensively used in military and industrial applications due to its high energy density and low sensitivity to external stimulations. Considerable attention has been devoted to the study of the detonation initiation, with particular interest in the mechanism by which energy is transferred from a shock wave to the internal molecular vibrations so as to begin endothermic decomposition. During the whole process, phonons as the primary carriers of heat may play an important role. Experimentally, inelastic neutron scattering (INS) technique provides a means of studying the dynamics of motions of atoms and molecules in the crystal, especially in the low frequency region which contains most phonon lattice modes. In this work, neutron diffraction pattern of polycrystalline RDX under ambient condition has been measured and compared with the calculated results, showing reasonable agreement with and thus confirming the structure of RDX. Subsequently, the vibrational INS spectrum of RDX has been measured at T=10 K over the region of 10-104~cm-1 by using cold neutron triple-axis spectrometer. On the basis of the solid-state density functional calculations with the generalized gradient approximation (BLYP and BP functionals), it is possible to perform normal-mode analysis, which agrees with previous assignments. A total of 9 phonon lattice modes and 3 internal vibrations have been identified. Eight possible doorway modes may be predicted in the energy range between 100 and 148~cm-1, which arise from the combinations of phonon lattice modes 38.3, 40.3, 50.2, 61.5~cm-1 and fundamental vibrations 86.6, 88.6, 101.4~cm-1. The doorway modes are the proposed bridge by which the energy of initial shockwave can pass from the external degrees of freedom into those of the molecule. It is shown that all of these eight modes have fundamental vibrational components that consist of nitro-group deformation vibrations. This point is of particular importance and supports the theory that the initial bond broken in detonation is the NN bond. This work may shed light on the mechanism of detonation initiation from a microscopic viewpoint.

Rapid detonation initiation by sparks in a short duct: a numerical study

Rapid onset of detonation can efficiently increase the working frequency of a pulse detonation engine (PDE). In the present study, computations of detonation initiation in a duct are conducted to investigate the mechanisms of detonation initiation. The governing equations are the Euler equations and the chemical kinetic model consists of 19 elementary reactions and nine species. Different techniques of initiation have been studied for the purpose of accelerating detonation onset with a relatively weak ignition energy. It is found that detonation ignition induced by means of multiple sparks is applicable to auto-ignition for a PDE. The interaction among shock waves, flame fronts and the strip of pre-compressed fresh (unburned) mixture plays an important role in rapid onset of detonation.

Initiation of detonation in a tube with parabolic contraction and conic expansion

On the basis of results of two and threedimensional numerical simulation, we proposed for the firsttime to use the profiling of axisymmetrictube walls inthe shape of parabolic contraction and conic expansion for initiating the detonation by means of a relatively weak shock wave. The detonation initiationmechanism is revealed, and its basic stages are analyzed. For simulating the propane–air mixture, wefound the paraboliccontraction shape and selectedthe conicexpansion parameters providing the initiation of detonation for the Mach number of an originalshock wave equal to 2.65.According to the theoretical results of V.P. Korobeіnikov, V.A. Levin, and V.V. Markov, the externaldisturbance of a reacting flow can become the cause offormation of a selfsustained detonation [1, 2]. Thesedisturbances can result in substantially nonlinearoscillatory processes due to the instability of the exothermicreaction front. Apparently, only detailednumerical experiments can provide the solution of theproblem under consideration on detonation initiationand indicate ways of its practical realization.Two processes at which the detonation wave can begenerated are wellknown and used in practice; theseare the direct initiation [1] and deflagrationtodetonation transition [3]. In [4] we proposed, investigated,and justified an original approach to shock detonationinitiation with the help of a regular shaped obstacles inplanar channel in which the leading shock wave(LSW) propagates. On the basis of numerical investigations and experiments, we showed that the regularparabolic structure of walls of the channel makes itpossible to reduce considerably the time and the distance before detonation initiation in comparison withthe wall structure in the shape of rectangular irregularities. The subsequent numerical investigation of theshocktodetonation transition (SDT) in an axisymmetric tube with the geometry of the walls investigatedin [4] for the planar case showed that for the Machnumbers in the initiating shock wave (ISW) importantfrom the viewpoint of practical applications, the process differs from that in the planar channel. It provedto be that the local explosion occurs in the region offocus of already the first parabolic element of thestructure instead of in the vicinity of the fifth one asoccurred in [4] for

Density functional theoretical study of transition metal carbohydrazide perchlorate complexes

The geometry, electronic structure and thermochemical properties of transition metal carbohydrazide perchlorates MCP [M = Co(II), Ni(II), Zn(II), Cd(II)] are investigated via density functional theory (DFT). The results show that the Heyd–Scuseria–Ernzerhof (HSE) functional yields the most accurate geometry. Detailed NBO analyses indicate that the metal–ligand interactions of CoCP and NiCP are covalent, whereas those of ZnCP and CdCP are ionic in nature. The mechanism of detonation initiation in covalent and ionic complexes is entirely different. There is a relationship between the energy gap and impact sensitivity. The thermochemical properties show that the formations of these complexes are exothermic; moreover, CoCP and NiCP are easier to decompose than ZnCP.

Numerical simulation of detonation initiation in a contoured tube

The effect of the wall contours in an axisymmetric tube on the transition from the shock wave to the detonation wave is studied numerically. Qualitative features and quantitative characteristics of the detonation initiation mechanism realized in a tube with a parabolic segment of the wall contour and conical expansion are found. The calculated results are presented in the form of “detonation curves” (angle of inclination of the contoured segment versus the Mach number of the initial shock wave) for various levels of tube blockage ratio.

Comparative analysis of physical mechanisms of detonation initiation in HMX and in a low-sensitive explosive (TATB)

Based on the concept of hot spots, the basic physical factors determining the macrokinetics of chemical reactions in heterogeneous crystalline explosives are estimated. The macrokinetics in plasticized TATB is shown to be basically determined by the velocity of combustion propagation from microscopic reaction spots (hot spots), whereas the macrokinetics in plasticized HMX is determined by the density of hot spots.

Effects and phenomena in filamentary heavy-metal azide crystals exposed to heat, light, and electric and magnetic fields

Heavy-metal azides (lead, silver, and thallium azides) attract attention of scientists not only because they are common initiating explosives whose nature of explosion is still discussed, but also because their properties are close (on a number of parameters) to those of halogenides whose physical properties are well studied. In the present work, based on the analysis and correlations of polarization-and magnetostrictive effects found in heavy-metal azides, an attempt is made to substantiate the physical-chemical mechanism of detonation initiation as ionization induced by the electric field.

An excitonic mechanism of detonation initiation in explosives

A novel mechanism for detonation initiation in solid explosives is proposed. This is based on electronic excitations induced by an impact wave propagating through the crystal. We illustrate the model by using the RDX (C3H6N6O6) crystal as an example. In our model, a key role belongs to lattice defects, in particular edge dislocations, which promote dramatic changes in the electronic structure, primarily a reduction of the optical gap due to the splitting off of local electronic states from both valence and conduction bands. The pressure inside the impact wavefront further reduces the band gap, making it close to zero. This promotes highest occupied molecular orbital–lowest unoccupied molecular orbital HOMO–LUMO transitions resulting in N–NO2 bond breaking and the creation of favorable conditions for the initiation of a chain reaction. Experimental facts supporting the suggested mechanism are discussed.