Overdriven Detonation Research Articles

The Performance of Pressure Vessel Using Concentric Double Cylindrical High Explosive

The detonation pressure from the steady detonation of high explosives is a characteristic. Nevertheless, in materials processing using high explosives, there are cases when the detonation pressure does not match the intended pressure. In this investigation, as a new method of generating the overdriven detonation effectively, a double cylindrical high explosive set up using two kinds of explosives was developed, and its basic performance is analyzed. The concentric double cylindrical high explosive set up was composed of a high velocity explosive and a low velocity explosive, and the overdriven detonation was performed in the low velocity explosive. In this experiment, the ion gap was set up in the high velocity explosive and low velocity explosive respectively, and the detonation velocity was measured. The detonation pressure was also measured by setting up a manganin gauge (Kyowa Electric Instrument Co., Ltd.,) at the position where the generation of the overdriven detonation phenomenon was expected. Furthermore, the overdriven detonation process of the concentric double cylindrical high explosive was continually observed by numerical analysis and the framing photography. From the experimental results, the very high pressure region including the mach stem was observed in the low velocity explosive, and the overdriven detonation phenomenon was confirmed. The maximum pressure value of the concentric double cylindrical high explosive set up was 2.3 times higher than the Chapman-Jouguet pressure of the single explosive.

A Method for Producing Extra-High Dynamic Pressure Due to the Efficient Use of High Explosive

In the material processing such as shock synthesis and powder consolidation by shock waves the method for generating dynamic pressure is a vital factor for the quality of the final recovered materials. A general and convenient way for producing shock wave demanded in such applications is to take advantage of the explosion effect from high explosive. Under normal conditions, a given high explosive can only provide some kind of magnitude of dynamic pressure after its explosion. Therefore, it is whether possible to obtain the higher dynamic pressure by adequately changing the form of the explosion of high explosive. Starting from this motivation, we put forward a new method for producing high dynamic pressure from the use of the overdriven detonation of high explosive. The proposed device consists of the following configurations. A metal flyer accelerated by the high explosive is used to impact another layer of high explosive to incur an overdriven detonation in this layer of explosive. The overdriven detonation of high explosive acts on the powder materials, bringing out high dynamic pressures to the materials studied. To examine the efficiency of this combination on the improvement of dynamic pressure, a numerical computation is performed on this system. The details on the illustration of this method as well as the results of numerical investigation will be given.

An adaptive numerical scheme for high-speed reactive flow on overlapping grids

We describe a method for the numerical solution of high-speed reactive flow in complex geometries using overlapping grids and block-structured adaptive mesh refinement. We consider flows described by the reactive Euler equations with an ideal equation of state and various stiff reaction models. These equations are solved using a second-order accurate Godunov method for the convective fluxes and a Runge–Kutta time-stepping scheme for the source term modeling the chemical reactions. We describe an extension of the adaptive mesh refinement approach to curvilinear overlapping grids. Numerical results are presented showing the evolution to detonation in a quarter-plane provoked by a temperature gradient and the propagation of an overdriven detonation in an expanding channel. The first problem, which considers a one-step Arrhenius reaction model, is used primarily to validate the numerical method, while the second problem, which considers a three-step chain-branching reaction model, is used to illustrate mechanisms of detonation failure and rebirth for the channel geometry.

SELF-SUSTAINED MEAN STREAMING MOTION IN DIAMOND PATTERNS OF A GASEOUS DETONATION

A weakly nonlinear analysis of an overdriven detonation is carried out in the neighborhood of the instability threshold. The result leads to a nonlinear integral-differential equation describing the dynamics of the cellular front and exhibiting "diamond" patterns similar to those observed in experiments. An unexpected outcome of the analysis is a self-sustained mean streaming motion associated with the nonlinear dynamics of a weakly unstable detonation front.

Structure and stability of weak–heat–release detonations for finite Mach numbers

The stability of an overdriven detonation wave for a one–step Arrhenius reaction in an ideal gas is examined in the limits of a finite detonation Mach number, small modified heat release γ−1)β and large activation energy θ, limits that are relevant to the generation of the regular cellular detonation structures observed experimentally in highly diluted mixtures. Here, γ is the specific heat ratio and β is the formation energy. The limiting case, β = 0, corresponds to a finite Mach number reactionless shock, which in an ideal gas is always stable. Nevertheless, for small but finite values of (γ−1)β, we will show that detonation instabilities can arise when perturbations in the reaction rate generated by a small disturbance at the shock front are sufficient in magnitude to balance the corresponding acoustic fluctuations that are also generated. These acoustic disturbances are also those that govern the stability of a reactionless shock (β = 0). Such reaction–rate perturbations are only generated for sufficiently large activation energies, and it is the precise magnitude of the activation energy that leads to detonation instability that we identify here for various choices of β and γ. In particular, subject to the restriction on the modified heat release (γ−1)β ≪ 1, three new weakly exothermic ordered limits are identified that are relevant to detonation instability: (1) θ(γ−1) = O (1) and (γ−1) = O (1); (2) θ(γ−1) = O (1) and (γ−1) = o (1); and (3) θ(γ−1) ≫ 1 and (γ−1) = o (1).

Numerical study of wedge supported oblique shock wave-oblique detonation wave transitions

The results of a numerical study of premixed Hydrogen-air flows ignition by an oblique shock wave (OSW) stabilized by a wedge are presented, in situations when initial and boundary conditions are such that transition between the initial OSW and an oblique detonation wave (ODW) is observed. More precisely, the objectives of the paper are: (i) to identify the different possible structures of the transition region that exist between the initial OSW and the resulting ODW and (ii) to evidence the effect on the ODW of an abrupt decrease of the wedge angle in such a way that the final part of the wedge surface becomes parallel to the initial flow. For such a geometrical configuration and for the initial and boundary conditions considered, the overdriven detonation supported by the initial wedge angle is found to relax towards a Chapman-Jouguet detonation in the region where the wedge surface is parallel to the initial flow. Computations are performed using an adaptive, unstructured grid, finite volume computer code previously developed for the sake of the computations of high speed, compressible flows of reactive gas mixtures. Physico-chemical properties are functions of the local mixture composition, temperature and pressure, and they are computed using the CHEMKIN-II subroutines.

Numerical Investigation of Deflagration to Detonation Transitions

Transitions from deflagration to detonations are studied. A model consisting of the Navier-Stokes equation and a one-step reaction equation is used. Arrhenius kinetics are utilized, and the diffusive parameters of the problem are chosen so that the smallest scale of the problem can be resolved on the chosen grid. It is shown that for certain parameter regimes the transition can be described by three phases. First, an ordinary flame accelerates and a shock wave of increasing strength is produced ahead of and separated from the flame. In this phase, the shock wave "preconditions" the unburnt media. The pressure in the fluid between the flame and the shock wave increases with time. In the second phase, a "convected explosion" in the compressed state occurs, which is responsible for the transition to an overdriven detonation. In this phase peak pressures substantially larger than the Zeldovich Neumann Döring (ZND) pressure are obtained. Third, a relaxation to an ordinary Chapman-Jouget (CJ) detonation occurs. The convected explosion is driven by a gradient in the source term in the reaction equation. One-dimensional calculations show that the extension of the gradient required for a transition to occur is reduced if the strength of the shock wave is increased. The transition process is very sensitive to perturbations, and examples of this are shown by numerical experiments. It is also shown that the maximum separation between the shock wave and the flame zone increases with increasing activation temperature.

Relationship between the structure of vapor explosion and fragmentation mechanisms

A computer code PROVER-I for propagation phase of vapor explosion is developed to couple with MPS simulation. From the simulation results by MPS method, a new thermal fragmentation model is proposed with three types of time scale for modeling instant fragmentation, spontaneous nucleation fragmentation and normal boiling fragmentation. The structure of vapor explosion is investigated based on different fragmentation models. The vapor explosion with spontaneous nucleation fragmentation is characterized as a non-equilibrium overdriven detonation, and vapor explosion with hydrodynamic fragmentation is characterized as a non-equilibrium spontaneous burning. Sensitivity analyses are carried out for some parameters such as fragmentation time scale, delay time and vapor volume fraction. Spontaneous nucleation fragmentation with shorter delay time results in a vapor explosion structure situated between a non-equilibrium overdriven detonation and a non-equilibrium spontaneous burning, while a longer delay time gives a composite structure of vapor explosion which includes a shock and a non-equilibrium deflagration. The energy conversion ratio of a non-equilibrium overdriven detonation is much larger than that of a non-equilibrium spontaneous burning. The composite structure has the same destructive potential as non-equilibrium overdriven detonation. The variation of energy conversion ratio with the vapor volume fraction is consistent with the observations in fuel-coolant interaction experiments.

Diamond Patterns in the Cellular Front of an Overdriven Detonation

A nonlinear integral-differential equation describing the cellular front of an overdriven detonation is obtained by an analysis carried out in the neighborhood of the instability threshold. The analysis reveals both an unusual mean streaming motion, resulting from the rotational part of the oscillatory flow, and pressure bursts generated by the crossover of cusps representative of Mach stems propagating on the detonation front. A numerical study of the nonlinear equation exhibits the "diamond" patterns observed in experiments. An overall physical understanding is provided.

Numerical study on hypervelocity acceleration of flyer plates by overdriven detonation of high explosive

In most applications of explosives to flyer acceleration, the detonation of explosives is usually regarded as a steadily progressing wave phenomenon in which the pressure of the detonation products immediately behind the wave front is characterized by the so-called Chapman-Jouguet (C-J) pressure value. This type of detonation is therefore routinely referred to as the C-J detonation behavior and the detonation products start to expand from this C-J state to accelerate the flyer plate to a high velocity status. Overdriven detonation, however, is a detonation process that can provide a higher or much higher pressure than does the C-J detonation. Taking use of the detonation products from the overdriven detonation to push the plate may lead the plate to reach a hypervelocity status not achievable by means of the usual explosive acceleration techniques. This paper presents a numerical study on two acceleration systems for hypervelocity acceleration of plates by the application of overdriven detonation of explosives. The first acceleration system is the so-called planar acceleration system, which some researchers also call as the multi-stage launcher system. The other acceleration system is the improved technique that is named as the converging tunnel acceleration system. The numerical method used for the study mainly follows the formulation of HEMP computer code. Through numerical study, it is found that both systems can give an obvious improvement on the ability to hypervelocity acceleration of plates. Moreover, the comparison of two acceleration systems shows that the converging tunnel system has more superiority over the planar acceleration system for hypervelocity acceleration of plates.

Numerical simulations of pulsating detonations: II. Piston initiated detonations

Extremely long time, high-resolution one-dimensional numerical simulations are performed in order to investigate the evolution of pulsating detonations initiated and driven by a constant velocity piston, or equivalently by shock reflection from a stationary wall. The results are compared and contrasted to previous simulations where the calculations are initiated by placing a steady detonation on the numerical grid. The motion of the piston eventually produces a highly overdriven detonation propagating into the quiescent fuel. The detonation subsequently decays in a quasi-steady manner towards the steady state corresponding to the given piston speed. For cases where the steady state is one-dimensionally unstable, the shock pressure begins to oscillate with a growing amplitude once the detonation speed drops below a stability boundary. However, the overdrive is still being degraded by a rarefaction which overtakes the front, but on a time-scale which is very long compared with both the reaction time and the period of oscillation. As the overdrive decreases, the detonation becomes more unstable as it propagates and the nature (e.g. period and amplitude) of the oscillations change with time. If the steady detonation is very unstable then the oscillations evolve in time from limit cycle to period doubled oscillations and finally to irregular oscillations. The ultimate nature of the oscillations asymptotically approaches that of the saturated nonlinear behaviour as found from calculations initiated by the steady state. However, the nonlinear stability of the steady detonation investigated in previous calculations represents only the very late time (O(105) characteristic reaction times) behaviour of the piston problem.

Chapman-Jouguet Oblique Detonation Structure Around Hypersonic Projectiles

How to generate a steady-state detonation around a hypersonic projectile in stoichiometric hydrogen-oxygen premixed gases is studied. The speed of the hypersonic projectiles was beyond the Chapman ‐Jouguet(C‐J) detonation speed. The e owe eld around the projectile was visualized by using a gate intensie ed charge-coupled device camera(single-frame schlieren picturesand OHradical self-emissionimages ). Threeparameters are varied:1 ) the projectile e ight length from diaphragm rupture location, 2 )the initial pressure of mixture below/above the critical pressureforsteady detonation initiation around a hypersonicprojectile, and3 )theprojectilespeed.Atthepressure condition below the criteria, the detonation structure is composed of three different shock and detonation waves, which appear just after a diaphragm rupture and evolve in time: an overdriven bow detonation wave, a strong detonation wave, and a diffracted shock wave. Their propagation after diaphragm rupture is investigated, and it is found that the C ‐J detonation wave moved away from the projectile and only a reactive bow shock wave remained around projectile far away from the diaphragm. At the pressure condition above the criteria, a steady oblique detonation wave was generated around the projectile as soon as the projectile broke the diaphragm. In this steady-state detonation-wave case, with respect to the e ow Mach number behind the wave front, the whole detonation wavewas divided into fourparts: 1 )strong overdriven detonation wave, 2 ) weak overdriven detonation wave, 3) quasi-C‐J detonation wave, and 4 ) C‐J detonation wave. It has been found that a rarefaction wave is generated at the projectile shoulder and that curvature of the wave has a signie cant effect on the structure of the detonation wave.

One-dimensional overdriven detonations with branched-chain kinetics

The dynamics of time-dependent, planar propagation of gaseous detonations is addressed on the basis of a three-step chemistry model that describes branched-chain processes. Relevant nondimensional parameters are the ratio of the heat release to the thermal enthalpy at the Neumann state, the nondimensional activation energies for the initiation and branching steps, the ratio of the branching time to the initiation time and the ratio of the branching time to the recombination time. The limit of strong overdrive is considered, in which pressure remains constant downstream from the leading shock in the first approximation, and the ratio of specific heats γ is taken to be near unity. A two-term expansion in the strong overdrive factor is introduced, and an integral equation is derived describing the nonlinear dynamics and exhibiting a bifurcation parameter, the reciprocal of the product of (γ−1), the nondimensional heat release and the nondimensional branching activation energy, with an acoustic correction. A stability analysis shows that, depending on values of the parameters, either the mode of lowest frequency or a mode of higher frequency may be most unstable. Numerical integrations exhibit different conditions under which oscillations die, low-frequency oscillations prevail, high-frequency oscillations prevail, highly nonlinear oscillations persist, or detonation failure occurs. This type of parametric analysis is feasible because of the relative simplicity of the model, which still is more realistic than a one-step, Arrhenius chemical approximation. In particular, by addressing the limit of slow radical recombination compared with branching, explicit results are derived for the critical value of the bifurcation parameter, involving the ratio of the recombination time to the induction time. The results help to clarify the general nature of one-dimensional detonation instability and provide simplifications that can be employed in efficiently relating gaseous detonation behavior to the true underlying chemistry.

Initiation of a Porous Explosive by Overdriven Gas Detonation Products

This paper reports results of experiments on initiation and development of detonation in cylindrical charges of a porous explosive by overdriven detonation products of a gas mixture C2H2 + 2.5 O2. Explosive charges with a bulk density of about 1 g/cm3 in fragile shells were studied. For PETN and RDX charges, the critical initial pressure of the gas mixture at which detonation initiation still occurs is determined and the pressures acting immediately on the charge are given. For PETN, critical initial pressures and initiation delays were measured for the first time for charges with particles of various diameters. The obtained dependence characterizes the following abnormal property of porous charges: there is an optimum particle size for which the explosive sensitivity is maximal. Streak records of self‐luminosity for typical initiation modes are given. Mass velocity profiles in initiation waves at different depth of the charge are obtained using an electromagnetic procedure.

Experimental study of interaction between oblique detonation wave and rarefaction wave around a hypersonic free projectile

Hypersonic projectiles whose speed was beyond the Chapman-Jouguet (C-J) detonation speed, were fired into stoichiometric hydrogen-oxygen premixed gases. The flowfield around the projectile was visualized using a gated CCD camera. A steady-state detonation wave was generated around the projectile. With respect to the flow Mach number behind the wave front, the whole detonation-wave was partitioned into four parts; (i) strong overdriven detonation wave, (ii) weak overdriven detonation wave, (iii) quasi C-J detonation wave, and (iv) C-J detonation wave. It has been found that a rarefaction wave generated at the projectile shoulder has a significant effect on the structure of the detonation wave.

Numerical resolution of pulsating detonation waves

The canonical problem of the one-dimensional, pulsating, overdriven detonation wave has been studied for over 30 years, not only for its phenomenological relation to the evolution of multidimensional detonation instabilities, but also to provide a robust, reactive, high-speed flowfield with which to test numerical schemes. The present study examines this flowfield using high-order, essentially non-oscillatory schemes, systematically varying the level of resolution of the reaction zone, the size and retention of information in the computational domain, the initial conditions, and the order of the scheme. It is found that there can be profound differences in peak pressures as well as in the period of oscillation, not only for cases in which the reaction front is under-resolved, but for cases in which the computation is corrupted due to a too-small computational domain. Methods for estimating the required size of the computational domain to reduce costs while avoiding erroneous solutions are proposed and tested.

Reaction Zones of Detonating Solid Explosives

The reaction zones of normal and overdriven detonation waves in a number of solid HE were studied by recording the shock–wave luminosity in chloroform placed at the end of a high–explosive (HE) charge. The data obtained have led to some conclusions on the regularities of HE decomposition in a detonation wave. Thus, in a powerful solid HE, the heterogeneity of the charge plays a decisive role in the formation of a chemical spike. In this case, the time of reaction of heterogeneous HE correlates with the Jouguet pressure rather than with the sensitivity of the HE. The experimental parameters of the chemical spike are in good agreement with calculations on an extrapolated shock adiabat of the HE. This, however, does not indicate that the fraction of the HE decomposed directly at the detonation front is small but only shows that it depends smoothly on the front parameters. In overdriven detonation waves, an increase in the overcompression pressure is accompanied by an increase in the fraction of the HE decomposed directly at the front, and with a relatively large increase in pressure, the chemical spike completely disappears. In TATB and TATB–based HE, this occurs at a pressure of 40 GPa.

Linear stability of pathological detonations

In this paper we investigate the linear stability of detonations in which the underlying steady one-dimensional solutions are of the pathological type. Such detonations travel at a minimum speed, which is greater than the Chapman–Jouguet (CJ) speed, have an internal frozen sonic point at which the thermicity vanishes, and the unsupported wave is supersonic (i.e. weak) after the sonic point. Pathological detonations are possible when there are endothermic or dissipative effects present in the system. We consider a system with two consecutive irreversible reactions A→B→C, with an Arrhenius form of the reaction rates and the second reaction endothermic. We determine analytical asymptotic solutions valid near the sonic pathological point for both the one-dimensional steady equations and the equations for linearized perturbations. These are used as initial conditions for integrating the equations. We show that, apart from the existence of stable modes, the linear stability of the pathological detonation is qualitatively the same as for CJ detonations for both one- and two-dimensional disturbances. We also consider the stability of overdriven detonations for the system. We show that the frequency of oscillation for one-dimensional disturbances, and the cell size based on the wavenumber with the highest group velocity for two-dimensional disturbances, are both very sensitive to the detonation speed for overdriven detonations near the pathological speed. This dependence on the degree of overdrive is quite different from that obtained when the unsupported detonation is of the CJ type.

極超音速飛行体近傍に発生する定常斜めデトネーション波の構造

Hypersonic projectiles, whose speed was beyond the Chapman-Jouguet (C-J) detonation speed, were fired into stoichiometric hydrogen-oxygen premixed gases. The flowfield around the projectile was visualized using a gated CCD camera. Around the projectile, a steady-state detonation wave was generated. With respect to the flow Mach number behind the wave front, the whole detonation-wave was partitioned into four parts; (i) strong overdriven detonation wave, (ii) weak overdriven detonation wave, (iii) quasi C-J detonation wave, and (iv) C-J detonation wave, It has been found that a rarefaction wave delivered from the projectile shoulder has a significant effect on the structure of the detonation wave.

Evaluation of various theoretical equations of state used in calculation of detonation properties

The thermodynamic properties of detonation products are computed by allowing for the presence of several phases of gases and solids in chemical equilibrium. We briefly describe the main perturbation theories found in the literature and we discuss their reliability in including consideration of the adiabatic gamma. We also consider an equation of state of carbon in detonation products which allows a smooth transition from an effective graphite phase to an effective diamond phase. We extensively compare the most reliable calculated results with measured detonation properties of explosives made up of carbon, hydrogen, oxygen, and nitrogen (detonation velocity, kinetic energy transferred in a cylinder test, overdriven detonation, and Chapman–Jouguet pressure). Several subjects for further study of a priori calculation of detonation properties are indicated.