Mechanism of detonation of emulsion explosives with microballoons

A mechanism of detonation of emulsion explosives containing microballoons in finite-diameter charges is described. A parametric dependence of the detonation velocity on the charge characteristics is obtained. The fact that the reaction-zone width increases with decreasing charge porosity is explained. It is shown that the emulsion does not completely burn out at the Chapman-Jouguet point. Final formulas for calculating the reaction time and reaction-zone width are given.

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Experimental and numerical studies on transmission of gaseous detonation to a less sensitive mixture

A phenomenon of detonation transmission from one gaseous mixture (donor) to another of lower sensitivity (acceptor) was studied experimentally and numerically. The objective was to study effects of a donor mixture length and acceptor mixture sensitivity on the possibility of detonation transmission. Experiments were carried out in detonation tube 9.5–12 m long and 174 mm id. Three types of donor mixtures were used in the driver: stoichiometric acetylene/air, stoichiometric hydrogen/air, and 20% of hydrogen/air. Air mixtures with 14–29.6% of hydrogen were used as acceptors. Driver length varied from 0.17 to 5.6 m. Detonation transmission was studied for an abrupt opening of interface between two mixtures. Series of 1D and 2D calculations were made to simulate the problem numerically. Both, results of experiments and calculations revealed a set of parameters that effect transmission process. Critical conditions were determined as minimum driver length expressed in terms of characteristic chemical reaction length scales of acceptor mixture. They were shown to depend on differences in reaction rates and energy contents of donor and acceptor mixture. Received 6 January 1997 / Accepted 20 March 1997     

Experimental study on detonation parameters and cellular structures of fuel cloud

In this paper,detonation parameters of fuel cloud,such as propylene oxide(PO),isopropyl nitrate(IPN),hexane,90 # oil and decane were measured in a self-designed and constructed vertical shock tube.Results show that the detonation pressure and velocity of PO increase to a peak value and then decrease smoothly with increasing equivalence ratio.Several nitrate sensitizers were added into PO to make fuel mixtures,and test results indicated that the additives can efficiently enhance detonation velocity and pressure of fuel cloud and one type of additive n-propyl nitrate(NPN) played the best in the improvement.The critical initiation energy that directly initiated detonation of all the test liquid fuel clouds showed a U-shape curve relationship with equivalence ratios.The optimum concentration lies on the rich-fuel side(φ > 1).The critical initiation energy is closely related to molecular structure and volatility of fuels.IPN and PO have similar critical values while that of alkanes are larger.Detonation cell sizes of PO were respectively investigated at 25 C,35 C and 50 C with smoked foil technique.The cell width shows a U-shape curve relationship with equivalence ratios at all temperatures.The minimal cell width also lies on the rich-fuel side(φ > 1).The cell width of PO vapor is slightly larger than that of PO cloud.Therefore,the detonation reaction of PO at normal temperature is controlled by gas phase reaction.     

Effect of reflection type on detonation initiation at shock-wave focusing

Abstract. From practical and theoretical standpoints, the initiation of combustion in gaseous media due to the shock waves focusing process at various reflectors is a subject of much current interest. The complex gas flowfield coupled with chemical kinetics provides a wide spectrum of possible regimes of combustion, such as fast flames, deflagration, detonation etc. Shock wave reflection at concave surfaces or wedges causes converging of the flow and produces local zones with extremely high pressures and temperatures. The present work deals with the initiation of detonation due to shock waves focusing at parabolic and wedge reflectors. Particular attention has been given to the determination of the critical values of the incident shock wave (ISW) Mach number, parameters of the combustible mixture, and geometrical sizes of reflector at which different combustion regimes could be generated. Received 30 August 1999 / Accepted 23 February 2000     

Ignition of liquid and dust fuel layers by gaseous detonation

Abstract. Ignition of a liquid layer and dust fuel layer by a detonation wave propagating in hydrogen-oxygen and acetylene-oxygen mixtures is reported. Experiments were carried out using a shock tube equipped with optical-quality observation windows. A schlieren system and a high-speed camera were used for measurements of ignition delay. Pressure transducers provided data necessary for measurements of the detonation wave velocity and pressure variation within the front of the interacted detonation wave and fuel layer. Kerosene, nitroglycerin and PETN were used as fuels. Investigation shows that the layer of liquid fuel can be efficiently ignited by detonation wave. It was found that the ignition delay of the fuel layer depends mostly on the detonation wave velocity and sensitivity of igniting fuels, and slightly on the layer thickness. Received 12 August 2001 / Accepted 1 July 2002 Published online 4 February 2003 Correspondence to: P. Wolanski (e-mail: wolanski@itc.pw.edu.pl) An abridged version of this paper was presented at the 18th Int. Colloquium on the Dynamics of Explosions and Reactive Systems at Seattle, USA, from July 29 to August 3, 2001     

Hydrogen detonation and fast deflagration triggered by a turbulent jet of combustion products

Initiation of detonation by a turbulent jet of combustion products has been studied in a detonation tube of 141 mm inner diameter. Jet formation techniques based on either a perforated plate or bursting membrane subjected to the impact of a stable detonation wave were utilized. Critical conditions of detonation initiation in hydrogen–air and hydrogen–oxygen–nitrogen mixtures have been found to depend on both the mixture sensitivity and the geometrical parameters of the arrangement. PACS 47.70.Fw; 82.33.Vx; 82.40.Fp This paper was based on work that was presented at the 19th Inter-national Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003     

Optimization study of spray detonation initiation by electric discharges

Development of air-breathing pulse detonation engines is faced with a challenging problem of detonation initiation in fuel sprays at distances feasible for propulsion applications. Extensive experimental study on initiation of a confined n-hexane spray detonation in air by electric discharges is reported. It is found that for direct initiation of spray detonation with minimal energy requirements (1) it is worth to use one discharger located near the closed end of a detonation tube and at least one additional discharger downstream from it to be triggered in-phase with primary shock wave arrival; (2) the discharge area should be properly insulated to avoid electric loss to metal tube walls; (3) discharge duration should be minimized to at least 50 μs; (4) discharge channel should preferably occupy a large portion of a tube cross-section; (5) test tube should be preferably of a diameter close to the limiting tube diameter; (6) gradual transition between the volume with electric discharger and the tube should be used; and (7) a powerful electric discharger utilized for generating a primary shock wave can be replaced by a primary shock wave generator comprising a relatively low-energy electric discharger, Shchelkin spiral, and tube coil. With all these principles implemented, the rated electric energy of about 100 J was required to initiate n-hexane spray–air detonation in a 28-mm tube at a distance of about 1 m from the atomizer. PACS 02.60.Cb; 05.10.Ln; 47.11.+j; 47.15.Cb; 47.40.Nm This paper was based on work that was presented at the 19th Inter-national Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003     

Detonation initiation developing from the Richtmyer–Meshkov instability

Detonation initiation resulting from the Richtmyer–Meshkov instability is investigated numerically in the configuration of the shock/spark-induced-deflagration interaction in a combustive gas mixture. Two-dimensional multi-species Navier–Stokes equations implemented with the detailed chemical reaction model are solved with the dispersion-controlled dissipative scheme. Numerical results show that the spark can create a blast wave and ignite deflagrations. Then, the deflagration waves are enhanced due to the Richtmyer–Meshkov instability, which provides detonation initiations with local environment conditions. By examining the deflagration fronts, two kinds of the initiation mechanisms are identified. One is referred to as the deflagration front acceleration with the help of the weak shock wave, occurring on the convex surfaces, and the other is the hot spot explosion deriving from the deflagration front focusing, occurring on the concave surfaces. The project supported by the National Natural Science Foundation of China (90205027 and 10632090).     

A study on jet initiation of detonation using multiple tubes

A detonator consisting of a dense bundle of small-diameter tubes (4.4–19 mm) is tested experimentally using stoichiometric mixtures of hydrogen–oxygen and hydrogen–air. Tests are conducted in a 5,200-mm long detonation tube fitted with a schlieren photograph section and smoked foil to record the deflagration to detonation (DDT) transition. It is confirmed that the flame jet emanating from the tube assembly causes detonation initiation immediately downstream of the detonator, with little dependence on the size of the detonation tube. For the fuel–air mixture, the insertion of Shchelkin spirals into each of the smaller tubes enhances the development of the turbulent flame jet, leading to a shorter DDT distance. Multi-point spark ignition is also shown to provide a further reduction in the DDT distance compared to single-point ignition. PACS 47.40.-x; 47.40.Nm; 47.70.Fw; 82.40.-g; 82.40.Fp     

Gaseous nitromethane and nitromethane-oxygen mixtures: A new detonation structure

Detonation in gaseous nitromethane (NM) and mixed with O₂ has been studied. Experiments were performed in a preheated steel tube at an initial temperatureT ₀∼=390 K for different initial pressuresP ₀ (1.7≥P ₀≥5 10⁻² bar). Different selfsustained detonation regimes were obtained, from multiheaded mode to spinning and galloping mode in marginal conditions. These chemical systems were characterized by a specific detonation cellular structure very different from that currently observed with classical gaseous C _n H _m /O₂/N₂ mixtures. All modes of detonation propagation in rich NM/O₂ mixtures exhibit a double scale cellular structure. The pattern of this double scale structure is particularly clear in the case of spinning mode. An abridged version of this paper was presented at the 15th Int. Colloquium on the Dynamics of Explosions and Reactive Systems at Boulder, Colorado, from July 30 to August 4, 1995     

Meshfree particle simulation of the detonation process for high explosives in shaped charge unlined cavity configurations

The numerical simulation of the detonation of a high explosive (HE) is generally not an easy task for traditional grid based methods. Smoothed particle hydrodynamics (SPH) method, as a meshfree, Lagrangian and particle method, provides a very attractive approach in dealing with large deformations and large inhomogeneities in the extremely transient high explosive detonation and later expansion process. This paper presents the application of SPH to simulate and analyze the detonation process of high explosive in shaped charge. A three-dimensional SPH code is developed and applied to simulate the shaped charge detonation process in different scenarios. It is observed that for high explosive in a shaped charge, the detonation produced gaseous products experience strong convergence that forms an extremely high-pressure gas jet. Factors such as different charge cavity shapes and different detonation models lead to quite different behavior of the gas jet convergence and later divergence. Further analyses reveal that a critical value for the charge head length exists. Beyond this critical value, increasing the charge head length will not result in improvement on the gas jet convergence performance.Received: 11 March 2002, Accepted: 9 December 2002, Published online: 28 April 2003     

A numerical simulation of reflection processes of a detonation wave on a wedge

Abstract. A two dimensional numerical simulation has been performed to study reflection processes of detonation waves on a wedge. The numerical scheme adopted is the flux corrected transport scheme and a two-step chemical reaction is assumed for a stoichiometric oxyhydrogen mixture diluted with argon. Transverse wave structures of the detonation are produced by artificial disturbances situated in front of a one-dimensional Chapman-Jouguet detonation wave. Numerical grids are generated by solving a Laplace equation. Results show that in the case where Mach reflection occurs, the cells in the Mach stem are smaller than those in the incident wave and are distorted in shape. There is also an initiating stage during which the cells in the Mach stem are created. The critical angle beyond which Mach reflection cannot occur is discussed. Received 15 October 1999 / Accepted 27 March 2000     

Reinitiation process of detonation wave behind a slit-plate

The propagation phenomenon of a detonation wave is particularly interesting, because the detonation wave is composed of a 3D shock wave system accompanied by a reaction front. Thus, the passage of a detonation wave draws cellular patterns on a soot-covered plate. The pressure and temperature behind the detonation wave are extremely high and may cause serious damages around the wave. Therefore, it is of great significance from a safety-engineering point of view to decay the detonation wave with a short distance from the origin. In the present study, experiments using high-speed schlieren photography are conducted in order to investigate the behaviors of the detonation wave diffracting from two slits. The detonation wave produced in a stoichiometric mixture of hydrogen and oxygen is propagated through the slits, and the behaviors behind the slit-plate are investigated experimentally. When a detonation wave diffracts from the slits, a shock wave is decoupled with a reaction front. Since the two shock waves propagate from the slits interact with each other at the center behind the plate, the detonation wave is reinitiated by generating a hot-spot sufficient to cause local explosions. Furthermore, it is clarified that the shock wave reflected from a tube-wall is also capable of reinitiating the detonation wave. The reinitiation distance of the detonation wave from the slit-plate is correlated using a number of cells emerged from each slit.      

Visualization of turbulent combustion of TNT detonation products in a steel vessel

Mixing and afterburning of TNT detonation products in a steel vessel are recorded by the use of the Schlieren visualization system and high speed photography. The vessel is filled with air or 50% oxygen enriched air. Overpressure histories at the vessel wall are also recorded by using pressure transducers. In these experiments nitrogen, air or 50% oxygen enriched air are used as vessel fillers. The Oppenheim-Kuhl theory of thermodynamics of closed systems is applied to estimate the released energy on the basis of pressure histories. Received 29 August 1999 / Accepted 21 January 2000     

Detonation in heterogeneous mixtures of liquids and particles

The mechanisms of detonation propagation in heterogeneous systems comprising closely packed particles and a liquid explosive are not fully understood. Recent experimental work has suggested the presence of two distinct modes of detonation propagation. One mode is valid for small particles (which is the regime we will address in this paper) with another mode for large particles. In this work we model numerically the detail of the wave interactions between the detonating liquid and the solid particles. The generic system of interest in our work is nitromethane and aluminium but our methodology can be applied to other liquids and particles. We have exercised our numerical models on the experiments described above. Our models can now qualitatively explain the observed variation in critical diameter with particle size. We also report some initial discrepancies in our predictions of wave speeds in nominally one dimensional experiments which can be explained by detailed modelling. We find that the complex wave interaction in the flow behind the leading shock in the detonating system of liquid and particles is characterised by at least two sonic points. The first is the standard CJ point in the reacting liquid. The second is a sonic point with respect to the sound speed in the inert material. This leads to a steady state zone in the flow behind the leading shock which is much longer than the reaction zone in the liquid alone. The width of this region scales linearly with particle size. Since the width of the subsonic region strongly influences the failure diameter we believe that this property of the flow is the origin of the observed increase in failure diameter with particle size for small inert particles. Received 3 December 1999 / Accepted 5 July 2000     

Ignition and growth of a detonation by a high energy plasma

Many experimental and numerical studies have been achieved to describe the transition process of deflagration to detonation when a projectile impacts an explosive. Also a large work has been done for the determination of various parameters — such as the impact pressure, the efficiency factors, etc. of the laser — material interaction. When a laser beam impacts an explosive, the P² criterion, characteristic of shock initiated detonations, is no longer valid due to the generated hot plasma whose effect is to decrease the DDT (Deflagration to Detonation Transition) duration. The present paper deals with a modelling of the plasma-explosive medium allowing the determination of distances and times of the DDT process. The two phase modelling of the granular explosive takes into account the creation of hot spots. The pressure of the plasma is computed using a semi empirical model, while the temperature is obtained from Maxwell Boltzmann statistics. The authors focused their attention on the equation of state for the detonation products and the numerical process.     

Ignition and combustion study of JP-8 fuel in a supersonic flowfield

The ignition and combustion process of fuels in a supersonic combustion chamber plays an important role in the design of hypersonic propulsion system. However, it is a quite complicated process, due to the large variation of inlet air velocity, temperature, oxygen concentration, and shocks in the supersonic combustion chamber. The purpose of this paper is to observe the ignition delay and combustion phenomenon of the JP-8 fuel droplets in a supersonic flowfield experimentally. A shock tube is used as a basic test facility to create a high-speed and high-temperature flowfield as a supersonic combustor. In the experiments, several test parameters are controlled, such as shock velocity, gas temperature, fuel droplet size and distance, initial fuel temperature, and oxygen concentration, etc. The test results show the influence of these parameters on ignition delay, ignition limitation, and detonation. The most important factor in the experiment is the initial fuel temperature effect, which is influenced by the altitude variation during a flight. Received 4 August 1995 / Accepted 12 December 1995     

Structure and multiplicity of detonation regimes in heterogeneous hybrid mixtures

The problem of propagation of steady nonideal detonations in heterogeneous hybrid mixtures is studied in the case of a hydrogen-air gaseous mixture with suspended fine aluminum particles. Due to the difference in the order of magnitude of the characteristic induction and combustion times of gaseous mixture and solid particles, the process of energy release behind the leading shock front occurs over an extended period of time and in a nonmonotonic way. An approximate numerical model has been improved to find the steady propagation regimes and investigate their structure. The problem is analyzed in the frame of the theory of the mechanics of multiphase media with mass, momentum and heat exchanges between particles and gases. The one-dimensional ZND model of detonation with losses to the lateral boundaries is used. It is shown that three different steady propagation regimes may exist: the Pseudo-Gas Detonation (PGD), the Single-Front Detonation (SFD) and the Double-Front Detonation (DFD). The numerical results match the available experimental results obtained previously. The influence of the fundamental parameters of the system on the domains of existence of the different regimes is displayed. Moreover, it is shown that, according to the theory of nonideal detonations with nonmonotonic energy release, there may exist a multiplicity of detonation modes. However, the total number of solutions actually obtained by numerical calculations differs from that predicted by the theory. The reasons for these discrepancies are discussed.     

The evolution of a detonation wave in a variable cross-sectional chamber

A two-dimensional numerical simulation has been performed to study the interaction of a gaseous detonation wave with obliquely inclined surfaces in a variable cross-sectional chamber. The weighted essentially non-oscillatory (WENO) numerical scheme with a relatively low resolution grid is employed. A detailed elementary chemical reaction model with 9 species and 19 elementary reactions is used for a stoichiometric oxy-hydrogen mixture diluted with argon. In this work, we study the effect of area expansion and contraction on the main/gross features of the detonation cellular structures in the presence of detonation reflection, diffraction and localized explosion. The result shows that there exists a transition region as the detonation wave propagates through the converging/diverging chamber. Within the transition region, the initial regular detonation cells become distorted and irregular before they re-obtain their regularity. While the ultimate regular cell size and the length of the transition region are strongly affected by the converging/diverging angle, the width/length ratio of the cells is fairly independent of it. A localized explosion near the wall is found as the detonation wave propagates in the diverging chamber.      

Some observations of the controlled generation and onset of detonation

Detailed observation of deflagration to detonation transition (DDT) is inherently difficult. This is primarily due to the stochastic nature of flame acceleration and shock formation processes that in most practical situations give rise to the conditions required for detonation to emerge. The present paper describes how shock tube techniques have been used to control the conditions required for the onset of detonation. The paper first outlines some initial experiments involving turbulent flame acceleration before concentrating on experiments in a reflected shock mode. To aid interpretation of the observations the paper also presents a simple gasdynamic analysis of particle trajectories and considers the various physical and chemical processes that could lead to the onset of detonation. Received 27 November 2001 / Accepted 28 January 2002