Model Detonation of Solid Explosives by the Homogeneous Mechanism

A twophase detonation model of solid porous explosives, which takes into account the compression of solidstate particles and the presence of a solid component in detonation products, is developed. The homogeneous detonation mechanism based on the Arrheniustype reaction is studied. Detonation is initiated by the region of highpressure and hightemperature gases modeling the detonator explosion. A numerical experiment confirms that there exists an initiationpressure limit below which no homogeneous mechanism of detonation is active. The detonation wave has the leading front in which the explosive is precompacted and the gas in the porous volume is compressed up to a pressure of 100 GPa without any significant change in particle density. Then the gas compresses the particles themselves up to the pressure and temperature of the thermal explosion. As a result, the leading front is followed (after a certain delay) by a narrow reaction zone where detonation products are formed with a further increase in pressure.     

Passage of a Bubble‐Detonation Wave into a Chemically Inactive Bubble Medium

Passage of detonation waves from a chemically active bubble medium into a chemically inactive bubble medium is studied experimentally. The structure of incident (detonation) and transmitted (postdetonation) waves is investigated, and the pressures of these waves for different parameters of bubble media are measured. The evolution of postdetonation waves is traced. Decay constants of postdetonation waves are determined. The speeds of propagation of detonation and postdetonation waves are measured. The energydissipation mechanisms for detonation and postdetonation waves in bubble media are analyzed qualitatively.     

Discoveries in Detonation of Molecular Condensed Explosives in the 20th Century

A number of experimental results that could not be satisfactorily explained within the framework of the Grib—Zel'dovich—Neumann—Döring detonation theory are reviewed, namely, the oscillating detonation of some liquid high explosives (HE), the weak dependence of the time of detonation transformation of heterogeneous charges on their structure (particle size, liquid or solid state, etc.) with a strong dependence of the critical diameter of detonation on the structure, and the extremely weak dependence of the detonation rate of liquid HE on the charge diameter with a significant value of the critical diameter of detonation. These studies yielded the following results: 1) for each heterogeneous HE, a typical shock–wave pressure p ^* and a typical initial density ₀ ^* were found, such that, for their low values, HE transformation follows the mechanism of hot points (depends on the charge structure), and for high values of these parameters, HE transformation obeys the homogeneous mechanism (does not depend on the charge structure); 2) two new theoretical notions were discovered and used in the detonation theory: the phenomenon of breakdown of the chemical reaction in the shock–wave front by rarefaction waves and the notion of a shock jump, which reflects the specific character of action of shock waves on complex multiatomic molecules of condensed HE. It was also shown that the discovery of the parameters p ^* and ₀ ^* and the breakdown and shock jump phenomena allowed one to confirm experimentally the explanations of the above observations, which are incompatible with the Grib—Zel'dovich—Neumann—Döring detonation theory, to propose the structure of the front of detonation waves both in homogeneous (stable and oscillating) and heterogeneous HE whose basic property is HE transformation (partial or complete depending on the HE power and initial density) already in the shock–wave front, and to proposed principally new ideas on the nature of the critical diameter of detonation of homogeneous and heterogeneous HE.     

Detonation Properties and Electrical Conductivity of Explosive–Metal Additive Mixtures

Detonation properties of mixtures of condensed high explosives with metal additives are studied. A scheme of measurement of high electrical conductivity of detonation products ( > 10 ^–1 · cm^–1) with a time resolution of 10 nsec is developed. It is shown that the properties of detonation products depend significantly on the content of the additive in the HE and on dispersion and density of the mixture. The electrical conductivity of detonation products of the compositions examined reaches 5 · 10^{3 –1} · cm^–1, which is more than three orders higher than the electrical conductivity of the HE without the additive. Significant variation of electrical conductivity of detonation products over the conducting region thickness has been found. The main conductivity corresponds to a sector 1 mm long near the detonation front. The overdriven state of the detonation wave has a strong effect on electrical conductivity and conducting region thickness. It is assumed that the behavior of electrical conductivity with time is caused by successive processes of shock compression of the HE, excitation of the chemical reaction (including the reaction of the additive with detonation products), and expansion of detonation products. The measurement technique used is highly informative due to the possibility of studying detonation in various regimes.     

Current Waves Generated by Detonation of an Explosive in a Magnetic Field

The structure of the electromagnetic field in detonation of a condensed explosive in a magnetic field is analyzed qualitatively. Propagation of a detonation wave in a magnetic field leads to generation of an electric current in explosion products. The physical reason for current generation is the freezing of the magnetic field into the conducting substance at the detonation front and subsequent extension of the substance and the field in the unloading wave. The structure of the current layer depends on the character of the boundary magnetic fields and conditions on the surface of initiation of the explosive. Detonation of the explosive in an external magnetic field B₀ generates a system of two currents identical in magnitude but opposite in direction. The structure of the arising current and its absolute value are determined by the parameter R₁ = _{0 0}D²t (₀ is the magnetic permeability of vacuum, ₀ is the electrical conductivity of detonation products, D is the detonationfront velocity, and t is the time). The value of the current increases with the detonationwave motion, and the linear current density is limited from above by 2B₀/₀. For R₁ 1, the electric field in the conducting layer is significantly nonuniform; for detonation products with a polytropic equation of state, a region of a constantdensity current is adjacent to thedetonation front. The results of this analysis are important for interpretation of experiments performed and development of new methods for studying the state of the substance in the detonation wave.     

Experimental Investigation and Numerical Simulation of an Expanding Multifront Detonation Wave

Results of experimental investigations of an expanding multifront detonation wave are presented. Two stages of spontaneous formation of new disturbances and transverse waves on the expanding detonationwave front are observed. The main mechanisms of reinitiation of detonation waves are discussed. Twodimensional numerical simulation of the dynamics of a multifront detonation wave in a linearly expanding channel is performed. The effect of spontaneous formation of new disturbances and new transverse waves is confirmed by computations, and the main mechanism of multiplication of transverse waves is the instability of detonationwavefront elements at the stage they cease to be in the overdriven state and are attenuated during expansion.     

Triggering of Detonation upon Flame Interaction with an Expansion Wave

The transition of a deflagration wave into an abruptly expanding part of a plane channel, where a quasisteady supersonic underexpanded jet of an unburned gas is formed, is studied for a propane–oxygen mixture using schlieren pictures. Two explosioninitiation modes (weak and strong) are registered. In the first case, almost instantaneous onset of the detonation wave occurs when the flame front enters the expanding section; the initial velocity of this wave is approximately 1.5 times the Chapman–Jouguet detonation velocity (D_CJ) and then decreases to a value corresponding to selfsustaining detonation. In the second case, the front velocity gradually increases from 0.4D _CJ to 1.0D _CJ. It is established that the starting pulse triggering the transformation of turbulent combustion to explosion and detonation regimes is generated by interaction of the flame front with expansion waves, which are elements of the structure of the initial section of the jet.     

Possibility of determining the velocities of turbulent and laminar flames at high initial temperatures

A low-velocity detonation regime without self-ignition is discovered in which turbulent flame is held at a distance of several channel diameters behind the leading-shock wave due to gas suction to the turbulent boundary layer at the tube wall. The structure of such a detonation agrees principally with the structure of low-velocity detonation in a capillary with a laminar boundary layer. Calculation results for the distance from the shock wave to the flame agree with the experimental data. It is proposed to use the experimental value of the distance to determine flame velocities in a nonturbulent shock-heated gas under conditions of extremely short ignition delays. The domains of existence of the initial pressures of multiheaded and low-velocity detonations partially overlap.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 4, pp. 43–46, July–August, 1996.     

Mechanical Sensitivity and Detonation Parameters of Aluminized Explosives

Experiments were performed to study the effect of the species particle size and structure of aluminized mixture samples on the sensitivity and detonation parameters of HMX, nitroguanidine, bis(2,2,2trinitroethyl)nitramine, and their mixtures with an Al powder with a mean particle size of 0.1 – 150 m. The addition of ultrafine Al to HMX and bis (2,2,2trinitroethyl)nitramine substantially increases the sensitivity to mechanical effects and decreases the detonation velocity. In compositions with nitroguanidine, the detonation velocity practically does not vary. For nitroguanidine, the width of the chemicalreaction zone and Chapman–Jouguet parameters were determined by recording the detonationpressure profiles. The pressure profiles for bis(2,2,2trinitroethyl)nitramine show that detonation decomposition can occur in two stages. A twopeak detonationwave structure was detected for mixtures of HMX with Al. Temperature measurements indicate that Al interacts with detonation products in the immediate proximity to the front. The highest temperature was recorded for compositions containing ultrafine aluminum and an aluminum dust.     

Mechanism of Formation of a Fast Gas Jet

The formation of a fast gas jet from not too long, plane, Ushaped charges is studied by radiography. It is shown that a shockcompressed region is formed in the air cavity as a result of collision of explosion products. After detonation of the charge, a jet flows from this region with a velocity exceeding the detonation velocity. The jet exhibits a cumulative effect, which is maximal for a square air cavity.     

Application of Synchrotron Radiation for Studying Detonation and Shock‐Wave Processes

A new method of remote investigation of detonation and shockwave processes with the use of synchrotron radiation is proposed. The facility used for the first experiments with measurement of density and smallangle xray scattering in detonation of condensed explosives is described. The high time and spatial resolution of the techniques proposed allows one to determine the character and mechanism of destruction of the condensed phase and the growth dynamics of new structures, including crystalline ones, in detonation flows. The capabilities of the new technique are described.     

Thrust Performance of an Ideal Pulse Detonation Engine

Quasisteady and twodimensional unsteady formulations of the problem on the operation cycle of a pulse detonation engine are derived. A formula for the specific impulse is obtained, and the thrust performance of the engine is calculated. It is found that the thrust performance of this engine for flight Mach numbers M [0; 3.6] and compression ratios p ₂/p ₁ [1; 80] are always higher than those of the ramjet and onespool turbojet. As the compression ratio increases, the advantage of the pulse detonation engine becomes less noticeable.     

Effect of Channel Geometry and Mixture Temperature on Detonation‐to‐Deflagration Transition in Gases

Results from theoretical and experimental studies of deflagrationtodetonation transition in hydrocarbon–air mixtures are reported. The effects of internal geometry, turbulence transition of the flow, and the temperature and fuel concentration in the unburned mixture on detonation initiation are considered.     

Relationship between brisance of explosives and their molecular-atomic structures

The explosives with various molecular-atomic structures substantially differ by their detonation velocities and brisance but often are similar by the expansion of their detonation products (DP's) which mainly consist of the same molecules. Such explosives referred to as “usual” show the relationship between ϱD and brisance determined by different methods. There are linear correlation relations between the results obtained. This relationship is not observed with the “unusual” explosives which differ from the “usual” ones by the chemistry of detonation processes. These explosives include liquid explosives, explosive-oxidants. CNO- and HNO-explosives and also CHNOF-explosives. Their calculation of thc detonation parameters and brisance from the same criterions which characterize the chemical composition of the explosives and the detonation products, results in some errors. Taking these differences into account it is possible in some cases markedly to increase the accuracy of the detonation parameters. As an example is the calculation of the detonation pressure to within 3% based on the linear correlation relation between the pressure (P_J) and the relative detonation impulse (I_rel) which characterizes the charge ability to do work at the initial stages of thc expansion of the detonation products: The relative impulse, in its turn, may be calculated both for “usual” and “unusual” explosives from the atomic composition of an explosive, its density and the enthalpy of the formation with the error that does not exceed the experimental (2%).     

Detonation and Physicochemical Properties of Cannon Powders

Detonation and physicochemical properties of several cannon powders were studied. Dependences of the critical diameter and detonation velocity of heptachannel pyroxylin powders on the powder element diameter were determined. The mechanism of detonation propagation over a charge was studied for various positions of the powder elements relative to one another and to the shockwave front.     

The Use of Utilizable Explosives to Increase the Efficiency of an Explosion

The possibility of using the utilizable explosives to increase the efficiency of an explosion of industrial high–explosive (HE) charges is studied under laboratory and working conditions. To do this, the explosive charges were used as linear initiators of the elongated charges of industrial HE. It is shown that the placing of an NB–40 ballistite powder rod of diameter 10 mm in a bulk–density TNT charge of diameter 40 mm increases the velocity of acceleration of an aluminum shell by 14% (the ratio between the detonation velocities of the powder and TNT is 1.8 : 1.0). The use of ShZ–1 TNT–based and ShZ–2 RDX–based hose charges in well charges of industrial HE, such as 79/21 Grammonit (79% granular ammonium nitrate/21% scale–shaped TNT), 30/70 Grammonit (30% granular ammonium nitrate/70% granular TNT), and ammonium nitrate, as linear initiators leads to a decrease in the output of bulky rock by 15—20% and allows one to increase the grid of the wells of diameters 160 and 220 mm by 20—25% with preservation of the rock output. The ratio of the detonation velocities of ShZ–1 and ShZ–2 and industrial HE charges is within 1.5—1.7 in the case of 79/21 Grammonit and 2.2—2.6 in the case of ammonium nitrate. The results obtained are explained by the fact that the detonation of a linear initiator from utilizable materials changes the form of the detonation wave front of the basic charge; as a result, it arrives at the surface of an ambient medium at a large angle and a more intense shock wave enters the medium compared to the case without a linear initiator.     

Phase Diagram of Ultrafine Carbon

A three-dimensional phase diagram of carbon has been built in the coordinates pressure–temperature—dispersivity on the basis of the published data on detonation-diamond properties. Key words: three-dimensional phase diagram, detonation diamond, nanoscale diamond particles, ultrafine carbon.     

Size Effect and Detonation Front Curvature

A simple theory relates the size effect (decrease of the detonation velocity with decreasing radius) of a cylinder with its average sonic reaction zone length, 〈x_e〉, i.e. the distance from first reaction to the sonic plane. The size effect is described by where R₀ is the radius, U_s and D the detonation velocities at R₀ and at infinite size and σ is a function describing the extent of wall motion, which is calibrated using four explosives. In this theory, the cylindrical symmetry imposes a quadratic shape to the detonation front. The lag distance at the edge of the cylinder, L₀, is related to the reaction zone length by 〈x_e〉 ≈︁ L₀. Collected results are presented for 56 measured curvatures on 26 explosives, with reaction zone lengths varying from 0.1 mm to 30 mm.     

On the Hydrodynamic Theory of Detonation

When the enthalpy of the unreacted explosive is correctly taken into account, the detonation rate D depends to some extent on the explosive's sound speed a₀. With a perfect or Abel gas equation, correction terms of the order $ 1 - \frac{{{\rm a}_{\rm 0}^{\rm 2} }}{{{\rm D}^{\rm 2} }} $ with respect to the classical theory are obtained. They increase D, but lower density and pressure in thc C-J plane. This agrees with the finding that data derived from experiments on the basis of the classical theory correspond better to a weak Than to a C-J detonation. The observed difference in density coefficients, dD/dϱ₀, of liquid and solid TNT as well as the anisotropy of D of elastically stretched rubberized high explosives are quantitatively understood. It is shown that the extended equations, though based on a gas equation of state, hold approximately for condensed explosives too, provided their compressibility is not too low.     

Combustion and the transition to detonation in a gas mixture in a closed space

The possibility of going from deflagration to detonation in mixtures of CH₄+2(O₂+N₂) and 2H₂+O₂ ±N₂ (03.76) was investigated experimentally in a space enclosed by various combinations of three thin-walled metal concentric turbulator-sphere (diameters in a ratio 124) with large numbers of openings (permeability 0.1–0.4). Transition from deflagration to detonation was observed for 1 in the first mixture and for 3.2 in the second.Balashikha. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 171–174, May–June, 1993.