Home – Home – Jornals – Combustion, Explosion and Shock Waves 2015 number 1

The state of the art in the field of initiation of combustible mixtures is overviewed. Recommendations on using various formulas for estimating the critical energy (of both ignition of the mixture and initiation of detonation) are given. The importance of the spatial and temporal components in the energy release law is noted, especially for initiation optimization. Unsolved and new problems of the theory of combustion and detonation are formulated.

The current status of experimental research of continuous detonation of fuel-air mixtures in flow-type annular combustors is described. Experimental data for C₂H₂—air, H₂—air, and CO/H₂—air mixtures are analyzed, systematized, and generalized. Basic specific features of continuous spin detonation and the influence of geometric parameters of flow-type combustors are described. The effect of physical and chemical parameters on the domain of continuous detonation formation is analyzed. It is concluded that the fundamental knowledge of continuous detonation processes in hydrogen and hydrocarbon fuels gained up to now allows one to consider the possibility of using them in air-breathing detonation engines.

Three-dimensional detonation in channels of different cross sections and spinning detonation in channels of circular cross sections are numerically studied. A possibility of galloping detonation in a supersonic flow of a mixture with a variable fuel concentration over a plane channel width is discussed. Conditions of detonation formation under the action of moving boundaries of the flow region due to rotation of an elliptical cylinder and in a variable-size square chamber are obtained. Formation of three-dimensional detonation in a supersonic flow in a helical channel of elliptical cross section and in a variable-size square channel is calculated.

Analytical solutions of the problem of collapsing of a spherical shell or cavity in an ideal compressible liquid having a constant density during its motion are constructed. The influence of the gas located in the cavity on the motion of the cavity boundary is studied. A quantitative characteristic of energy cumulation is proposed. An expression for energy cumulation in the case of shell or cavity collapsing is derived. The energy cumulation obtained in this study is compared with Zababakhin's results.

This paper presents an analytical review of modern quasihomogeneous and discrete models of gasless combustion. Particular attention is given to experiments that make it possible to distinguish between homogeneous and microheterogeneous regimes of this process. It is shown that in the cases where different theoretical models predict different behavior of the combustion wave at the macroscopic or microscopic level, experiments provide data in support of discrete models. The development of these models allows for a fresh look at the problem of controlling the propagation parameters of gasless combustion waves and the development of reaction compositions with specified strictly reproducible characteristics.

This paper presents a review of studies of the characteristics of condensed explosives in the reaction zone of which the distribution of parameters does not correspond to the classical detonation theory. An increase in the pressure and particle velocity behind a shock in liquid explosives and the possiblity of the existence of a stationary detonation wave without a Von Neumann spike in pressed explosives are interpreted withi the framework of models that take into account the possibility of chemical reactions directly at the shock wave front. It is noted that in the absence of the Von Neumann spike, both Chapman-Jouguet detonation or an underdriven detonation regime can occur.

A phase-plane method is proposed to model flow fields bounded by constant-velocity detonation waves propagating in TNT charges. Similarity transformations are used to formulate the problem in the phase plane of non-dimensional sound speed Z versus non-dimensional velocity F . The formulation results in two coupled ordinary differential equations that are solved simultaneously. The solution corresponds to an integral curve Z(F) in the phase plane, starting at the Chapman-Jouguet (CJ) point and terminating at the singularity A, which is the sonic point within the wave. The system is closed by computing thermodynamic variables along the expansion isentrope passing through the CJ point, forming, in effect, the complete equation of state of the thermodynamic system. The CJ condition and isentropic states are computed by the Cheetah thermodynamic code. Solutions are developed for planar, cylindrical, and spherical detonations. Species profiles are also computed; carbon graphite is found to be the predominant component (≈10 mol/kg). The similarity solution is used to initialize a 1D gas-dynamic simulation that predicts the initial expansion of the detonation products and the formation of a blast wave in air. Such simulations provide an insight into the thermodynamic states and species concentrations that create the initial optical emissions from TNT fireballs.

This paper generalizes the experimental data of the authors on the production and properties of thin-layer nanostructured explosives obtained by thermal vacuum sublimation. The method involves sublimation of explosive under heating in high vacuum, followed by deposition (condensation) of the explosive vapor on the substrate. Under these conditions, it has been shown that nanostructured polycrystalline layers of explosives containing a large number of micro-defects (pores and dislocations) are formed. In the explosive transformation in the deposited explosive layer, nano- and submicron-sized defects of the structure act as hot spots. The result is a significant reduction in the critical detonation dimensions. The nanostructured explosives studied by the authors are able to detonate at a layer thickness of 20–100 μm. Furthermore, their detonation velocity is substantially less dependent on the layer thickness than that of charges of the same explosives made by traditional technologies. Nanostructured explosives can also be used as components of explosive compositions with improved detonability.

Preparation of mechanically activated energetic composites (MAECs) based on solid fuels (Al, Mg, and Si) and oxidizers (S, MoO₃, (—C₂F₄—)_n , KClO₄, NH₄ClO₄, etc.) is considered. Compared to conventional mechanical mixtures, the burning rate of MAECs is significantly increased, and in some cases high-velocity detonation can be obtained. The propagation of the reaction in MAECs is accompanied by high energy release comparable to the heat of explosion of powerful aluminized explosive materials. The composites are highly sensitive to heat treatment and are capable of rapid transition from combustion to detonation. The results obtained in this work show that MAEC based formulations are promising energetic materials for a wide range of applications, from igniting and initiating compositions to components to small charges in microsystem devices.

Aluminum particles with a diameter of ≈50 nm were synthesized by means of the Gen-Miller flow-levitation method with alumina or trimethylsiloxane coatings formed on the surface of these particles. Aluminum/HMX nanocomposites manufactured by suspension atomization drying or dry mechanical mixing were investigated by x-ray diffraction analysis, scanning electron microscopy, and local x-ray analysis. The combustion of these mixtures with changing particle size of the components and composition of the coating on the metal particles was studied. It was found that, when the composites produced by atomization drying were stored as loose powder, HMX crystals grew, which increased the burning rate of compressed samples from 19 to 55 mm/s in the pressure range 3–10 MPa, and the pressure exponent varied from 0.34 to 0.84, depending on how the burning rate correlates with the pressure.

This paper presents a two-dimensional model of the propagation of a filtration combustion wave in a flat channel with cocurrent flow of a gas containing an oxidizer. It is shown that the increase in the permeability of the porous medium with fuel burnup leads to instability of the flat front and the formation of a structure called a finger. The reasons for the occurrence of the finger are explained, and the dependences of its most important characteristics on the permeability ratio of the initial fuel and combustion products, the specific heat of the injected gas and the width of the channel in which the filtration combustion occurs are determined.

This paper presents a new look at the structure of the radiance signal recorded by an optical pyrometer in measuring the brightness temperature of the detonation front of an emulsion explosive with glass microballoons as a sensitizer. The structure of the optical signal is typical of heterogeneous explosives: first a short temperature spike of up to 2500–3400 K occurs related to the formation of a layer of hot spots igniting the matrix capable of releasing energy, after which the radiance decreases to the quasi-equilibrium level corresponding to a temperature of 1880–2370 K at a detonation pressure of 0.7–11 GPa. There is satisfactory agreement between the experimental data and the results of independent calculations.

The dynamic elastic limit and spall strength of high-alloy chromium-manganese-nickel steel in the martensitic-austenitic transformation induced by a change in the temperature from –120 to 200 ˚C is measured by recording the complete wave profiles with a VISAR laser interferometer and subsequently analyzing them. The spall strength of the investigated steel in the martensitic phase is found to be 25–30% higher than the strength of steel in the austenitic phase. In this case, the strength decreases in a stepwise manner in a narrow temperature range approximately from –50 to 20 ˚C, where, apparently, basic changes in the internal structure of steel occur due to the martensitic-austenitic transformation. The measured values of the dynamic elastic limit of high-alloy steel have a sufficiently large scatter and hardly decrease with increasing temperature without any features associated with the martensitic-austenitic transformation of the structure.

The formation of high-velocity compact elements of shaped charges with a liner of a combined hemisphere-cylinder shape has been analyzed by numerical simulations of a two-dimensional axisymmetric problem of continuum mechanics. The influence of the radius and thickness of the hemispherical portion of the combined liner on the parameters of the compact elements has been studied. It is shown that in implosion of hemispherical liners whose thickness decreases from top to bottom, there is an increase in the velocity of the resulting jet flow. When using a hemispherical portion of variable thickness (to increase the velocity of the compact elements formed), it is necessary to solve two additional problems-to implement timely separation of the head part of the jet flow with increased velocity and increase its mass. For this, it is proposed to use the jet-forming part of the combined liner in the form of a truncated sphere or a slightly elongated ellipsoid of revolution. Combined shaped-charge liners in the form of a truncated ellipsoid of variable thickness were developed, allowing the formation of compact elements having a velocity of 7.5–10 km/s.