Home – Home – Jornals – Combustion, Explosion and Shock Waves 2015 number 3

The structure of a premixed methyl decanoate/oxygen/argon flame stabilized on a flat-flame burner at atmospheric pressure was studied by molecular beam mass spectrometry. The results of the experiment are compared with the results of numerical simulations using two different mechanisms of chemical reactions proposed in the literature. The main intermediate combustion products of methyl decanoate were identified by gas chromatography-mass spectrometry. Analysis of the primary stages of decomposition of methyl decanoate shows that reactions involving free radicals play a decisive role in its oxidation, which agrees well with the~results of the experiments.

Gas flow from a reservoir with porous permeable walls in which combustion occurs is considered. Gas combustion in the reservoir causes an increase in the pressure in it and filtration gas flow through the permeable porous wall into the surrounding space. A model is proposed which simultaneously describes the pressure increase in the reservoir due to combustion and the associated filtration gas flow through the porous walls. Data on the maximum gas pressure in the reservoir, the discharge time, and flow characteristics as functions of the permeability of the medium were obtained by numerical solution of a one-dimensional problem.

The nucleation of the ionized combustion products of small (d₁₀ ≈ 5 mm) particles of Al, Mg, Zr, Fe, and Ti under laminar dust flame conditions at atmospheric pressure is considered in an isothermal approximation. It is shown that under conditions close to the experimental ones, the condensation of the products of gas-phase combustion of these metals is “rapid.” Description of the “rapid” nucleation regime requires a nonstationary approach and knowledge of the kinetics of nucleation of the condensed phase and does not need a detailed analysis of the influence of environmental parameters on the free energy of formation of small nuclei. It is shown that the characteristic nucleation time of the gas-phase combustion products of metal particles is several orders of magnitude smaller than the residence time of the products in the combustion zone of the flame dust. This allows coagulation to be considered as the basic process which determines the degree of dispersion of primary particles of the metal combustion products.

Novel hydrogen-containing alloys based on aluminum, boron, and magnesium hydride (MgH₂) are fabricated in a special process. The properties of their dust clouds (minimum ignition energy, minimum explosive concentration, and maximum explosion pressure) are studied. The results show that the minimum ignition energies of the novel hydrogen-containing alloys are 20-40 mJ, which is 50 mJ lower on the average than those of flake aluminum. The minimum explosive concentration values are between 20 and 30 g/m³. The maximum explosion pressure of the novel hydrogen-containing alloys is up to 0.88 MPa at the dust cloud concentration of 750 g/m³, which is obviously higher than that of flake aluminum (0.78 MPa). Magnesium hydride existing in the alloy can change the way of energy releasing and improve the efficiency.

A method of increasing the efficiency of mixing in an air-breathing rocket engine is discussed. Three-dimensional simulations confirm the advantages of the proposed method based on using normal supply of products of incomplete combustion of a pasty propellant into the wake zone behind a bluff body in an air flow.

A transformation of an annular flame into rotating flames in an annular combustion chamber and their attenuation with changes in the flow rate and concentration of the propellant are studied.

A possibility of ballistic efficiency of space rockets with main rocket propulsion engines by means of developing a system of gasification of unburned residues in propellant tanks with the use of gas-generating compositions with self-sustained combustion is considered. The criterion for choosing gas-generating compositions is the increment of the characteristic velocity of the rocket stage acquired due to combustion of gasified propellant residues. The results of the present study show that the proposed method of gasification of liquid propellant residues increases the energy characteristics of the rocket.

Molecular dynamics modeling of melting of aluminum nanoparticles with the use of the DL_POLY simulation package and two types of parametrization of the embedded atom potential is performed. Predicted melting temperatures are compared with available experimental and numerical data. A significant scatter of data (melting temperatures as functions of the nanoparticle size) is noted. The previously proposed semi-empirical model of molecular dynamics for the description of the thermal history of the aluminum nanoparticle is justified. The specific heats obtained in this study ensure a qualitatively correct description of their dependence on temperature and on the crystal rib size.

A combustion model for a thin heat-conducting film with discrete, chemically active hot spots distributed on its surface is proposed. An equation describing the combustion of such a system is derived, and exact analytical solutions of this equation for periodic arrangement of hot spots on the film surface are found. The sensitivity of burning rate to changes in the initial temperature of the system depending on its main parameters is determined. It is shown that the system has critical parameters (film thickness, hot spot concentration, and initial temperature) that define the limits of its combustion. With large concentrations of hot spots on the film surface, it is theoretically possible to reach burning rates that significantly exceed the speed of sound in the ambient gas. The proposed combustion model provides a qualitative explanation of the high burning rate of mechanoactivated nanocomposites and allows one to understand the influence of mechanoactivation on combustion of powder mixtures.

A method of an integrating sphere is used to study the optical characteristics of PETN containing 0.1% of aluminum inclusions with a particle size of ≈100 nm, depending on the concentration of inclusions and sample thickness. The study is carried out by using a stationary light source with a wavelength of 643 nm and a pulsed neodymium laser with a pulse duration of 14 ns. The extinction coefficients are calculated. It is concluded from the experimental results that light is absorbed by particles with the formation of “hot spots” while an increase in the path of photons in the matrix due to scattering at inclusions and an increase in the possibility of light absorption by the PETN matrix are secondary phenomena.

It is shown that a detonation wave is formed in needle-like pressed pellets of silver azide initiated by a neodymium laser pulse. The propagation velocity of the reaction was 3.65±0.12 km/s. Kinetic regularities of the initiation of detonation regimes of propagation of the explosive decomposition reaction were studied, and the spatial characteristics of the glow wave were determined: the width of the leading and rear edges of the wave at half-height were 100±18 and 112±15 mm, respectively.

The main features of detonation initiation in shielded thin layers of an explosive impacted by high-velocity metal shaped-charge jets are presented. It has been shown experimentally that the exclusion of the initial shock-wave stage of loading of the explosive by shielding the explosive layer by a compressible light material significantly reduces the initiating ability of the shaped-charge jet. The critical conditions of detonation initiation in thin layers of explosives placed between metal plates impacted by shaped-charge jets normally and at an angle of 30^o have been experimentally determined; maximum thicknesses of the shielding plates of different materials have been determined above which detonation in the explosive does not occur even upon exposure to the high-velocity head element of a shaped-charge jet.

Electrical conductivity of explosion products behind the detonation front of emulsion explosives is measured. The composition of the emulsion matrix and the amount of the additive consisting of sensitizing glass microspheres are varied. The peak value of electrical conductivity for the examined compositions is 0.5–0.05 W^–1 × cm^–1.

The present investigation examines the interaction of shock waves with closed cell aluminum foam samples in a conventional shock tube. The effect of the sample thickness on shock wave attenuation and/or enhancement and the use of the foam in the sandwich structure is studied. Results in terms of incident and reflected shock pressures are obtained, and the effectiveness of the samples with and without the foam is compared. It is demonstrated that the foam density and thickness, as well as the placement of cover plates of the same material in front of and behind the foam have the most significant effect on the reflected shock pressure. It is concluded that the closed cell aluminum metal foam can be effectively used as a sacrificial layer in blast protection of structures.

To study underwater explosions, a new type of an underwater pressure sensor using polyvinylidene fluoride as a sensing element is developed and well calibrated. The sensor is then applied in underwater explosion tests of RDX explosives with addition of aluminum in the fiber or powder form. The results show that the peak pressure of shock waves slightly decreases when aluminum is added to RDX, and the peak pressure of the fiber aluminum composite explosive is slightly inferior to that of the explosive containing the aluminum powder at the same measuring positions.

The effect of the liner material density and elongation on the shape of dual mode penetrators is studied by using the LS-DYNA software. Characteristics of liners made of five types of materials (aluminum, pure iron, copper, tantalum, and tungsten) under the same conditions of the liner volume or liner mass are compared. It is established that it is better to choose liner materials with good dynamic elongation and moderate density, such as pure iron and copper, to form good dual mode penetrators, namely, the explosively formed penetrator (EFP) and the jetting projectile charge (JPC). Characteristics of penetrators formed from aluminum, steel 20, and copper are considered. The simulation results are in good agreement with x-ray imaging experimental results.

The paper presents empirical and numerical models of characteristics of a crater emerged from a surface explosion of a small TNT charge on the soil. The proposed models can be useful for estimation of the explosive mass based on the crater characteristics. The models show satisfactory fitting with the experimental results and confirm the hypothesis that the crater characteristics depend on the explosive mass and contact area between the charge and the soil surface. Two equations for estimation of the explosive mass based on the crater volume are proposed.