Vol 54, No 4 (2018)

A numerical analysis of self-ignition of a hydrogen–air–water vapor mixture at different initial pressures is carried out. The results of this analysis are used to make a shortened list of reactions that make the largest contribution to process rate during induction. A simplified analytical description of the system state before self-ignition, which makes it possible to calculate the thermal power and adiabatic heating rate of the system, is presented. A method for estimating the self-ignition limits from the adiabatic heating rate of the mixture is described.

Flame propagation in a closed vessel containing a stoichiometric propane–air mixture and partially filled with a porous medium was studied experimentally. The porous medium consisted of steel balls of diameter 3.2 and 6 mm and ceramic balls of diameter 6 mm. An experimental dependence of the maximum pressure during flame propagation in the vessel on the degree of filling of the vessel with the porous medium was obtained. Theoretical estimates of the pressure were made, which were in satisfactory agreement with the experimental data. The estimates closest to the data of the experiment are based on the assumption that the gas burns adiabatically in free space and is isothermally compressed in the porous medium. The influence of heat losses from the gas into the porous medium and the walls of the vessel on the maximum pressure was analyzed.

The effect of shell material (copper and silicon carbide) on the detonation of a cylindrical explosive charge was analyzed. The wave patterns in the detonation products and the shells are substantially different, which is due to different sound velocities and the rapid destruction of the ceramic under explosive loading. The wave pattern at the explosive/ceramic interface was found to be affected by desensitization of the explosive due to its loading by an advancing wave from the shell side, resulting in a decrease in pressure, blurring of the detonation front, and an increase in particle velocity. Throughout the process, there is a continuous increase in the time of explosive decomposition near the interface between the explosive and the ceramic shell. An extended region with a constant pressure close to the Chapman–Jouguet pressure was observed on the axis of symmetry behind the detonation front of the explosive charge in the ceramic shell.

A few-parameter equation of state in the Mie–Grüneisen form is proposed to describe shock compression of condensed matter. The equation is based on a postulated dependence of the Grüneisen coefficient on the specific volume and temperature Γ(V, T), which provides a qualitative description of compression of metal samples in strong shock waves. The curve of cold compression is found on the basis of the dependence Γ(V, T) with the use of a generalized formula for the Grüneisen function. Heat-induced oscillations of the crystal lattice are described in the Debye approximation. The resultant Grüneisen function has two free parameters. The values of other coefficients of the equation of state are determined from the reference data for matter under normal conditions and also from limiting values under extreme conditions. The model is tested by an example of copper. The derived equation of state describes the cold compression curve, normal isotherm, shock compressibility, as well as the copper unloading curves in density, pressure, and internal energy ranges for which experimental data are available. The thermodynamic characteristics of copper (isentropic modulus of volume compression, velocity of sound, Debye temperature, specific heat, linear expansion coefficient, and melting temperature) are calculated. Comparisons with available experimental data show that the proposed model, despite its simplicity, ensures a consistent description of a large array of experimental data in the region of high energy densities.

The heating of energetic materials by the radiation of fiber-coupled continuously-pumped lasers at near IR-wavelengths of 0.98, 1.56, and 1.94 μm was studied. Samples of pressed secondary explosives and loose gunpowder were used. The length of the linear portion of the temperature rise and the rate of its rise immediately after exposure to laser radiation were measured. It was established that the rate of temperature rise at the initial time was proportional to the laser radiation power aP. For a 600 μm diameter of the laser beam emerging from the fiber, the coefficient of proportionality a for secondary explosives was 6–250 K/(s · W) at a wavelength of 0.98 μm and 40–2000 K/(s · W) at wavelengths of 1.56 and 1.94 μm. For gunpowder, a = 7000–15 000 K/(s · W), which is an order of magnitude or more higher than that for most of the secondary explosives we studied. The possibility of increasing the efficiency of laser heating of secondary explosives by applying an absorbing thin film on the surface of the samples was studied. The heating dynamics and the initial stage of ignition of energetic materials by laser radiation were investigated.

Owing to its high mass and volume heats of combustion, boron is a promising component of solid propellants for air-breathing engines. Its application is limited by difficulties of organizing high-efficiency combustion. Experimental investigations of combustion of individual boron particles demonstrate a large number of unique features, which are not typical for other materials: variable ignition temperature, two stages of combustion, and drastic reduction of the burning rate for particles with sizes of several micrometers or smaller. Models that cover the entire range of temperatures, concentrations, and particle sizes are physically non-obvious, can be hardly reproduced, and do not provide the accuracy needed for solving practical problems. In this paper, we propose a simple diffusion model of combustion, which ensures an adequate description of combustion of boron particles 34.5 and 44.2 μm in size at temperatures above 2240 K.

Single-phase aluminum diboride was obtained by thermal explosion of an aluminum–boron mixture mechanically activated in a planetary ball mill. The additional mechanical treatment of the products of thermal explosion was found to reduce the coherent scattering area size of AlB₂ to nanometer values. The results of x-ray phase and electron microscopy of the mechanocomposites and products of thermal explosion are presented.

In the original publication, the title was misspelled. It should read “Buoyant Turbulent Diffusion Flame near a Vertical Surface ” instead of “Natural Buoyant Turbulent Diffusion Flame near a Vertical Surface.”

It should also read “buoyant turbulent diffusion flame” instead of “natural buoyant turbulent diffusion flame” everywhere else in the text and “the upward flame spread rate” instead of “the velocity of the upward flame spread rate” in the abstract.

The original Russian text was translated by the journal.