Home – Home – Jornals – Combustion, Explosion and Shock Waves 2018 number 4

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 the 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 autoignition limits from the adiabatic heating rate of the mixture is described.

The upper flammability limit of ethane-air and ethane-oxygen mixtures for various initial temperatures and pressures was determined experimentally. In experiments, the upper flammability limit increased with increasing initial temperature and pressure, which is consistent with literature data. The limit was determined in a closed vessel with central ignition. The obtained limit corresponds to quenching of the flame during its downward propagation after rise under the action of the Archimedes force.

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 with a diameter of 3.2 and 6 mm and ceramic balls with a diameter of 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 pressure estimates are made which are in satisfactory agreement with experimental data. The estimates closest to the data of the experiments 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 to the porous medium and the walls of the on the maximum pressure was analyzed.

A modified physicomathematical model of ignition of silane/hydrogen/air mixtures is applied to calculate the ignition delay time for these mixtures at low initial temperatures (300-900 K) and pressures (0.4-1 atm) of the mixture. It is shown that the diagrams of the ignition delay time as a function of temperature contains a region of the so-called negative temperature coefficient. The influence of the pressure in the mixture and of the silane fraction on the length of this region is studied. It is found that an increase in both factors (silane concentration and pressure in the mixture) leads to an increase in the length of the negative temperature coefficient region.

A physicomathematical model of the Rayleigh-Benard convection in a chemically equilibrium gas containing chemically inert microparticles (Al₂O₃) is proposed. A linear analysis of convection in the Boussinesq approximation is performed. It is shown that addition of chemically inert microparticles increases the critical value of the Rayleigh number and stability of the convective process. The possibility of using chemically inert microparticles for control and monitoring of convection in a chemically reacting gas is demonstrated by an example of a chemically equilibrium gas.

Single-phase aluminum diboride was obtained by thermal explosion of an aluminum-boron mixture mechanically activated in a planetary ball mill. It was established that the additional mechanical treatment of the products of thermal explosion made it possible 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.

The effect of a NiO additive on the combustion and structure formation in a Ni-Al-W system in self-propagating high-temperature synthesis (SHS) is under study. The stages of the combustion of compositions containing a NiO high-energy additive are shown. The interaction of W particles with Ni-Al melts during SHS results in the formation of globular decoration of particles on the basis of solid solutions of tungsten on the particle surface. This effect is observed only in compositions with an equimolar mixture of Ni-Al. Should the amount of 1 at. % be present in the original sample of the NiO additive, the globular decoration on the surface of unreacted W particles never occurs. This effect can be associated with changes in the combustion temperature, deviation of the NiAl phase in the direction of a larger content of Ni, and the influence of oxide phases on diffusion processes.

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.

This paper describes a study of boron powders and powder compounds, obtained by various methods, including metallothermal, electrolytic, and boron hydride cracking methods. The crystal state, particle size and microstructure, presence and composition of impurities, and chemical composition of the oxide layer of boron particles are profoundly investigated. The effects of the above-mentioned characteristics on the particle oxidation parameters during heating with a constant velocity are analyzed. The determining influence of chemical composition of the particle surface layer on the initial temperature of their intense oxidation is established. It is shown that the maximum increase in the weight and heat release value during oxidation of the boron powders is almost independent of microstructural features, crystal state, and chemical composition of and oxide layer thickness of the particles, and cannot serve as indicators of completeness of boron oxidation during heating.

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 is established that the rate of temperature rise at the initial time is 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 wave lengths of 1.56 and 1.94 μm. For gunpowder, a = 7000-15000 K/(s × W), which is an order of magnitude or more higher than 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.

Regimes of continuous detonation of methane/hydrogen-air mixtures in spin and opposing transverse detonation waves are obtained for the first time in a flow-type annular cylindrical combustor 503 mm in diameter. A two-component (methane/hydrogen) fuel with the H₂ mass fractions of 1/9-1/2 in the range of specific flow rates of the mixture from 64 to 1310 kg/(s × m²) and the fuel-to-air equivalence ratio φ = 0.78-1.56 is considered. In methane/hydrogen-air mixtures with two compositions of the fuel (CH₄ + H₂ and CH₄ + 4H₂) one-wave and two-wave regimes of continuous spin detonation are obtained; the frequency of rotation of transverse detonation waves is 0.56-1.66 kHz at φ = 0.78-1.02. For the fuel compositions CH₄ + 2H₂ and CH₄ + 1.5H₂, continuous multifront detonation with two opposing transverse detonation waves rotating with the frequency of 0.86-1.34 kHz at φ = 1.0-1.23 is obtained. For the CH₄ + H₂ + air mixture, both combustion in the chamber and continuous spin detonation outside the combustor with transverse detonation waves rotating with the frequency of 1.01-1.1 kHz are observed. The lean limits of continuous detonation are obtained in terms of the specific flow rate of the mixture: 64, 100, 200, and 790 kg/(s × m²) for the fuel compositions CH₄ + 8H₂, CH ₄ + 4H₂, CH₄ + 2H₂, and CH₄ + 1.5H₂, respectively, for the mass fraction of hydrogen in the methane/hydrogen fuel of ≈0.16. Violation of regularity of the continuous detonation wave structure and the wave velocity with a decrease in the fraction of hydrogen in the two-component fuel is detected.

A small-parameter equation of state in the Lie-Gruneisen form is proposed to describe shock compression of condensed matter. The equation is based on a postulated dependence of the Gruneisen coefficient on the specific volume and temperature Г(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 Г(V,T) with the use of a generalized formula for the Gruneisen function. Heat-induced oscillations of the crystal lattice are described in the Debye approximation. The resultant Gruneisen function has two free parameters. The values of other coefficients of the equation of state are determined from the reference data for the substances under standard conditions and also from the 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 shows 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 acceleration ability of an emulsion explosive sensitized with expancel polymer microballoons in the initial density range 0.193-1.2 g/cm³ was measured using the end acceleration method and the method of acceleration of a cylindrical shell. The results were compared with those obtained for 6ZhV ammonite and with the results of ANSYS AUTODYN simulation.

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