Home – Home – Jornals – Combustion, Explosion and Shock Waves 2017 number 2

The Rayleigh-Benard convection in a chemically active gas in the chemical equilibrium state is numerically studied in the Boussinesq approximation. A flat layer with isothermal horizontal boundaries free from shear stresses is considered. Thermodynamic parameters of the gas (hydrogen-oxygen mixture) are calculated by the previously proposed model of chemical equilibrium. It is shown that the allowance for recombination and dissociation processes leads to the emergence of an additional factor at the Rayleigh number. An expression for the growth rate of infinitesimal perturbations and a relation for the critical Rayleigh number as a function of temperature are derived. It is found that the neutral curves consist of the upper (instability due to heating from below) and lower (instability due to heating from above) branches. Results calculated for a nonlinear steady mode are reported.

The interaction between natural convection and the heat explosion in porous media is studied. The model consists of a nonlinear heat equation coupled with the Darcy equation for the motion of an incompressible fluid in a porous medium. Numerical simulations are performed using the alternate direction finite difference method and the fast Fourier transform method. A complex behavior of solutions is observed, including periodic and aperiodic oscillations and an oscillating heat explosion. It is shown that convection can decrease the risk of the explosion due to additional mixing and heat loss, but it can also facilitate the explosion due to temperature oscillations arising as a result of instability of stationary convective regimes.

The characteristics of soot particles formed during combustion of liquid hydrocarbons in a laboratory version of an original burner with injection of a superheated steam jet into the combustion region are experimentally studied. The concentration and particle size of soot formed in the burner plume are measured by a diffusion aerosol spectrometer. It is shown that the majority of the primary particles have sizes ranging from 20 to 60 nm. The particle concentration in the external flame rapidly decreases with distance from the burner exit from 10⁸ to 5x10⁶ cm^-3. The images obtained by transmission electron microscopy demonstrate a chain-branching (fractal-like) structure of aggregates. The primary particles composing these aggregates have a bulbous structure with the interplane distance between the layers smaller than 1 nm. Compact aggregates with sizes up to 500 nm are observed in cooled combustion products. The content of soot in combustion products is 35 mg/m³, and the mean particle mass is 7x10^-12 mg. Results measured in the combustion modes with injection of a superheated steam jet and with injection of an air jet are compared.

Issues of explosion and fire safety of silane during its transportation and storage are intimately related to its possible catastrophic leak off from tanks if they become cracked. It turned out that silane self-ignition is possible at some leak off velocities. The interest of the aerospace industry to problems of ignition and combustion of silane combined with other fuels should be also noted. Experimental investigations of such problems, which are rather expensive and labor-consuming, provide primary information for the development and verification of mathematical models of physical and chemical processes, capable of predicting the characteristics of silane mixing, ignition, and combustion induced by its exhaustion. Recent experiments of Prof. Chen and his colleagues from Taiwan allowed them to determine the critical length of the silane jet (distance from the tube exit where ignition of exhausting silane occurred) and the time of silane ignition. In the present work, an attempt is made to simulate these phenomena within the framework of the concept of the critical concentration of silane with studying the dynamics of silane leak off from a tube.

This paper presents experimental dependences of the temperature, thermo-emf, speed and width of the combustion wave of a heterogeneous mixture of 5Ti + 3Si on the pressure of quasi-isostatic compression. Combustion parameters are calculated from measurements of the temperature gradient and the difference between the electrical potentials in the combustion wave. It is shown that under quasi-isostatic compression conditions, the maximum burning rate is 90 mm/s and the width of the combustion wave is 10 mm. It is found that the voltage-temperature characteristic of combustion of a heterogeneous mixture of 5Ti + 3Si has a hysteresis shape. The cause of the hysteresis related to a change in the bulk concentration of electric charge carriers in the combustion wave is discussed.

This work demonstrates that the heating of fracture fragments of rods made of VT1-0 commercial titanium and its alloys OT4-1 and PT3V in gaseous oxygen at a pressure p _O2, which is accepted in this paper and in the works of other researchers as critical pressure p *, only leads to melting of individual regions of the formed juvenile surface, which differ from each other only by obstructed heat sink, and the metalì-oxygen interaction does not transfer into combustion. However, this interaction does transfer into combustion during which the bulk of the metal burns out at a slightly higher pressure p **, which can be calculated from the thermal explosion equation for temperature T * that is equal to the melting point of titanium, with account for the dissociative absorption of oxygen molecules on the melt surface and the heat exchange coefficient corresponding to the case where the heat is transferred from the melt hemisphere to the semibounded solid body.

The ignition mechanism and the dependence of the composition of the products of combustion or thermal explosion in a mixture of 2Zr + Al + C on the initiation temperature and heat transfer conditions were studied. Heat transfer conditions were changed by varying the size of the samples and the gaseous medium in which the experiments were performed. Two contusion regimes were found: a low-temperature regime in which zirconium aluminides formed and carbon and part of the zirconium remained unreacted and a high-temperature regime in which the reaction products were carbide and zirconium aluminide. Upon re-initiation, the low-temperature combustion products reacted in the high-temperature combustion mode. The observed dependences are due to parallel reactions in the three-component system.

This paper presents a numerical study of the effect of burnout on the ignition delay of a typical thermoplastic polymer (polymethylmethacrylate) by a metal particle heated to a high temperature. The initial temperature of the power source was varied from 960-1150 K. Three ignition modes of the polymer are distinguished according to the temperature of the heat source, ignition delay, and the position of the ignition zone in the vicinity of the hot particle. It is found that under local heating conditions, the burnout of the heated area of the surface layer affects insignificantly (by less than 5%) the increase in the basic characteristics of the process - the ignition delay. At the time of initiation of the combustion, the degree of thermal degradation of the polymer (degree of conversion) does not reach even 15% in the section corresponding to the maximum heat flux from the heat source. It is shown that the ignition delay increases more significantly when accounting for the temperature dependence of the thermal properties of polymethylmethacrylate than when accounting for the burnout factor. The induction period is increased by 15- 25% due to an increase in the polymer storage capacity and heat transfer rate from the heated the area of the surface layer deeper into the material.

Multiwave regimes of continuous spin detonation in syngas-air mixtures in a flow-type annular cylindrical combustor 503 mm in diameter are obtained. Experiments are performed for mixtures of carbon oxide and hydrogen with the ratio of the components equal to 1/3, 1/2, or 1/1. The varied parameters are the flow rates of air and syngas, the ratio of these flow rates, and the combustor length. Scalability of the continuous spin detonation process is demonstrated: at identical values of the specific flow rate of air and the combustor expansion ratio, the number of transverse detonation waves increases with increasing combustor diameter. In the examined ranges of combustor lengths and specific flow rates of air, the frequency of these waves is independent of the combustor length, except for narrow regions where the number of waves (and, correspondingly, the flow regime) changes. The structures of transverse detonation waves in regular regimes are almost identical for all examined syngas compositions. It is shown that detonation can be initiated by a jet of combustion products. The minimum diameters of the detonation chamber for different flow rates of the mixture are estimated.

A VISAR technique was used to study the structure of the reaction region in PETN for various initial densities and dispersion of samples. The flow in the pressed charges corresponds to the classical denotation model. In the case of bulk density, the properties corresponding to an explosive combustion model are determined. In the vicinity of the initial density of 1.7 g/cm³, a kink is found on the curve showing detonation rate versus density, and the shock-wave initiation of PETN above and below the kink point is studied.

This results of investigation of the influence of the characteristics of porous inclusions on the detonation parameters of emulsion explosives (EMX) are presented. Glass and polymer microballoons, perlite grains, hollow cenospheres, and a gas-generating additive are used as sensitizers. It is shown that polymer microballoons with an ultra-thin wall filled with isobutane are the most efficient sensitizer that almost fully unock the potential of EMX.

Ignition of RDX, HMX, and TATB by a nanosecond laser pulse is numerically simulated. The heat conduction equation in a cylindrical coordinate system is solved with allowance for multiple reflections of the light beam, zeroth-order exothermic reaction, and melting. Despite a small temperature gradient caused by a small coefficient of radiation absorption, violation of thermal equilibrium due to the Arrhenius nonlinearity leads to ignition of energetic materials from the surface. The critical energy density of PETN, RDX, HMX, and TATB ignition by a nanosecond laser pulse is determined. For identical absorption and reflection coefficients, the calculations show that the most sensitive explosive is PETN, whereas the most heat-resistance explosive is TATB.

This paper describes the micro-hotspot model of laser initiation of energy materials. The relationship of the critical energy density and the temperature of the reaction hotspot in PETN with the radii of nanoparticles of 12 metals at a pulse duration at half-height of 14 ns is determined. It is established that, as the nanoparticle radius is about 10 nm, the critical energy density tends to a certain value independent of heat capacity of metal. This is due to the reduction of the ratio of the nanoparticle volume to the volume of the heated PETN layer, which leads to the fact that most of the energy is spent on heating the matrix. It is shown that the critical hotspot temperature depends on both pulse duration and nanoparticle radius. The analytical expressions for the relationships of the critical parameters of reaction initiation with the radius and heat capacity of metal nanoparticles and for the relationship of the critical hotspot temperature with pulse duration are obtained. The invariant binding the critical energy density and the characteristic development time of the reaction is discovered. The results of this paper are necessary for the optimization of the composition of the optical detonator cell.

This paper presents data on laser initiation of low-density PETN mixtures with metal additives with varying dispersion of PETN and particle size of the additives. A laser with a wavelength of 1.06  m and a pulse length of 40 and 30 ns was used. Curves of the threshold initiation parameters on the additive content are shown to have minima. For coarse additives, no significant dependence of the initiation threshold of the mixtures on the nature of the metal at its optimum content (except for aluminum) was observed. For PETN mixtures with an optimum amount of fine aluminum, a significantly greater decrease in the threshold initiation parameters (a factor of 6.2) compared to direct initiation of PETN was found.àIt is shown that the initiation thresholds of the mixtures do not depend on the degree of dispersion of PETN with optimum content of additives. Increasing the degree of dispersion of PETN extends the dependences of the threshold parameters on the additive content while the optimum content is shifted to higher values. The initiation thresholds are found to strongly depend on the density of the mixture charge. The key points of the mechanism of laser initiation of PETN mixtures with additives are formulated.

This paper presents the results of experimental studies of the effect of damage and initial porosity of desensitized HMX samples on the limiting initiation conditions and explosion parameters under impact of steel spherical projectiles weighing 2-100 g. It is shown that preliminary mechanical damage to the explosive samples, as well as reduction in the initial density, leads to a qualitatively similar result - an increase in the limiting impactor velocity, causing initiation of the explosive transformation. The damage structure of the samples in the absence of initiation and the dependence of the physical and mechanical properties of samples on their density are studied.

Depending on the geometry and amplitude-time characteristics of external impact, there may be cumulation of shock-wave energy in the samples, which reduces the time durind which the structural materials retain their functional properties. The dispersion of the free metal surface of samples without and with pre-applied disturbances in the form of pyramids. Under certain amplitude-time characteristics of the external impact, dispersion from the tops of pyramids occurs. The quantitative characteristics of dynamic destructive processes at different amplitude-time characteristics of the external impact are determined for the purpose of introducing them into two- and three-dimensional codes to predict the behavior of metals under extreme conditions.