Vol 53, No 2 (2017)

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.

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 laborconsuming, 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 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 pO₂, 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, while the metal–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.

This paper presents the results of 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 energy source was varied from 960–1150 K. Three ignition modes of the polymer can be 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 region of the near-surface layer of the polymer has an insignificant effect (less than 5%) the increase in the basic characteristic of the process—the ignition delay. At the time of initiation of combustion, the degree of thermal decomposition 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 accumulating capacity of the polymer and heat transfer rate from the heated region of the near-surface layer into the depth of the material.

A VISAR technique was used to study the structure of the reaction zone 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 peculiarities corresponding to an explosive combustion model are recorded. In the vicinity of the initial density of 1.7 g/cm³, a kink is found on the curve showing detonation velocity versus density, and the shock-wave initiation of PETN above and below the kink point is studied.

Numerical simulation of the ignition of RDX, HMX, and TATB by a nanosecond laser pulse was performed. The heat-conduction equation was solved in cylindrical coordinates with allowance for the multiple reflection of the light beam, a zero-order exothermic reaction, and melting. Despite the small temperature gradient due to the smallness of the radiation absorption coefficient, violation of thermal equilibrium due to Arrhenius nonlinearity leads to ignition of energetic materials from the surface. The critical energy density for ignition of PETN, RDX, HMX, and TATB by a nanosecond laser pulse was determined. Calculations have shown that with identical absorption and reflection coefficients, PETN is the most sensitive and TATB is the most heat-resistant.

This paper presents data on laser initiation of low-density mixtures of PETN with metal additives with varying dispersity of PETN and particle size of the additive. A laser with a wavelength of 1.06 μm 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 optimal content (except for aluminum) was observed. For PETN mixtures with an optimal amount of fine aluminum, a significantly greater (a factor of 6.2) decrease in the threshold initiation parameters compared to direct initiation of PETN was found. It is shown that the initiation thresholds of the mixtures do not depend on the dispersity of PETN with the optimal additive content. Increasing the dispersity of PETN extends the dependences of the threshold parameters on the additive content while the optimal additive 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.

Depending on the geometry and amplitude–time characteristics of external impact, there may be accumulation of shock-wave energy in the samples, which reduces the time during which the structural materials retain their functional properties. Dispersion from the free metal surface of samples without and with pre-applied perturbations in the form of pyramids is considered. Under certain amplitude–time characteristics of external impact, dispersion from the tips of the pyramids occurs. The quantitative characteristics of dynamic destructive processes at different amplitude–time characteristics of 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.