Vol 55, No 1 (2019)

Various publications dealing with physicomathematical modeling ol detonation processes in gas suspensions ol line, submicron, and nano-sized aluminum particles within the framework ol mechanics ol continuous and heterogeneous media are reviewed. Important issues ol the description ol thermal dynamics, transport properties, and ignition mechanisms are discussed. Specific features of combustion regimes of micron- and nano-sized particles are considered. Closing relations for a semi-empirical model of detonation of suspensions of aluminum nanoparticles in oxygen are presented.

A number of di-, tri-, and tetraazido-substituted azines as potential energetic dispersing agents of solid propellants for ramjet engines have been studied. The enthalpy of combustion and the enthalpy of formation of several azides (2,4,6-triazidopyrimidine, 2,4,6-triazidopyridine, 3,4,5-triazidopyridine-2,6-dicarbonitrile, and 3,4,5,6-tetraazidopyridine-2-carbonitrile) were experimentally determined. Eleven azides were compared with HMX in terms of the enthalpy of combustion in oxygen to CO₂ and water (in the case of the presence of hydrogen in the component), as well as in terms of the temperature of the products of adiabatic conversion of the investigated components due to the high enthalpy of formation in the absence of an external oxidizer and the amount of gases released in this process. The enthalpy of combustion of all the investigated azides burned in air was found to be significantly higher than that of HMX, and for seven of the azides studied, the combustion temperature is significantly higher. As regards the gas release volume (24–31 mol/kg), the azides are inferior to HMX (41.9 mol/kg). Based on the combination of properties, the investigated azides can be considered as promising dispersing agents of solid propellants for ramjet engines.

The combustion of monodisperse titanium particles with a characteristic size of 38 and 320 μm moving in air was studied. Pyrotechnic samples generating monodisperse particles were burned in a chamber with a nozzle to impart an initial velocity to the burning particles. The particles were accelerated by the jet of gaseous combustion products discharged from the nozzle. The maximum path-averaged particle velocity relative to the ambient air was 7.9 m/s. Combustion of moving particles was carried out in a quartz tube 2 m long. At the end of the combustion, combustion products (oxide aerosol) were sampled from the tube using a thermophoretic precipitator. Size distribution functions of nanometer-sized spherule particles were determined by processing electron micrographs of samples. The velocity and burning time of burning particles were determined by video recording at a speed of 300 fps. It was found that increasing the velocity of motion of agglomerate particles with a diameter of 320 μm relative to the gas from 0.9 to 7.9 m/s leads to a decrease in the size of the spherules from 28 to 19 nm and to a decrease in the burning time from 0.45 to 0.26 s.

The flame structure of submicron-sized aluminum dust particles and air is investigated through a two-phase mixture in a counterflow configuration. A mathematical model is developed to estimate the premixed dust flame location and velocity in terms of the strain rate. In order to simulate combustion of dust particles, a three-zone flame structure is considered, including the preheat, reaction, and post-flame zones. The governing conservation equations for each zone are derived and solved under appropriate boundary conditions. The effects of thermophoresis and Brownian motion of fuel particles are investigated. Moreover, the particle size and polydispersity impacts on the burning rate and flame position are taken into consideration. In general, the simulation results for the flame velocity are in reasonable agreement with the experimental data available in the literature.

The experimental study of the high-temperature interaction kinetics of a titanium-soot mixture under quasistatic compression in an electrothermal explosion (ETE) is described. Dependences of the heating rate of a sample mixture on temperature are obtained, and the effective values of the energy of ignition activation and thermal explosion are calculated. The formation of the microstructure of intermediate and final interaction products is investigated. It is shown that the formation of a final product occurs according to a shell-core mechanism. The high rate of the solid-phase interaction of titanium and soot is explained by the formation of micropores and microcracks in the intermediate product, which ensure the high rate of surface diffusion of carbon.

A mixture of SiO₂, C, and Mg powders is mechanically milled in a planetary ball mill during different milling times of 60, 90, 120, and 150 min. The milled powders are then used in a self-propagating high-temperature synthesis (SHS) reaction to produce the Si–SiC composite. The thermal properties of the milled powders are determined by using differential scanning calorimetry and thermogravimetry. The chemical composition and microstructure of both as-synthesized products and as-leached powders are characterized by the x-ray diffraction analysis and scanning electron microscopy, respectively. The results show that an increase in the milling times of the mixture of the reactant powders has a significant effect on the thermal properties, diffusion processes, and SHS reaction mechanisms, as well as on the phase conversion and the final yield of the products.

Electromagnetic measurements of the particle velocity are performed in a situation when the detonation wave reaches the interface between a powdered high explosive (HE) and a window made of an inert material (Plexiglas). PETN, RDX, and HMX with densities close to the natural bulk density are studied. In order to measure not only the averaged velocity profile, but also possible fluctuations at scales of the order of the HE grain size, small sensors with the working arm length approximately 1 mm are used. In most experiments, profiles with clear chemical spikes are obtained; in some of the measured results, however, the chemical spike cannot be detected on the background of strong signal oscillations, which may be considered as manifestation of the nonclassical mechanism of wave propagation (explosive burning suggested by Apin). Along withour previous study, the present results suggest parallel operation of the shock and convective mechanisms with domination of each mechanism at different segments of the wave front.

The nonequilibrium radiation occurring during ignition of a 10% stoichiometric hydrogen-oxygen mixture with flame suppressant additives diluted with argon behind shock waves was studied. Instead of the expected reduction in the super-equilibrium radiation of active radicals in the ignition zone, the addition of halogenated flame suppressants led to increased UV radiation around wavelengths of 220 and 411 nm characteristic of the HO₂ radical and H₂O₂ and H₂O molecules. Therefore, the hypothesis about the suppression mechanism due to quenching of the excited HO^*₂ radical is not confirmed, and the effect of flame-suppressing additives is due to the binding of H and O atoms.