Home – Home – Jornals – Combustion, Explosion and Shock Waves 2024 number 5

This paper presents a critical review of experimental results and theoretical models for the effect of mechanical activation on the ignition of powder mixtures in gasless and filtration combustion and the ignition of thermite compositions. It is shown that all available data clearly indicate an increase in the sensitivity of powder compositions for thermal initiation, but in the case of shock initiation, the sensitivity does not always increase. The change in the sensitivity is due to the combined effect of two factors: an improvement in the contact between reactants and a decrease in the activation energy of the chemical reaction. It is proposed to use the mechanical activation factor taking into account the change in contact area and activation energy to describe the sensitivity of activated compositions.

The effect of preliminary mechanical activation of powder compositions on the combustion rate and maximum temperature is considered. The burning rate can increase, decrease or pass through an extremum with increasing activation time. It is argued that this complex behavior is most fully described by a microheterogeneous combustion model; homogeneous and convective-conductive models are also considered. The questions of whether additional energy is accumulated in powder mixtures under mechanical treatment and why superadiabatic combustion temperatures are not observed in activated compositions are discussed.

Numerical data on chemically equilibrium parameters of detonation of a stoichiometric hydrogen-air mixture and the same mixture with partly dissociated components are reported. It is found that the dissociated components (hydrogen, oxygen, and nitrogen atoms) produce a significant effect on the system parameters, especially on reduction of the critical energy of detonation initiation. As the fraction of atomic nitrogen in the hydrogen-air system increases, its explosion hazard also increases, and the initiation energy becomes even smaller than the critical energy of detonation initiation in the hydrogen-oxygen mixture. The system parameters for heated air are analyzed.

An experimental study was performed to investigate flame propagation above a liquid fuel film on the surface of the solid phase under conditions where the vapor pressure of the liquid is below the lean limit and the solid phase consists of granules of a porous medium. The flame propagated at a speed of the order of the normal speed of a homogeneous stoichiometric mixture. The effects of the characteristic element size of the porous medium and the oxygen concentration in the gas phase on the flame speed in porous media wetted with n-butanol were experimentally investigated.

The study of ignition and thermal decomposition of solid waste of the oil industry was conducted using different methods of thermogravimetric analysis. The experiments were carried out on pre-dried, crushed and fractionated (from 40 to 100 m m) samples. Ignition was carried out in a vertical tubular furnace. The effect of mechanical activation on the processes of ignition and decomposition is shown. The results of analysis by different methods correlate with each other. The kinetic parameters of the thermal decomposition process are determined depending on the particle size and the degree of decomposition of the coke residue.

The oxidation of amorphous boron powders modified with vanadium pentoxide were studied by thermogravimetry, differential scanning calorimetry, X-ray phase analysis, and electron microscopy. The modification was carried out by mixing powders with vanadium-containing gels: V₂O₅ × nH₂O hydrogel and V(OCH₂)₂ × n(HOCH₂)₂ oleogel. It was found that adding 2 wt.% V₂O₅ to the amorphous boron powder led to a decrease of 200 °C in the temperature at the onset of its intense oxidation and to a 25% increase in the specific heat release during interaction under heating in air at a rate of 10 °C/min. The reaction of boron with vanadium pentoxide is considered within the framework of a boron thermal process model.

Combustion of cylindrical Hf + 0.5 C samples pressed from hafnium powder mixtures with soot in nitrogen is investigated. Non-activated mixtures and compositions that had undergone preliminary mechanical activation in a planetary mill were studied. The nitrogen pressure in the experiments was 0.2-4 MPa. It was found that the combustion mode of samples from the non-activated mixture depends on the nitrogen pressure. At a pressure below 2 MPa, the front propagates in a self-oscillating mode, and at high pressures - in a stationary mode. The combustion of samples from the mechanically activated mixture proceeds in a stationary manner over the entire range of studied pressures, while the combustion rate is 50-100 times higher than the combustion rate of samples from the non-activated mixture. This is due to the difference in the microstructures of these mixtures and, accordingly, to different reaction mechanisms. During mechanical activation of the mixture, composite particles are formed, the contact area of carbon and hafnium in which is orders of magnitude greater than the contact area between them in the case of a non-activated mixture. As a result, the leading role in the combustion of samples from mechanically activated mixtures is played by the interaction of hafnium with carbon, which leads to an improvement in the gas permeability of the sample and subsequent nitriding. In samples from a nonactivated mixture, the leading role is played by the nitriding of hafnium, which occurs at a low rate due to filtration difficulties.

The combustion of powder and granulated mixtures (1 - X)(Ti + C) + X (5Ti + 3Si), 0≤X≤1 was studied. The experimental values of the combustion rate of powder mixtures depended on the fraction X of the binary mixture 5Ti + 3Si, on the characteristic size of titanium particles d(Ti) in the charge and on the value of the free volume above the charge in the reactor. The convective-conductive combustion model was used to explain the results. It was shown that the nature of the change in the combustion rate of powder mixtures with increasing X is associated with the fulfillment or nonfulfillment of the conditions for heating the charge particles and desorption of impurity gas in front of the combustion front. A large amount of liquid phase at 0.4<X<0.6 prevents equalization of gas pressure in front of and behind the melt layer, which ensures maximum combustion rates of the ternary mixture at d(Ti) = 120 μm (or minimum at d(Ti) = 20 μm according to other authors). Based on the experimental combustion rates of mixtures with granules measuring 0.6-1.7 mm, the combustion transfer time between granules and the combustion rate of the substance inside the granules were calculated, i.e., the combustion rate of powder mixtures with the effect of gas evolution leveled out. The dependence of the combustion rate of the granule substance on X is close to linear. For the 5Ti + 3Si composition, it is shown that, in contrast to the Ti + C mixture, due to the release of impurity gases behind the melt layer, the combustion front velocity in the powder mixture exceeds the combustion velocity of granulated mixtures and the combustion velocity of the substance inside the granules.

Model experiments on shock-wave synthesis in mechanocomposites of the composition 64% Ti + 36% Al were carried out in the developed flow-type pulse reactor. Composites after 3,5 and 7 min of mechanical activation, separated into four fractions, were subjected to extreme thermal action. A planetary ball mill “Activator-2SL” was used to activate the mixture. It was experimentally established that different times of mechanical activation action and different granulometry of powders do not affect the qualitative phase composition of the synthesis products. The reaction products include amorphized Al, underreacted Ti, intermetallic compounds TiAl, TiAl₃ and Ti₃Al, as well as nuclei of metastable phases or solid solutions based on Ti, which are in a nonequilibrium weakly ordered state. It was revealed that varying the time of mechanical activation and granulometric composition changes the quantitative content of the phase composition of the final synthesis products. The microstructures of the obtained samples confirm the formation of a multiphase product with a partially ordered structure, having amorphous and crystalline components.

The process of detonation propagation in semi-annular charges made of plasticized TATB with a steel shell inside during initiation of normal detonation along a line on the outer surface of the charge was studied using the X-ray method. In the experiments, the shape of the detonation wave front was determined using the X-ray method at different moments in time. The experiments recorded the effect of a layer of plastic explosive based on hexogen located on the surface of the main charge and having a detonation velocity ≈10% higher than that of TATB on the shape of the detonation front. The experimental position and shape of the detonation wave front in a cylindrical TATB charge are not described by the laws of geometric optics (Huygens principle) due to the peculiarities of detonation initiation in the initial section and the presence of a steel shell. Numerical modeling of the experiments was carried out using the LEGAK technique using macroscopic detonation kinetics. A similar to the experimental picture of the initiation and propagation of detonation in the TATB charge was obtained. A theoretical analysis of the features of the propagation of the detonation wave was carried out.

Detonation velocity was measured for an emulsion explosive with the addition of PAP-2 and ASD-4 aluminum powders in charges of different thicknesses with different initial densities. Adding PAP-2 aluminum powder was found to reduce the critical thickness of the emulsion explosive. At large charge thicknesses and densities of the aluminized emulsion explosive of more 1.0 g/cm³, the effects of PAP-2 and ASD-4 powders on the detonation velocity are equivalent. Replacing aluminum powder with talc powder markedly deteriorates the detonation characteristics of the emulsion explosive. The results of the work suggest that PAP-2 and ASD-4 aluminum powders in the emulsion explosive with a density of more than 1.0 g/cm³ react completely during detonation to the Chapman-Jouguet surface.

The case of an impact on a thin annular layer of incompressible viscoplastic material with a cavity filled with gas is considered. The layer is placed in a rigid assembly such as a press mold and is compressed by a piston at a constant speed. The solution of the corresponding strength problem is performed by a semi-inverse method with the specification of the rod type of medium flow and the governing equations of the viscoplastic substance. The obtained data on the accelerated motion of the cavity walls are considered as an example of focusing (energy accumulation) in a converging flow. Information on the temperature distribution under a load on the layer is used to calculate the parameters of explosion initiation by an impact on a ring charge of a solid explosive. The possibility of thermal ignition of reactive substances without the participation of self-heating and only due to dissipative heating due to the energy of mechanical action is discussed.

Emulsion explosives exhibit a strong nonideal detonation behavior. Their blast performance is commonly enhanced by inserting additives, such as aluminum powders. To investigate the nonideal detonation properties of aluminized emulsion explosives, a series of tests are conducted for emulsion explosives with 5% of an aluminum powder additive using mortar confinement. The velocity of detonation and the profile of the detonation front for charges of different diameters are measured. Then, the ignition and growth (I and G) model is employed to numerically investigate the detonation properties of aluminized emulsion explosives based on experimental results. A procedure combining the optimization LS-OPT code with the LS-DYNA hydrocode is first developed to calibrate the parameters of the I and G model. Then, the detonation pressure and temperature of charges of different diameters are predicted using the calibrated parameters. The results are in line with the existing literature data.

High-entropy alloys show noticeably better mechanical and physical properties than traditional alloys and have found widespread application prospects in the fields of national defense equipment, aerospace, high-pressure physics, and so forth. The preparation of high-entropy alloys largely involves the arc melting method, which displays defects like element segregation and low production efficiency in the preparation process. In this study, based on the mechanical alloying of a mixed refractory metal powder, the explosive sintering technology is adopted to prepare refractory high-entropy alloys (RHEAs). Through numerical calculations, the formation conditions of the solid dissolution phase of RHEAs and the minimum detonation pressure needed for explosive sintering are derived, and an explosive sintering test is performed with the explosive-powder tube ratio as a test variable. A dense bulk material containing the Mo-Nb-Re-Ta-W type of RHEAs is prepared through explosive sintering. The explosively sintered alloy products are characterized by X-ray diffraction analysis, scanning electron microscopy, energy dispersive spectroscopy, etc. The results show that the alloy products only contain the BCC phase and some Re elements that are not yet miscible, distributions of various elements are not completely homogeneous, and RHEAs are developed only in some parts of the product, implying the feasibility of the method for RHEA preparation.

The impact sensitivity of propellants is a significant concern during production, application, and storage. To study the impact sensitivity and reaction behavior of 3,3-bis(azidomethyl) oxetane and tetrahydrofuran copolymer (PBT propellants) subjected to a low-velocity impact, 20 sets of Susan tests are performed, and a simulation model is developed. The results show that the reaction is initiated by the impact and extrusion pyrolysis of PBT propellants. For impact velocities ranging from 120 to 300 m/s, the relative released energy of the Susan tests is greater than 20%, increases with the impact velocity, and reaches the maximum value of 57.67%. PBT propellants exhibit severe reactions under a low-velocity impact and are sensitive to crushing impact conditions. According to the simulation results, extruded PBT propellants dominate the reaction behavior of PBT propellants in a projectile of Susan tests under a low-velocity impact, and the reaction behavior of extruded PBT propellants is primarily dominated by coupling of both the impact velocity and confinement. The simulation results are in good agreement with the experimental results. The initiation impact velocity of partial detonation is about 260 m/s, based on experimental and simulation results.