Home – Home – Jornals – Combustion, Explosion and Shock Waves 2021 number 3

A review of investigations dealing with mathematical and numerical modeling of shock wave and detonation processes in gas suspensions of fine-grained inert or reacting particles is presented. The basic models of mechanics of continuous media that describe rarefied gas suspensions and saturated powder media are mentioned and analyzed. Models with internal pressure in the particle phase, including those with the description of particle collision dynamics within the framework of molecular-kinetic approaches, are identified. Problems of interphase interaction and equations of state are discussed. Various issues of the qualitative analysis of the characteristic properties of models and theoretical analysis of shock wave structures (conditions on shock waves, classification of shock waves and combined discontinuities) are outlines. Most popular numerical algorithms used for simulations of shock wave processes are mentioned. Some results of numerical studies of the processes of detonation initiation and propagation, interaction of shock waves with clouds and layers of particles, and dispersion of the layers) are noted.

Four detailed chemical mechanisms used to describe detonation combustion of hydrogen in oxygen are considered. Ignition delays for various temperatures and pressures are found, The Chapman-Jouguet velocity is determined, and the Zel'dovich-Neumann-Doring solution for different models is obtained. The effect of dilution of the stoichiometric mixture of hydrogen and oxygen by an inert gas is estimated. Direct numerical simulation of detonation wave propagation in a channel is performed; the emergence of instability of the plane wave and formation of a cellular (multifront) structure are studied. The results predicted by different chemical models are analyzed and compared with each other and with available experimental data.

Fire accidents in closed regions, transport carriers, and auditoriums are influenced by the confinement conditions. The current experimental investigation is intended to find the effect of obstacles, obstacle-free space, gas flow, and near-wall turbulence on the flame acceleration. Two rows of bench-shaped obstacles (replicate chairs in transport carriers) enhance the flame speed in the obstacle-free path. Turbulence intensity is higher in local regions where the flow becomes highly subsonic and mildly supersonic, which enhances the flame speed. The flame front-driven supersonic flow can form negative shock waves, which can further increase the flame speed. Careful design of confined geometry can prevent accidents or slow down the flame propagation.

The study of the ballistic properties of energetic materials remains vital to ensure safety during their transportation, handling, storage, and processing. It might be a complex, expensive, and time-consuming process to evaluate the ballistic properties experimentally. This work proposes a simple numerical method to predict the ballistic properties of deflagrating energetic materials. Adiabatic and one-dimensional propagation of waves is assumed for theoretical evaluation, and the Rankine-Hugoniot equations are used to obtain correlations. The correlations are obtained in terms of the reactant mass to determine the ballistic properties, namely, the pressure, temperature, and velocity of the particle at the point of ignition. For determining the particle velocity and combustion wave velocity, theoretical correlations are also derived in terms of the mass and distance from the point of ignition. The theoretical calculation is compared and validated with experimental results. The deviation is found to be smaller than 2%. This simple calculation can be used for evaluating and comparing the ballistic properties of various deflagrating energetic materials without the need of performing complicated experiments.

This paper presents the results of an experimental study of the ignition and slagging characteristics during combustion of boron-containing solid propellants with ammonium perchlorate as an oxidizer under conditions simulating working processes in a gas generator and in the afterburner of a rocket-ramjet engine. It is shown that the introduction of fluorine-containing additives into the propellant reduces the content and adhesion of primary condensed combustion products (slags). The dependences of the ignition delay on the density of the radiant heat flux in the range 20 ÷ 180 W/cm² are obtained for model solid propellants containing boron, carbon, boron carbide, and slag particles collected in the gasifier.

Thermogravimetric analysis data were used to determine the activation energy E = 205.9 kJ/mol, the pre-exponential factor A = 2.275 x 10^-15 m³/s, and the order of the reaction with respect to the oxidizer m = 1 for the oxidation of an aluminum diboride particle leading to the formation of aluminum borate on the surface. The ignition conditions for a single particle of aluminum diboride in air were estimated using the theory of ignition of metal particles proceeding by the thermal explosion mechanism. It is shown that a consequence of the formation of aluminum borates in the induction period is a strong positive dependence of the ignition temperature of particles on their size and the partial pressure of oxygen.

A study of various macrokinetic modes of interaction (self-ignition or combustion) of compact samples from non-passivated (pyrophoric) and passivated iron nanopowders with air has been carried out. Experiments have shown that the modes of interaction with air depend on the type of gaseous medium used (argon or air), in which the bottles with the samples were previously located, as well as on the duration of the exposure of the bottles in air. For the first time, the possibility of passivation of pressed samples from pyrophoric iron nanopowder was experimentally established when weighing bottles with samples in air. Various experimental methods have been used to study the dynamics of heating the sample and the effect of density nonuniformity along the length of the sample on it. It was found that the heating of pyrophoric samples is nonuniform, although it begins simultaneously over the entire surface of the sample.

The influence of the content of polyvinyl butyral (0 ÷ 2.3 %) on the combustion of a granular mixture of Ti + C with titanium of different grades is investigated. The experiments were carried out in the absence of an external gas flow; therefore, due to the low decomposition temperature and a small amount of polyvinyl butyral, a conductive combustion mode was expected. However, for rapidly burning mixtures, a convective combustion mode was found due to the ignition of the granule surface with hot gaseous decomposition products of polyvinyl butyral. The mechanism of falling of undecomposed polyvinyl butyral behind the ignition front is explained. It was found that the combustion mode of the Ti + C granular mixture depends on the rate of its combustion in the absence of a gas flow through the sample. Based on the experimental and theoretical analysis of the combustion process, it has been established that the ignition of granules in the convective mode occurs at the temperature of the α®β-transition in titanium. A qualitative explanation is given for the different effect of the content of polyvinyl butyral on slow and fast burning mixtures Ti + C.

Structure of detonation waves in mixtures of tetranitromethane with methanol and nitrobenzene is studied using a multipoint laser interferometer. There is a poor reproducibility of the mass velocity profiles measured in different experiments with a fixed composition of mixtures. Simultaneous registration of wave profiles at several points of the detonation front and different sections of the sample showed that in this case the flow is one-dimensional and stable with respect to longitudinal perturbations. This means that in each experiment a stationary detonation mode is realized, the parameters of which differ from experiment to experiment. Along with the lack of reproducibility of the mass velocity profiles, nonclassical detonation modes were recorded in the mixtures under study, which are manifested in the absence of a chemical peak in the reaction zone. A possible relationship between these two phenomena is discussed.

The explosive tetrazene (1-amino-1(tetrazol-5-yl-diazenyl)guanidine monohydrate) has been actively used in initiating compositions since the beginning of the 20th century, but it has low thermal and hydrolytic stability. As an alternative to tetrazene it was proposed to use 2-(tetrazol-5-yl-diazenyl)guanidine (MTX-1), which is its more thermally stable derivative. However, the decision on the applicability of a particular explosive is primarily made based on its explosive properties, which are not well known for both tetrazene and MTX-1. The objectives of this study was to determine the detonation velocity and heat of explosion of MTX-1 in comparison with tetrazene and explain the results obtained. Calorimetric measurements were carried out in a modified steel bomb. Samples of MTX-1 and tetrazene »1 g) were exploded in helium to determine the heat of explosion and the volume of gases. Experimental data on the critical diameter of both substances were obtained. The detonation velocities of MTX-1 and tetrazene were determined by an electromagnetic method.

Detonation of PETN and HMX with a particle size of about 1  m was investigated by an electromagnetic method. At an initial density of 0.9 ÷ 1.2 g/cm³, the von Neumann spike was weak or not observed at all. This indicates a fast reaction, whose time is outside the experimental resolution (about 5 ns). Electrical conductivity measurements provided only a rather rough upper-bound estimate of the reaction time (less than tens of nanoseconds). Density measurements using synchrotron radiation showed that the initiation of PETN with an air shock wave led to almost instantaneous detonation initiation, without any acceleration region. In general, the results of the study confirm the acceleration of the chemical reaction in ultrafine explosives.

This paper presents the results of a study of the initiation of explosive transformation of flat charges 40 mm in diameter and 5 mm thick, made of a low-density (&rho:ρ» 0.9 g/cm³) light-sensitive explosive composition based on fine RDX and aluminum. It is shown that laser radiation (LR) can be used to initiate explosive transformation on a large area (»1 000 mm²) with a small difference in the time of arrival of the detonation wave front. It is determined that at a LR energy density Q_opt = 10 J/cm²) over the entire LR spot on the charge surface, the difference in the time of arrival of the detonation wave front at the rear surface of the charge is not more than 50 ns.

The ignition behavior of the polytetrafluoroethylene/aluminum/tungsten reactive projectile impacting a fuel-filled tank is experimentally investigated. In ballistic experiments, the reactive projectiles and the steel projectiles are launched from a smooth bore powder gun barrel to impact fuel-filled tanks at different velocities. The processes are recorded by a high-speed video camera. The result of the ignition-enhanced behavior of the reactive projectile relative to the steel projectile is presented. When the reactive projectile impacts the fuel-filled tank, the combined effects of the kinetic energy impact and the chemical energy release are achieved to improve the ignition probability. Especially, the flame (ignition kernel of the fuel) caused by the reactive projectile has a longer duration and a greater expansion region

The question of whether the physical state of shock-compressed copper is in equilibrium is discussed. For this, experimental data on defect electrical resistivity are used to estimate the concentration of point defects. Quantitative information on the concentration of defects can be obtained if the type of arising defects is known. Assuming the predominant formation of vacancies, the dependence of the concentration of defects in copper on the pressure of the shock wave is obtained. It is shown that the number of defects monotonically increases with increasing pressure of the shock wave. The found concentration of vacancies »0.8 % at a pressure of 20 GPa) exceeds the corresponding equilibrium value by ten orders of magnitude. Thus, the state of copper under shock compression is highly defective and highly nonequilibrium. Comparison of data for shock-compressed copper and silver shows the general features of the state of these metals. Comparison of the data obtained immediately after the passage of the shock wave through the sample (in situ) with the known results for the samples preserved after the experiment demonstrates that in the first case, noticeably higher (up to two orders of magnitude) defect concentrations are recorded. Thus, the technique of stored samples does not provide objective information on the state of matter directly behind the shock front. The problem of constructing the equation of state under conditions of nonequilibrium physical state is briefly discussed.