Home – Home – Jornals – Combustion, Explosion and Shock Waves 2021 number 4

The review is devoted to the possibilities of combining self-propagating high-temperature synthesis (SHS) and electrospark sintering (EIS) for obtaining monophase ceramic materials and composite materials with ceramic and metal matrices. The materials covered in this review contain compounds formed in the SHS mode: carbides, borides, metal silicides, and intermetallides. Factors in the structure formation of materials obtained by sintering of SHS products and the influence of EIS conditions on the characteristics of the materials (relative density, grain size) are analyzed. Advantages of combining SHS and EIS methods, including the possibility of additional processing of the SHS product (grinding, adding components) to modify the composition of the material and its properties, are discussed.

Results of a numerical study of mixing, ignition, and combustion of a cold hydrogen jet propagating along the lower wall of a channel parallel to a supersonic (M = 2) flow of an inert gas mixture/wet hot air are reported. The computations are performed with the use of the ANSYS CFD Fluent commercial software by means of solving unsteady Favre-averaged Navier-Stokes equations supplemented with the k -w SST turbulence model and several kinetic schemes of hydrogen combustion. Two brutto schemes and three detailed kinetic schemes including 16, 38, and 37 direct and reverse reactions are considered. The goal of the study is to choose a computation method and kinetic mechanism that ensure good agreement with experimental data on supersonic combustion of a cocurrent hydrogen jet. In the case of a non-reacting flow, it is demonstrated that the computational algorithm can accurately predict the parameters of mixing of the hydrogen jet and external flow. In the case of a reacting flow, the flow characteristics are significantly affected by large vortex structures developing at the boundary of the combustion layer with the external flow. If the flow unsteadiness is taken into account and a detailed kinetic scheme with 37 reactions is used, good agreement of the mean characteristics of the flow with experimental data on the distributions of pressure, temperature, Mach number, and species concentrations at the combustor exit is provided.

The propagation of a flame over the surface of a liquid fuel deposited on a substrate with low thermal diffusivity has been studied. It has been shown that this system-fuel and substrate - is not thermally thin. Heat transfer in front of the flame edge due to the motion of the liquid under the temperature gradient in the liquid layer (the Marangoni effect) has been analyzed. Estimates of temperature gradients in the condensed phase, the thickness of the liquid fuel layer in front of and below the flame front are given. The velocity of the diffusion flow of the fuel in the gas phase at the flame edge is estimated. It is shown that the temperature gradient along the surface of the liquid film determines the velocity of the film and the rate of diffusion of the evaporated fuel to the flame edge.

The initial (linear) stage of the development of rotating transverse detonation waves in a flat-radial annular combustion chamber is determined and simulated. The problem of linear mode stability of the cylindrical front of Chapman-Jouguet deflagration combustion in a radially diverging subsonic flow with a small Mach number in the presence of perturbation waves of the flow entropy is solved. The steady flame front is described by discontinuity of the gas-dynamic parameters provided that the combustion products are in chemical equilibrium. It is revealed that the flame front is unstable for some types of small perturbations of the main flow of the combustible mixture and the flame front. Instability is determined under the condition of a constant flow rate in the mixture injection system. The spatial forms of oscillations and perturbation waves of the combustion front in the annular combustion chamber are obtained by numerical and analytical methods.

Zero-dimensional computations of nanosecond-order ignition using a nanosecond discharge are performed with two constraints. The effects of these constraints are assessed to study the experimental rapid pressure change properly at the initial stages. The computations are carried out with the following constraints: constant internal energy and volume (U&V) and constant enthalpy and pressure (H&P), revealing differences between the two solutions. As the pressure remains constant under the H&P constraint, the total number density of all species decreases during ignition. In this case, O radicals are less generated and consumed. The progression of all reactions and temperatures increases under the H&P constraint less intensely than under the U&V constraint. Significant differences are found between the results calculated under the U&V and H&P constraints. Therefore, large discrepancies with real phenomena can be caused if the loss due to pressure reduction is not treated well.

Provision of stability of the operation process in solid-propellant rocket motors is an integral aspect of engine design and operation. The present paper describes a numerical and experimental method of determining the linear stability of the operation process in a solid rocket motor with an axisymmetric combustor. The method is based on solving a linearized system of equations that describe the dynamics of solid propellant combustion products in the frequency domain. The values of acoustic conductivity of the solid-propellant combustion region obtained in a pulse T-burner are used as the boundary conditions. Verification of the method is performed on the basis of numerical and experimental data obtained for six model engines. Results of stability computations for the operation process in an engine with a large aspect ratio operating on a metallized propellant are reported.

The effect of the ratio of the components of mixtures of nickel (II) oxide with titanium on the modes and rate of combustion of compositions based on them has been studied. It is found that under normal conditions, a change in the mass content of nickel oxide from 75 to 30 % leads to a natural change in combustion modes: fire flame, a multi-stage mode, and a self-oscillating mode with periodic stripping of combustion products from the burning surface. It is shown that the maximum burning rate of such mixtures (82 mm/s) is achieved at an equal mass ratio of nickel oxide and titanium. In condensed combustion products of a mixture of nickel oxide with titanium, Ti₂Ni intermetallic and Ni₂Ti₄O _x double oxide identified.

The stages of chemical transformation in the combustion wave of an exothermic Fe₂O₃/Al/AlN mixture in nitrogen atmosphere is studied using an original method for terminating a combustion front. It is revealed that, in a stoichiometric Fe₂O₃/6Al mixture diluted with 35 % (wt.) AlN, the combustion front stops following the combustion of a 30 ÷ 40-mm mixture column. The zones of a stopped combustion wave are investigated by the local analysis methods. It is shown that the alumothermal reduction of iron (III) oxide to iron aluminide proceeds in stages via the formation of a (FeAl) _x O _{ydouble oxide having varying chemical composition. The final synthesis products include aluminum oxynitride, iron aluminide, and unreacted aluminum nitride.}

Mathematical models constructed in the macroscopic approximation are used to perform a theoretical study of volume and wave synthesis in a hybrid mixture consisting of nonactivated and activated powders of the same composition. Synthesis dynamics depending on the proportion of activated powder at different values of the preliminary mechanical activation parameters is considered. Analytical formulas are obtained for approximate estimates of the temperature and time of synthesis in nonactivated and activated compositions.

A model for synthesizing a composite during combustion is proposed and numerically investigated. It is assumed that a set of chemical transformations can be described by a kinetic scheme with two total parallel reactions. One of the reactions corresponds to the matrix synthesis, and the other one to the inclusion synthesis. It is taken into account that the mixture components melt in a certain temperature range rather than at a fixed melting point. The possibility of the reaction front propagation in a self-oscillating mode is shown. The critical values of the parameters that separate the stationary and self-oscillating modes of the reaction front propagation are revealed. Computational results in extreme cases correspond to known theoretical concepts.

This paper presents the results of an experimental study of shock compression of titanium hydride TiH₂ and the deuterides of zirconium ZrD₂, tantalum TaD_0.8, and titanium TiD₂, TiD_1.6, and TiD_1.1 in the pressure range 30 ÷ 220 GPa. The synthesis of titanium and zirconium deuterides from titanium and zirconium powders and tantalum deuterides from tantalum rods is described. The Hugoniots of deuterides and hydrides were determined using the well-known reflection method. Shock-wave compression was performed using shock-wave generators with explosive charges of different power. A description of the obtained experimental data using simple equations of state is proposed.

Results of numerical simulations of fracture of titanium and aluminum nanocrystals by the molecular dynamics are reported. The nanocrystals are subjected to uniaxial tension in a wide temperature range (300 ÷ 1 270 K). It is demonstrated that tension of titanium nanocrystals heated to temperatures above 0.7 of the melting temperature in a non-stressed nanocrystal first leads to a phase transition from the crystalline to liquid state, followed by their fracture. This effect is not observed in the case of tension of the aluminum nanocrystal.

The influence of the velocity of compact metal projectiles of spherical and cylindrical shapes on the size of craters formed in steel targets of different strength was studied in the velocity range 2 ÷ 10 km/s by numerical simulations using two different computing systems. The behavior of the materials of the projectile and target is described using a model of a compressible elastoplastic medium with a constant yield point. The materials of projectiles are copper, titanium, and tantalum. The mass and velocity of projectiles are specified assuming that their kinetic energy is constant. It is found that the dependences of the crater depth on the velocity of projectiles with a constant kinetic energy have a maximum point, and the volume of craters decreases monotonically with increasing velocity.