Home – Home – Jornals – Combustion, Explosion and Shock Waves 2024 number 1

The initial part of a hydrogen jet exhausting upward from a 2 × 20 mm slot and burning in air is studied. The profiles of the streamwise and transverse velocity are determined by means of particle image velocimetry; the local temperature distribution and the composition of stable substances are obtained by the Raman scattering method with the use of a focusing resonator system. Based on experimental data, the contributions of molecular and convective transfer mechanisms to the heat release intensity is determined. It is shown that the maximum intensity of heat release depends mainly on gas-dynamic and thermophysical characteristics of the gas flow during its macroscopic motion.

The combustion of rich mixtures of methanol and hydrogen with air has been studied by experimental and numerical methods. It has been shown that the deviation from the Le Chatelier rule for the rich flammability limits is due to two factors: inhibition of flame propagation by methanol in rich mixtures of hydrogen and the presence of superadiabatic temperatures in rich mixtures of methanol. It has been found that the effect of adding small amounts of hydrogen to rich mixtures of methanol is the same as the effect of adding inert nitrogen and carbon dioxide. Numerical simulation has shown that adding small amounts of hydrogen to rich mixtures methanol leads only to physical effects on the normal flame speed. Additives of H₂ affect the implementation of above-adiabatic temperatures in a methanol flame in the same way as Inert additives of CO₂ and N₂.

A mathematical method is developed for solving the problem of detonation initiation in a hydrogen-air mixture by a small-diameter sphere flying with a velocity greater than the Chapman-Jouguet detonation velocity. The mathematical model verification is performed against experimental data on the detonation cell size in hydrogen-oxygen and hydrogen-air mixtures. Depending on the pressure in the mixture, which is varied from 100 to 250 kPa, three regimes of oblique detonation waves are obtained: (1) stabilized oblique detonation wave at 250 kPa; (2) stabilized oblique detonation wave of the “straw hat” type at 200 kPa; (3) periodic regime with a detached oblique detonation wave, which was not observed in previous experiments, at 125 kPa. At 100 kPa, a regime of shock-initiation combustion is observed. Based on an analytical dependence, the energy of detonation initiation by a high-velocity body is estimated, and the analytical and numerical data are found to be in good agreement.

Organometallic complexes of transition metals with inorganic anions are considered as promising precursors for the synthesis of nanoscale materials used in various fields, including chemical catalysis. In this work, organometallic complexes of nickel, iron, and copper with an organic ligand (imidazole) and with an inorganic ligand (nitrate anion) were synthesized and characterized. The kinetic parameters of thermal decomposition of the synthesized organometallic complexes were determined by low heating rate thermogravimetric analysis and high-speed dynamic mass spectrometric thermal analysis. The main gaseous products of thermal decomposition of the complexes under high-speed heating were identified. The chemical and phase composition of condensed combustion products of organometallic complexes in air was studied.

There are various methods for synthesizing composites from mixtures of powders of metals forming intermetallic phases, as well as from mixtures of powders of metals and non-metals (for example, carbon or boron). In the 1970s, an approach was proposed to describe combustion and explosion processes with identification of reaction cells, which from modern positions can be classified with two-level synthesis models. The approach has been actively developed for binary systems, but remained rather conditional. This paper presents an overview of similar models; it is suggested that they can be developed by taking into account accompanying processes (not only diffusion, but also the evolution of the stress-strain state), and modified in relation to synthesis controlled by a laser or electron beam.

Mathematical modeling was used to study the modes of electrothermal explosion of a gasless system surrounded by a dielectric medium. The influence of the intensity of conductive heat transfer and Joule heating power on the formation of dimensionless temperature and concentration profiles, the integral depth of transformation and the speed of propagation of the reaction front is considered. To separate the stages of ignition and propagation of the reaction, the criterion was to achieve a conversion depth of 0.99 at any point in the sample. The amount of product formed at the ignition stage was determined. It is shown that near critical conditions during ignition on the axis of the sample, a large depth of transformation is achieved, leading to a displacement of the ignition zone from the axis to the surface of the sample.

This paper presents the results of thermodynamic calculation of adiabatic temperature and the equilibrium composition of products of HfO₂ reduction with calcium depending on carbon and calcium content at different pressures. The calculations are based on the possibility that solid HfN-HfC solutions could be formed, and their formation is identified with that of hafnium carbonitride. It is shown that adiabatic temperatures lie in a range of 2000 ÷ 2900 K, and its elevation is limited by the melting of CaO at 2900 K. The introduction of carbon often reduces the adiabatic temperature, and a pressure rise leads to its increase. A connection is revealed between the composition of products and the type of temperature curves. The main reason why adiabatic temperature rises along with pressure is a displacement of equilibrium toward the formation of condensed phases and an increase in the HfN proportion in the products.

Targeted synthesis of oxynitride composites is implemented by means of organizing coupled processes. Interaction of ferrosilicon with nitrogen in the combustion regime is considered as the main (inducing) process. The phase composition of the products of the coupled processes is determined by the chemical and phase compositions of the components added to ferrosilicon before performing self-propagating high-temperature synthesis. The influence of the basic products of the synthesis on the burning rate, fraction of nitrogen, phase composition, and morphology of synthesis products is considered. Chemical stages of ferrosilicon interaction with additives of natural minerals (zircon, ilmenite, and shungite) and aluminum in a nitrogen medium are demonstrated. The phase composition is determined by chemical transformations in the combustion wave. It is found that addition of aluminum leads to reduction or elimination of the Si₂N₂O phase in synthesis products with an increase in the aluminum fraction and obtaining composites based on the Si₃N₄ (SiAlON) solid solution. The microstructure of combustion products is presented by aggregates (5-10 μm) composed of small faceted crystals, shapeless structures, and crystal flakes. Oxynitride composites with an open porosity value of 51.0÷68.8% are obtained.

The possibility of obtaining a composite material from a ferroalloy based on silicon, aluminum, and zirconium nitrides by the method of self-propagating high-temperature synthesis is considered. It is shown that introduction of a nitrogen-containing additive on the basis of ferroalumosilicozirconium in amounts up to 35% to the initial ferroalloy leads to an increase in the nitrogen fraction, emission of the basic nitride phases in combustion products, deceleration of motion of the combustion wave front, and reduction of the maximum combustion temperature. It is found that introduction of a preliminary nitridized material in amounts of more than 20% makes it possible to convert combustion wave front propagation to a steady regime and to obtain combustion products with a macroscopically homogeneous composition. Under the conditions of natural filtration of nitrogen, combustion of a powder mixture based on ferroalumosilicozirconium and a nitridized material yields a composite consisting of AlN, Si₃N₄, ZrN, and α-Fe phases.

A study of the process of mechanochemical synthesis of titanium nitride under conditions of double mechanical activation of titanium powder: in argon and in nitrogen gas is presented. In the experiments, the time of mechanical activation of titanium in argon and nitrogen was varied. Analysis of the morphology of powder mixtures, X-ray diffraction and microanalysis data showed that preliminary mechanical activation of titanium in argon helps to accelerate the mechanochemical synthesis of titanium nitride. Analytical relationships are obtained and theoretical estimates are given that make it possible to predict the process of double mechanical activation of titanium.

The formation of compounds of refractory metal foils (Ti, Hf, Ta, Ni) with ceramic layers formed as a result of combustion of reaction tapes rolled from powder mixtures (Ti + 0.65C, Ti + 1.7B and 5Ti + 3Si) was experimentally studied. The microstructure, elemental and phase compositions of multilayer composites obtained by self-propagating high-temperature synthesis were studied using scanning electron microscopy and X-ray diffraction analysis. The influence of synthesis conditions (initial temperature, applied pressure) and the initial structure of the samples on the speed of propagation of the combustion wave front, microstructure, phase composition and strength characteristics of the resulting layered materials was revealed. It has been shown that connections in the combustion mode between metal foils and reaction tapes rolled from powder mixtures are ensured due to reaction diffusion, mutual impregnation and chemical reactions occurring in the reaction tapes and on the surface of metal foils. The strength characteristics of the resulting materials (up to 275 MPa at 25 °C, up to 72 MPa at 1100 °C) were determined using a three-point loading scheme. The results are of interest for the development of structural materials operating under extreme conditions.

The effect of preliminary low-energy mechanical activation (MA) of nickel powder on the thermal explosion of the Ni₃Al intermetallic compound has been studied. Two synthesis methods are considered. In the first, the mixture was continuously heated by an external energy source. In the second, when the set temperature was reached, the external source was turned off. It has been shown that low-energy MA of nickel promotes the intensification of the synthesis of the Ni₃Al intermetallic compound. With continuous heating, the ignition temperature does not depend on the activation time and is equal to the melting point of aluminum. When heating with the external source turned off, preliminary activation of nickel reduces the solid-phase ignition temperature. It has been established that the activation of nickel in a laboratory mill allows one to avoid the factors of its passivation.

Boron is one of the most valuable fuels for rocket propellants. However, the boron oxide (B₂O₃) on the B surface has a high boiling point (1860 °C), which hinders the contact between the internal active B and the oxidation component during the ignition and combustion process. As a result, the ignition energy of B is higher and the combustion reaction rate is lower. In order to improve the ignition performance and reaction rate of B, a surface modification of B by means of surface purification by a solvent, surface grafting, and making a composite with aluminum (Al), which has a high combustion temperature, has been prepared. Composite particles of B purified by ethanol and thick flake Al (TF-Al), denoted by EB/TF-Al, have the fastest reaction rate higher by 96.6% than that of raw B and TF-Al composite particles (RB/TF-Al). Surface grafting of B with TF-Al composite particles (KHB-3/TF-Al) ensure the minimum ignition energy, which is 29.1% lower than that of raw B. As the ignition performance and reaction rate of B are improved by means of surface modification, the performance of B-based rocket propellants is expected to be improved.

The widespread use of flat electric heaters in technology and household appliances requires the search for simpler and cheaper technologies for their production. This work proposes a method for producing an electrically conductive coating (NiAl) and an electrically insulating layer (glass ceramics) in one stage - by the method of self-propagating high-temperature synthesis (SHS) in a thermally conjugated powder system (Ni + Al)/(PbO₂ + B + Al₂O₃ + glass). The process of propagation of the combustion wave front has been studied, the influence of layer thickness, the ratio of components in the powder mixture PbO₂ + B + Al₂O₃ + glass on the propagation speed and temperature of the wave front has been studied. combustion. An expansion of the wave front of the exothermic process was discovered when a mixture of PbO₂ + B was added to the lower layer. The addition of this mixture makes it possible to reduce the thickness of the NiAl layer and ensure the formation of a uniform dielectric coating. The phase composition and microstructure of the coating were studied. The optimal ratios of layer thicknesses and the composition of the powder mixture of layers have been established. The fundamental possibility of forming an electrically conductive and dielectric coating by the method of self-propagating high-temperature synthesis in one stage is shown.

Model expressions for the concentration diffusion coefficient and thermal diffusion factor of a binary mixture obeying the van der Waals equation of state are obtained. Model calculations at elevated pressure (density) require the equation of state parameters and thermal diffusion factor of the mixture in a low-density ideal gas state. The behavior of model relations is studied depending on pressure along isotherms. Near the critical state, the model diffusion coefficient has a minimum, and the thermal diffusion coefficient has a maximum. The diffusion and thermal diffusion models at elevated pressure are compared with experimental data for a number of mixtures. It is shown that they are in qualitative and generally quantitative agreement at different temperatures and concentrations of components. The van der Waals equation is not quite suitable for describing the experiment on compressibility and phase equilibrium at high pressure. This requires more complex and flexible modifications of the equation. Generalized expressions for model characteristics of the diffusion of the binary mixture in the case of such equations of state are given.