Home – Home – Jornals – Combustion, Explosion and Shock Waves 2022 number 2

Results of numerical simulations of reacting turbulent flows in a channel with a rectangular cross section and sudden expansion (backward-facing step) are reported. The simulations are performed for test conditions in a high-enthalpy hotshot wind tunnel with the Mach number at the channel entrance M = 3.85. Hydrogen is used as a fuel; it is injected transversely to the main flow ahead of the step from the upper and lower walls of the channel. The computations are performed in an unsteady three-dimensional formulation with the use of the Fluent 2020R1 software with reactions of hydrogen combustion in air being ignored or taken into account. The structure of reacting turbulent flows is studied; the flow parameters at different stages of the unsteady process of ignition and flame stabilization are determined. The computed data are compared with the results of an experiment in which the static pressure distribution over the channel walls were measured. It is demonstrated that the simulations correctly predict the unsteady pattern of ignition and flame propagation along the channel.

To reveal the influence of vent parameters on the dynamic mechanism of an external explosion induced by a vented premixed methane-air explosion, the evolution process of an outdoor flow field under different vent opening pressures ( p_v ), opening times ( t_v ), and scaled vent size ( K_v = A_v / V ^2/3) is studied by methods of computational fluid dynamics. With an increase in K_v , the shape of the unburned gas cloud and vented flame gradually changes from a jet shape to a depression toward the vent. The outdoor peak specific turbulent kinetic energy increases by 36.5 and 4 times with an increase in p_v and t_v , respectively. At t_v = 0.1 s, the peak specific turbulent kinetic energy reaches 1 411 m²/s² and the turbulence range reaches 3 times the length of the room. With an increase in p_v , t_v , and K_v , the occurrence time interval of the external explosion exhibits a decreasing trend. The external explosion is located at a distance of less than 1.4 times the length of the room. At K_v = 0.05, the external explosion occurs at the furthest location.

Regimes of continuous multifront detonation in a methane mixture with heated air in a flow-type annular combustor 503 mm in diameter are obtained and studied for the first time. Air is preheated by straight firing from 600 to 1 200 K in the settling chamber by means of burning a stoichiometric hydrogen-oxygen mixture entering the combustor; the air flow rate is 6 ÷ 20.9 kg/s. The fuel (methane)-to-air equivalence ratio is 1.15 ± 0.1. The influence of the air heating level on the domain of continuous detonation, pressure in the combustor, and specific impulse is analyzed. Regimes of continuous multifront detonation with one pair of colliding transverse waves with a frequency of 1.2 ± 0.1 kHz are observed in all experiments at air temperatures ranging from 600 to 1 200 K. Based on the stagnation pressures measured at the combustor exit, the specific impulse of continuous detonation is determined as a function of the air flow rate and its temperature. As the air heating temperature increases, the specific impulse of the thrust force is found to decrease owing to the increase in the degree of dissociation of the detonation products. The maximum specific impulse with allowance for the heated air energy, equal to 1 630 s, is obtained for the air temperature in the settling chamber equal to 600 K.

The use of nanosized metal powders is a promising direction in the development of modern energy compositions due to their high reactivity and intense heat release upon contact with an oxidizer and during combustion. The results of a combined TG-DSC analysis of Alex aluminum nanopowders and a Al-Cu compound, obtained via electrical explosion of conductors, are presented at constant heating rates of 2, 4, and 20 °С/min in air in a temperature range of 30 ÷ 1 300 °С. It is revealed that Alex and Al-Cu nanopowders are intensely oxidized when heated in air to a temperature of 600 °С due to the oxidizer diffusion through the porous oxide layer Al₂O₃ and the possible formation of open surfaces of an active metal during a phase change in the crystal lattice of the metal oxide. The Friedman and Kissinger-Akahira-Sunose (KAS) methods were used to obtain dependences between the activation oxidation energy on the degree of conversion (oxidation) of nanosized metal powders. It is shown that the activation energy of Alex and Al-Cu nanopowders depends on the degree of conversion (oxidation stages) and lies in ranges of 78 ÷ 307 and 99 ÷ 430 kJ/mol, respectively.

This paper describes a study of the effect of a Fe + Co + Cr + Ni + Al metal binder content and mechanical activation (MA) on burning rate, maximum combustion temperature, the elongation of samples during combustion, the mixture yield after MA, the size of composite particles after MA, and the morphology and phase composition of combustion products in a Ti + 2B + x (Fe + Co + Cr + Ni + Al) system. Self-propagating high-temperature synthesis (SHS) is used to obtain a composite material consisting of ceramics and a high-entropy alloy. The MA increases from 60 to 80 % the maximum content of a metal binder in the mixture, at which SHS is carried out at room temperature. The addition of a Fe + Co + Cr + Ni + Al binder to the activated Ti + 2B mixture prevents mechanochemical synthesis from proceeding during 5-min long MA.

This paper describes the combustion of 90 % (Ti + C) + 10 % Ni and 80 % (Ti + C) + 20 % Ni granular mixtures with a binder (polyvinyl butyral) content of the order of 1 %, used to obtain composite materials. Experimental data and calculations show that combustion waves in grains of fractions of 0.4 ÷ 0.8, 0.8 ÷ 1.2, 1.4 ÷ 2, and 0.6 ÷ 1.6 mm propagate in the conductive mode. The micro- and macroscopic characteristics of combustion (namely the combustion transfer time from one granule to another and the burning rate of the granule substance) are calculated on the basis of the experimental burning rate of granular mixtures from granules of different fractions for the first time. The combustion transfer time from one granule to another for mixtures with 10 and 20 % Ni turns out to be the same for all fractions: approximately 0.006 s. Regardless of the granule size, X-ray diffraction patterns of combustion products contain only TiC and Ni phases. Brittle agglomerates easily crushed under laboratory conditions are obtained as a result of the synthesis. The resulting data indicate the possibility of possibility of safe scaling of the process of obtaining (Ti + C) + x Ni cermets with a Ni content of 10 and 20 % in the combustion mode from a granular charge with granule sizes up to 1.7 mm.

The synthesis of an intermetallic compound in a mixture of Ni + Al + Al₂O₃ powders with an inner metal plate oriented in the direction of propagation of the combustion wave has been studied by experiments and theoretical calculations as applied to the problem of self-propagating high-temperature synthesis of functionally gradient materials. Copper, titanium, and steel plates are considered. The propagation of the exothermic reaction front is analyzed for various thermophysical characteristics of the inner metal layer. In samples with a copper plate, the combustion front near the contact boundaries is stretched in the direction of the combustion wave. Increase in the average burning rate of the sample with the heat recovery effect is observed when using copper plates 1 ÷ 3 mm thick. During combustion of samples with titanium and steel plates, there is a deceleration of combustion wave propagation along the contact boundary.

Dioctyl sebacate (DOS), paraffin (PW), fluororubber (FR), and polyurethane (PU) are widely used as binders in aluminized propellants and explosives. In this study, ignition and combustion of aluminum powders coated with DOS, PW, FR, and PU are investigated. The flame speeds of the aluminum powders coated with DOS, PW, and PU are found to decrease with an increase in the content of these binders. The flame speed of the aluminum powder coated with FR is greater than those of the aluminum powders coated with DOS, PW, and PU. In this case, combustion improvement is due to the reaction between the alumina shell and FR.

The combustion of a mixed metallized solid fuel containing boron particles is numerically studied. The mathematical model is based on the approaches of two-phase reacting medium mechanics for describing processes in a two-phase flow above the fuel surface and the Hermance model for describing the decomposition of a mixed solid fuel. The model of combustion of a mixed metalized solid fuel takes into account the oxidation and combustion of boron particles. The dependence of the burning rate of a metallized mixed solid fuel on the gas pressure above the combustion surface and the size of boron particles is determined from a parametric study.

The destruction of an energetic condensed system based on glycerol trinitrate, polyether urethane, and an aluminum powder from exposure to a relativistic electron beam with a maximum energy of 310 keV, a total duration of 170 ÷ 180 ns, and an average flux density of 200 ÷ 215 J/cm² was studied by nanosecond electron-optical chronography, scanning electron microscopy, and energy dispersive analysis. The effect of the generation of pulsed electric fields and shock-wave loads accompanying relativistic electron beam absorption on the mechanical damage in samples of energetic condensed systems is discussed.

The values of the detonation velocity in thin layers of the VS-2 pyrotechnic composition are given. The Jones-Wilkins-Lee equation of state parameters of explosion products were determined. The obtained equation of state was used to perform numerical calculations and comparison with the experimental parameters of the air shock wave formed upon initiation of the VS-2 composition. Computational and experimental studies were carried out to determine the dependence of the pressure pulse on the thickness of the VS-2 layer. The dependence of the specific pressure pulse of the explosion products on the surface density of the VS-2 composition was determined.

A method of constructing the curve of cold compression and the isotherm for the metal based on the generalized equation for the Gruneisen coefficient is considered. In contrast to available approaches, it is proposed to take into account the change in the character of interaction of atoms in the crystal due to compression. For this purpose, the dimensionless parameter t in the generalized equation is considered as a function of compression. The experimental data on metal compressibility testify that the parameter t increases with an increase in density. Free parameters of the model for aluminum are found. The proposed approach allows the aluminum isotherm to be described more accurately and in a wider range of density. For the cold compression curve, the working region of the model increases almost by an order of magnitude in terms of density and by more than two orders of magnitude in terms of pressure (as compared to the model where the parameter t is assumed to be constant).

Honeycomb ceramic-metal catalysts were studied before and after stability tests in a coal fluidized-bed reactor with a capacity of 3 Gcal/h. The catalysts were studied by X-ray phase analysis and scanning microscopy with elemental analysis; their textural and strength characteristics were studied. The test reaction of octane oxidation was used as a measure of catalytic activity. A 30 ÷ 40 % decrease in activity was found with decreasing strength from 26 to 18 MPa. Based on the obtained changes in properties, the time of possible operation of the catalyst was predicted to reach at least two winter seasons.

This paper presents an analysis of the hydrodynamic and physico-chemical features of flow and combustion in a low-emission combustion chamber (LECC) which differs from other combustion chambers in design and combustion modes of a lean air-fuel mixture. The influence of the composition and residence time of the mixture in the LECC on the emission of NO _x and CO was considered. The organization workflow was proposed, and an LECC prototype was developed whose emission level of NO _x and CO less than 5 ppm is 2 ÷ 5 times lower than that in available LECCs. The results of experimental studies of the LECC prototype with variations in flame temperature, pressure, and residence time in the LECC confirm the validity of the proposed solutions.