Home – Home – Jornals – Combustion, Explosion and Shock Waves 2023 number 4

Results of investigations of flame acceleration and deflagration-to-detonation transition in circular smooth tubes performed for several last years at the ITMO University (Minsk, Belarus) are briefly reviewed and systematized. All stages of the flame acceleration process from initiation with a weak source to detonation are demonstrated and described.

Results of simulations of a high-velocity reacting flow of a non-premixed hydrogen-air mixture in a channel with sudden expansion in the form of backward-facing steps with transverse injection of hydrogen jets are reported. The computations are performed with the Ansys Fluent software package based on solving three-dimensional unsteady Reynolds-averaged Navier-Stokes equations with the κ-ω SST turbulence model and equations of detailed chemical kinetics of hydrogen combustion in air. The simulations predict self-ignition of the hydrogen-air mixture subsequently transforming to intense combustion with upstream motion of the flame from the ignition region. It is demonstrated that combustion occurs in thick subsonic regions, which merge at the channel axis in areas of elevated heat release, thus, forming a thermal throat. As a result, a system of normal shock waves is formed, which separate the boundary layer from the channel wall. The reverse flow transfers hot reaction products toward the step wall; thus, the thermal throat and shock waves are shifted upstream. As a result, the combustion wave and the shock wave enter the injector area, the “knocked-out” shock wave merge with the bow shock ahead of the jets, and thermal choking of the channel occurs.

Results of numerical simulations of flame acceleration in a semi-open channel filled by acetylene-based mixtures are reported. The computations are performed by the advanced dissipationless method CABARET. The effects of the channel width and the roughness of the inner wall of the channel on the flame evolution dynamics at various stages of the flame acceleration process are demonstrated based on comparisons of results obtained in different formulations. In particular, it is shown that the flame velocity and the amplitude of velocity fluctuations at the quasi-steady stage of flame propagation increase with an increase in the channel width. It is also demonstrated that flow deceleration at the channel walls produces the most pronounced effect at the stage of quasi-steady propagation of the flame owing to faster development of the boundary layer and vortex generation in the near-wall region and vortex interaction with the flame extended along the channel walls.

This paper presents experimental data on the ignition induction period of synthetic hydrocarbons at various temperatures and pressures obtained using a shock tube. The influence of the ignition induction period on the combustion completeness of hydrocarbons in high-enthalpy flows for diffusion-kinetic regimes was determined based on the experimental results. An integral mathematical model is presented that takes into account the influence of the kinetic factors of ignition and combustion on the completeness of physicochemical processes in the air flow. The results of calculations of the combustion completeness of synthetic hydrocarbons in flows with different parameters.

The gasification of a two-layer solid porous fuel in a combined low-temperature charge of a gas generator has been studied using the developed mathematical model and computational algorithm implemented in the OpenFOAM software. The influence of the two-layer nature of the solid porous fuel on its gasification has been studied in computational experiments using as an example gasifiers containing a fuel based on polymethylmethacrylate and polyethylene with different mutual arrangement of their layers. It has been shown that in a two-layer fuel, two waves of gasification propagate simultaneously, due to which the relative mass flow rate of gasification products can have two local maxima. The operating time of a gas generator using a two-layer porous fuel ambiguously depends on the mutual arrangement of the layers and can go beyond the range between the times of operation of the gasifier using each of the fuels.

Dispersed metallic fuels are energy-intensive components of various gel-like and mixed solid fuels, which significantly increase the performance of propulsion systems. This article presents the combustion characteristics of a high-energy material (HEM) containing an oxidizer, a polymeric combustible and a dispersed metallic fuel - aluminum Al, aluminum borides AlB₂ and AlB₁₂, amorphous boron. In a constant-pressure bomb, the combustion rates of HEM were measured in the pressure range of 0.7-4.0 MPa, the effect of aluminum dispersion and the nature of the metallic fuel on the combustion rate and temperature, the fuel sensitivity to pressure changes in the chamber, and the composition of condensed combustion products was established. An increase in the fineness of Al particles in the HEM significantly increases the burning rate and the sensitivity of the fuel composition to pressure changes. Replacing microsized Al powder with amorphous boron, AlB₂ or AlB₁₂ in HEM increases the combustion rate by 2.1-2.2 times at a pressure of 4.0 MPa, while the power exponent in the combustion rate law u(p) = Bp^ν increases from 0.22 to 0.45.

The processes occurring during the induction of a thermal explosion in mixtures of titanium and aluminum powders are analyzed. The role of the oxide film on aluminum particles and the heating rate of samples during the interaction of titanium with aluminum is considered, and various mechanisms of destruction of the oxide film at temperatures near the melting point are proposed. It is shown that, depending on the heating rate of the samples, three mechanisms for the formation of direct contact between titanium and aluminum are possible: mechanical destruction of the oxide film on aluminum, the reaction of aluminum oxide with titanium, and the reaction of aluminum oxide with aluminum. Mechanical activation of the powder mixture lowers the ignition temperature by 20-30 °C. Reducing the average particle size of titanium from 90 to 10 microns lowers the ignition temperature by 100 °C. It is shown that in a number of cases a two-stage ignition mechanism is observed. After the isothermal section of aluminum melting, the stage of a slow temperature rise to 700-800 °C begins, after which the temperature rise rate increases by an order of magnitude.

A promising direction in barrel ballistics to increase the muzzle velocity of a projectile is the use of new fuels as an attached charge. The paper presents an experimental and theoretical method for determining the law of fuel combustion in a manometric experiment (in a closed volume), which allows, by a small number of experiments, to determine the change in the law of combustion as the fuel burns and depending on pressure. The technique is demonstrated on the example of processing three experiments with model high-density fuel. The obtained laws of combustion of high-density fuels can later be used to calculate the gas-dynamic parameters of a shot from a barrel system or a nozzle bomb (in a semi-closed volume) using these fuels.

This paper presents a technique for non-contact diagnostics of on solid rocket motors (SRM) based on the analysis of acoustic vibrations generated by the supersonic jet of products combustion discharged from the nozzle. The combustor pressure in a model rocket motor fueled by E-5-0 solid propellant was determined experimentally by a non-invasive control method using a dynamic microphone located at a predetermined distance from the object of study and recording acoustic fields generated by the running engine. Experiments confirmed the possibility of non-contact determination of combustor pressure from the frequency of acoustic vibrations and the sound pressure generated by the jet of combustion products of the model SRM. Calculated pressures are in satisfactory agreement with the values recorded by intra-chamber pressure sensor.

Electrospark initiation of thermite mixtures based on nanosized powders of aluminum and copper oxide has been studied. Data on the effect of the energy of an electric spark discharge on the ignition delay have been obtained. When initiating long cylindrical samples of mixtures with a low-current spark, two types of unsteady combustion were observed. The first type is characterized by an exponential nature of the establishment of a constant burning rate. The second type is characterized by regions with incomplete reaction along the main direction of propagation. To obtain stationary modes of a thermite mixture with a minimum ignition delay time, it is necessary to use a discharge with energy of more than 5 mJ per 1 mm² mixture surface.

Experimental data are presented on the rate of pressure change during the combustion of an air suspension of coal dust in a closed spherical volume at various mass concentrations of coal dust. On the basis of the equations of mechanics of disperse media in a one-velocity one-temperature approximation, a physico-mathematical model for the combustion of an air suspension of coal dust in a closed spherical volume is formulated. The combustion wave propagation velocity relative to the gas suspension and the combustion velocity of a coal dust particle are model parameters and are determined by matching the calculation results with experimental data. Their agreement was good. The proposed approach can be used to assess the effect of coal dust combustion on the intensity of shock waves in coal mines during accidental methane explosions involving coal dust.

A pioneering study of the characteristics of combustion of liquid hydrocarbons in a superheated steam jet under conditions of a closed burner is performed by an example of the diesel fuel. An undated structure of an original low-power burner is presented. It is based on the principle of fuel atomization by a high-velocity steam jet and offers a possibility of controlling injection of primary and secondary air. Amounts of pollutants ejected into air are determined for different air-to-fuel ratios inside the burner. First comparisons of the developed burner with a production model of the liquid-fuel burner (Weishaupt burner) are performed. It is shown that addition of superheated steam ensures a high combustion efficiency and low contents of CO and NO_x in combustion products satisfying the most rigorous European standards.

Experiments are performed for studying the process of shock wave initiation of detonation in pure tetranitromethane and its mixtures with acetone, nitrobenzene, and methanol. Glowing of the detonation front is detected by high-speed cameras operating in slit scan and frame-by-frame modes. It is found that detonation both in pure tetranitromethane and in its mixtures with dilutants is initiated at localized sites. The number of these sites, the character of their formation, and the features of their growth and merging depend on the nature of dilutants. The evolution of the wave profiles is registered by a velocity interferometer system for any reflector (VISAR). The resultant velocity profiles are noticeably different from those predicted by the classical pattern of detonation initiation and evolution under a shock wave action.

The structures of detonation waves in mixtures of tetranitromethane with acetone was studied in experiments using a VISAR interferometer and a NANOGATE-22 electron-optical camera. At a diluent concentration of 10-40%, there was a sharp change in the flow in the reaction zone, manifested in a decrease in the amplitude of the von Newman spike up to its complete disappearance. Detonation waves are stable in almost the entire range of concentrations, except in the range near the limiting value of 52%. In approaching the limiting concentration, the waves become unstable, which is manifested in the formation of both the cellular structure of the front and reaction failure waves. The obtained experimental dependences of the detonation velocity on acetone concentration are in good agreement with thermodynamic calculations.

When using the method of multipoint initiation of the side surface of a cylindrical charge in a section perpendicular to the axis, a detonation wave with a complex gas-dynamic structure is formed, which has the shape of a polygon with vertices at the points of wave conjugation. The sides of the polygon are always convex towards the charge axis. To obtain a smooth cylindrical detonation wave with reverse curvature, it is proposed to use special devices at the points of initiation - lenses made of an inert material. The dynamic characteristics of the material are experimentally determined, and the method for constructing the lens profile is substantiated. Mathematical modeling of the operation of the node of initiation and formation of a cylindrical detonation wave in a charge has been carried out. The features of the operation of a single initiating node and a node that is part of the experimental assembly are shown. The dynamics of axisymmetric compression by a converging detonation wave is presented and compared with calculations.

The conditions of suppression of flame combustion and thermal decomposition in model forest and peat fires using water, foam concentrate emulsion (5%), bischofite solution (10%), FR-Les 01 solution (20%), flame retardant solution (vol. 5%), OS-5 solution (15%), and bentonite slurry (5%) were determined. Temperatures in the process of ignition of model fires and extinguishing of the latter by specialized water-based compositions were measured. Minimum volumes of the extinguishing compositions and the time necessary and sufficient to suppress the pyrolysis of plant biomass were established. Minimum irrigation densities of model forest and peat fires using water-based compositions with specialized additives were determined.