Home – Home – Jornals – Combustion, Explosion and Shock Waves 2014 number 6

The problem of stabilization of the wave of filtration combustion of a premixed mixture of gases in a porous cylindrical burner is solved numerically. Two formulations of the problem are considered. In the first case, a one-dimensional steady problem of gas combustion in a porous cylindrical burner with allowance for radiative heat transfer inside the porous skeleton and emission of radiation from the burner surface is solved. In the second case, the problem is solved within the framework of the conventional one-dimensional steady diffusion-thermal model of filtration combustion of gases with allowance for radiative heat losses only from the external surface of the burner. The results obtained by two models are compared. It is shown that radiative heat transfer exerts a significant effect on the process of filtration combustion in a porous body, in particular, on the temperature distributions in the gas and the solid, as well as on the radius of stabilization of the combustion front. Heat losses in the form of emission into the ambient medium are found to be caused to a large extent by emission from internal layers of the porous body owing to radiative heat transfer, not only by emission from the external surface of the burner.

Ignition of a stoichiometric methane-air mixture in a piston-driven setup at temperatures of 900–1200 K and pressures of 1–1.2 MPa is experimentally studied. The experiments reveal the emergence of bright points in the duct of the facility. The measurements show that luminescence in the volume appears almost immediately after the end of the compression stage, whereas the methane-air mixture ignites only after 4–5 ms. Presumably, these bright points appear owing to ignition of fine particles. It is experimentally found that ignition of these particles reduces the ignition delay time of the gas mixture. A physicomathematical model of ignition of methane—oxygen—nitrogen/argon mixtures in the presence of fine metal particles is proposed to obtain a theoretical description of this phenomenon. This model takes into account both detailed kinetic mechanisms of chemical conversions in the reacting gas mixture and reduced kinetic mechanisms of oxidation of metal particles. Calculations by this model show that ignition of particles at low temperatures (less than 1100 K) leads to reduction of the ignition delay time of the gas mixture.

A thermal explosion and self-propagating high-temperature synthesis in low-calorific SiO₂–Al mixtures subjected to preliminary mechanical activation are obtained. Results of x-ray and electron-microscopic investigations of the microstructure and phase composition of activated mixtures and reaction products are reported. It is found that complete reduction of quartz by aluminum occurs only in the thermal explosion mode.

The effect of the phase composition of an aluminum-based alloy powder containing 1.3% barium and the products of its interaction with air on the kinetics and mechanism of oxidation was first studied by x-ray diffraction using synchrotron radiation. It is found that the high surface and reactivity of barium lead to the activation of aluminum oxidation due to the heterogeneity of the phase composition of the barrier layer of the reaction products on the particle surface.

A thermal explosion synthesis method for production of magnesium diboride, MgB₂, has been investigated, revealing the formation of a high-quality material via an exothermic reaction after heating a mixture of magnesium and boron powders to a temperature of ≈600 ºC. The reaction product has been characterized by different thermal and x–ray analyses. It has been also revealed that aluminum diboride, AlB₂, is not formed from aluminum and boron powders in the range of temperatures applied.

Combustion of micron-sized magnesium dust particles with spatially discrete sources is studied numerically. The flame propagation speed for various dust concentrations and different particle diameters is estimated. Experimental results for two different burning times are compared. The minimum ignition energy as a function of dust concentration for different particle sizes is studied. The results show good agreement with experimental data.

Ignition, combustion, and oxidation characteristics of mixtures of amorphous and crystalline boron powders are studied by using a laser ignition system and a thermogravimetric analyzer. Yellow, green, and white-glow flames are observed. When the weight ratio of the amorphous boron powder (K) is lower than 0.33, burning of the mixtures is found to be at the ignition stage, while the combustion stage gradually appears with increasing K. The emission spectrum intensity is weakened by 3/4 as K decreases from 1 to 0.75. No obvious emission spectrum is detected at K < 0.33. The ignition delay time is found to decrease significantly with increasing K, but it does not change much at K < 0.8. Mixtures with larger values of K have a lower peak temperature of differential thermal gravity and a greater weight gain and maximum reaction rate. The Satava-Sestak method is applied to study the kinetics of the oxidation reaction of the mixtures. The calculated apparent activation energy decreases from 297 to 191 kJ/mol as K increases from 0 to 1. For mixtures with K ranging from 0.23 to 0.75, the apparent activation energy mainly stays constant at a value of 280 kJ/mol.

A numerical simulation of solid-phase ignition of a composite propellant by a single small disk-shaped metal particle heated to a high temperature is performed. In the “heat flux amplitude-ignition delay” coordinates, an region of stable initiation of combustion of a typical composite solid propellant under local heating by a source of limited energy capacity is selected. The limiting amplitudes of heat fluxes during ignition of the condensed substance under conductive and radiative heating are compared.

This paper presents a physicomathematical model and a method for calculating the parameters of the transition of a SRM to steady-state operation based on the design features of the propellant charge, the initial conditions in the motor, and the interrelated processes such as the formation and motion of compression waves and shock waves in the channel of the charge; the motion and mixing of the igniter combustion products containing a condensed phase with the initial gas and the combustion products of the charge; gradual heating and ignition of the surface of the charge. The developed program is used to explain the difference between the experimental pressure-time curves in the region of the transition of a large-size motor model to steady-state operation, obtained in bench tests under atmospheric conditions and on a high-altitude test facility at an initial motor pressure of 0.25×10⁵ Pa. It is shown that ignition of the charge in vacuum produces more favorable conditions for heat transfer than ignition at atmospheric pressure. In the channel, this is due to an increase in the velocity of the igniter combustion products behind the transmitted shock wave, and in the submerged region and the charge slots, due to the minimum absorption of the radiation from the combustion products by the rarefied initial gas (air) present there.

A physicomathematical model of shock wave initiation of an explosion of oxygen-containing bubbles in liquid hydrocarbon fuels is developed. The model is used to calculate an explosion of bubbles in which the initial concentration of fuel vapors is outside the flammability limits. A principal possibility of condensation of fuel vapors during compression of a fuel-rich bubble located in liquid cyclohexane with a comparatively high initial temperature is demonstrated. Condensation leads to a decrease in the fuel concentration in the gas lower than the fuel-rich flammability limit. As a result, an explosion of an initially non-combustible bubble becomes possible. The condensation rate is estimated. It is found to be sufficiently high to change the chemical composition of the gas during the period of the first oscillation of the bubble. The explosion limits of the bubble are calculated as functions of the initial pressure, temperature, and amplitude of the shock wave. If condensation is taken into account, the explosion limits are expanded. Bubble liquids (2-ethylhexanol and isopropyl benzene) where the fuel vapor concentration is initially lower than the fuel-lean flammability limit are considered. It is shown that liquid evaporation induced by mechanical mixing of the phases can shift the chemical composition of the gas inward the flammability region, resulting in a bubble explosion with a corresponding increase in gas temperature. Calculated and experimental results are found to be in good agreement. Explosive processes in bubble media containing a liquid monopropellant are considered at the qualitative level for the first time. An assumption is put forward about a possibility of multiple explosions of an individual bubble loaded by a series of shock waves and subjected to multiple transitions of the detonation wave over the liquid with distributed bubbles, which may be used for generating a series of powerful acoustic signals in the ambient space.

In this paper, we study a micro-hotspot model for the laser initiation of explosive decomposition taking into account the melting of the matrix of the energetic material and the nanometallic inclusion contained in its volume. The heating features of the nanoparticle in an inert matrix are investigated, and the dependence of the maximum temperature on the surface of the inclusion on its radius is constructed. It is shown that melting leads to a reduction in the maximum heating temperature and a slight change in the radius of the most heated nanoparticle. The dependences of the critical initiation energy density of explosive decomposition of pentaerythritol tetranitrate (PETN) with aluminum nanoparticles on the inclusion radius with and without melting are calculated. With melting taken into account, the model gives higher critical initiation energy density of explosive decomposition. In the case of inclusions of large radius, the formation of the reaction site of explosive decomposition occurs before complete melting of the metal inclusion, which results in solidification of the melt during the induction period.

The propellant performance and the velocity of expansion of detonation products of explosives based on RDX and HMX and their mixtures with particulate Al were investigated. Measurements were performed in air at atmospheric pressure and in rarefied air. Comparison of the results shows that the detonation products of explosives, including those containing no Al additives, react with the surrounding air. The completeness of the reaction increases with increasing velocity of detonation products in air.

The Hugoniot of an emulsion matrix based on aqueous ammonium nitrate was determined at pressures of up to 30 GPa, far exceeding the calculated detonation pressure. In the range of 13–15 GPa, the Hugoniot curve was found to have a jump singularity associated with the possible decomposition of the emulsion at high shock pressure.

Variants of experiments in which prefractured samples (in tests on spall fracture) are further compacted by shock reloading are considered on the example of aluminum. The results of the experimental-computational study and metallographic analysis of recovered samples in the tests are used to determine the pressure of compacting aluminum, which is ≈2 GPa.

This paper presents the results of experiments on the spalling and nonspalling acceleration of steel plates by HMX–based explosive charges of different initial thickness (with spall closure) in the presence and absence of an air gap between the explosive charge and the projectile plate. The plate acceleration dynamics was simultaneously recorded by two different independent laser interferometric techniques using lasers of different wavelengths. The experiments were carried out to perform a metrological certification of a new laser–heterodyne technique based on the data of a previously metrologically certified laser–interferometric technique based on the Fabry–Pérot scheme.

New computational and experimental methods for studying the thermal processes occurring during metal explosive welding are developed based on the use of natural and local thermocouples and allowing one to reliably construct temperature fields in the heat–affected zone and thermal welding cycles. The basic regularities of thermal processes in the heat-affected zone during explosive welding are revealed. It is theoretically shown and experimentally confirmed that, in this zone, the heat released during detonation of the explosive can significantly increase the temperature of the contacting metal layers and lead to melting of the latter.

This paper presents information on the development of a new contactless method for measuring the unsteady mass gasification rate based on dynamic recording of the amount of the test substance in a microwave sensor of the resonator type. The sensor design allows measuring the gasification rate with vigorous blowing of the surface of a sample with a bore for passage of gas flow.

In studying lifted diffusion flames in the transitional regime from laminar to turbulent combustion, structures with a reduced intensity of own luminescence and a clearly expressed contour are observed in the combustion front. The emergence of such structures in the flame has a spontaneous character and may occur at significant distances from the ignition points (region of lifted flame stabilization).