Effect of macroscopic heterogeneity of the medium on the solid-state combustion wave characteristics in thermally and chemically heterogeneous media

A macroscopically heterogeneous medium is modeled by a set of cylinders (rods) pressed from a mixture of solid reagents, which have coaxial cylindrical cores made from an inert material. A three-dimensional mathematical model describing propagation of combustion waves over one cylinder is studied by numerical methods. The influence of the distance between the centerlines of the neighboring cylinders and transverse sizes of the cylinders on characteristics of combustion waves propagating over a heterogeneous medium is considered. It is demonstrated that there exists an optimal distance between the inert rods, which ensures a much higher velocity of the combustion wave along the specimen than the theoretically predicted velocity of the classical combustion wave propagating over a solid specimen. New types of spinning waves are described, whose motion makes the high-temperature spot move inside the charge mixture. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 39–49, May–June, 2008.     

Fluctuating regime for combustion of azidoethanol

Fluctuating combustion of azidoethanol is observed in unthermostated glass tubes. The dependence of the combustion velocity of azidoethanol on the external pressure is determined. A distinctive feature of this dependence is that a density jump develops during the transition from normal combustion to a fluctuating regime. The periods of the fluctuations in the combustion velocity of azidoethanol are determined for different external pressures. The observed effect is explained qualitatively. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 4, pp. 23–25, July–August 1998.     

Energetic-material combustion experiments on propellant formulations containing prilled ammonium dinitramide

Energetic material combustion experiments were performed on propellant formulations containing ammonium dinitramide prills. The overall performance of the formulation was found to be sensitive to the prill microstructure. The combustion experiments show that the performance of the composition could be further improved by “tailoring” the microstructural characteristics of the prills. These parameters are not revealed by strand burning-rate measurements, which represent an average or overall measurement and pick up on the localized rate variations as “noise,” averaging them out. Microstructural characterization (such as filming of the propellant combustion at the highest magnifications and examination of quenched samples) of propellant formulations may thus provide some of the complementary data to the standard propellant characterization measurements to reduce or eliminate measurement errors. Furthermore, with prilling the errors are reduced and the performance is improved. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 1, pp. 131–137, January–February, 2000. This work was supported by ONR.     

Fireball during combustion of hydrocarbon fuel releases. I. Structure and lift dynamics

The formation, combustion, and thermal interaction of the fireballs which develop upon ignition of a cloud of hydrocarbon fuel near the Earth’s surface are simulated numerically. The axisymmetric nonstationary flow is described by a system of Favre averaged conservation equations invoking a (k−ε)-turbulence model, a model for turbulent combustion, and a global-kinetic scheme for formation and burnup of soot particles. The optical properties of the mixture of combustion products and soot are modeled by a weighted sum of gray gases. The radiation field is calculated using a combination of a volume emission approximation and a diffusion approximation. Calculations are done for fireballs formed during vertical releases of gaseous propane masses of 1 g to 10³ kg with ignition near the release point. The internal structure of a fireball is analyzed in detail at various stages of its evolution. The lift dynamics of a fireball is illustrated for release velocities corresponding to Froude numbers (defined as the square of the ratio of the linear outflow velocity to the characteristic velocity owing to buoyancy forces) ranging from 5–250. The temperature, concentrations, and reaction rates in the fireball are determined as functions of time. It is shown that for these ranges of fuel mass and release velocity, the dimensionless parameters introduced here can be used for scaling the results and using the calculated dependences obtained here in a unified fashion. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 7–19, May–June 1999.     

Filtration combustion of hydrogen-air, propane-air, and methane-air mixtures in inert porous media

The filtration combustion characteristics of hydrogen-air, propane-air, and methane-air mixtures in inert porous media have been studied experimentally. It is shown that the dependences of the combustion wave velocity on the fuel-air equivalence ratio are V-shaped. For hydrogen-air mixtures, the velocity minimum is shifted to the rich region, and for propane-air and methane-air mixtures, it is shifted to the lean region. For lean hydrogen-air and rich propane-air mixtures, the measured maximum temperatures in the combustion wave are found to be reduced relative to those calculated theoretically. For methane-air mixtures, a reduction in the measured temperatures is observed over the entire range of the mixture composition. The results are interpreted within the framework of the hypothesis of selective diffusion of gas mixture components. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 8–20, July–August, 2006.     

The mechanism of phase and structure formation of the Ti-B-Fe system in a combustion wave

The mechanism of phase changes in the Ti-B-Fe system in a combustion wave for a mixture of Ti, B, and Fe powders and a ferroboron alloy-titanium mixture with the same proportion of elements is studied. It is found that the mechanism of structure formation depends significantly on the type of contact between the initial components. An x-ray phase and x-ray spectrum and structural microanalyses of the quenched layers of specimens show that the first contact melts occurring in the combustion wave are ferroboron (the first type of mixture) or ferrotitanium (the second type) melts. In the first case, the calculated high-melting compound TiB₂ forms as a result of the interaction between the two melts; in the second case, it forms as a result of the interaction of the melt with solid ferroboron, which, in turn, determines the different type of microstructure of the final combustion products. The highly disperse and more homogeneous structure of the products forms after combustion of the second-type mixture. A method of producing the Ti-B-Fe pore-free composite produced during the self-propagating high-temperature synthesis (SHS-composite) by combining the combustion with rolling of the synthesis products is considered. In properties, theresulting material is similar to tungsten-carbide materials. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 5, pp. 27–34, September–October, 2000. This work was supported by the Federal Program “Integration.”     

Localization of combustion in a periodic velocity field

A direct numerical simulation of combustion-front propagation in a specified periodic velocity field of a medium was performed within the framework of a two-dimensional thermal diffusion formulation. The calculations showed that as the velocity amplitude is increased, the flame front separates into discrete burning segments, i.e., spatial localization of the combustion takes place. Only when heat losses are introduced into the model, flame quenching is observed at a certain amplitude of the medium’s velocity. The level of heat losses required to extinguish the combustion becomes lower with increase in the amplitude of velocity perturbations. On the whole, the obtained numerical results agree with results of an asymptotic analysis. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 19–28, May–June 1998.     

Mechanism and features of nitriding of ferrosilicon in the combustion regime

The nitriding of ferrosilicon in the combustion regime is studied. It is found that by diluting the starting alloy by the end product and by introducing halide salts of ammonium and magnesium, it is possible to increase the degree of conversion and to obtain a two-phase combustion product consisting of silicon nitride and iron. It is shown that the nitriding of ferrosilicon occurs in the temperature range from 900°C to the combustion temperature and includes several steps. The temperature at the beginning of interaction of ferrosilicon with nitrogen coincides with the temperature of the α-leboite → β-leboite phase transition. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 71–78, May–June, 2006.     

Mechanisms of combustion catalysis by ferrocene derivatives. 1. Combustion of ammonium perchlorate and ferrocene

Combustion of ammonium perchlorate (20–90%) mixtures with ferrocene is studied. It is demonstrated that, depending on the ratio of the components in the examined compositions, in addition to the usual gas-phase combustion model, another possible combustion mechanism exists. A somewhat unusual condensed-phase (c-phase) model may be realized, in which the heat-generating reaction occurs in a foam/aerosol layer at the temperature of evaporation of the less volatile component, which the surface temperature is defined by evaporation of the more volatile component. The efficiency of ferrocene depends on the propellant combustion mechanism: in systems that obey the gas-phase combustion mechanism, the influence of ferrocene addition is higher than the influence of addition of a hydrocarbon fuel; in systems with the c-phase mechanism of combustion, ferrocene addition produces a significant effect. Depending on the ratio of the components, ferrocene first acts in these compositions simply as a highly reactive fuel; it is only in fuel-enriched compositions with a high equivalence ratio that the burning rate increases owing to catalysis of the combustion process by the ferric oxide on the soot skeleton.     

Frontal regime of exothermal conversion of refractory condensed compounds

We consider a mathematical model of propagation of the combustion front in heterogeneous condensed compounds. For these compounds, the adiabatic combustion temperature is lower than the melting point of the initial reagents and condensed products; it is also lower than the minimum temperature of their eutectics. The chemical interaction in the combustion front is accomplished by a gas transport mechanism due to the gasification of additive oxides. Within the framework of the investigated model, values are obtained for the front velocity, which are of the same order as those observed experimentally. Periodic unstable combustion regimes (fluctuating and spinning) arise when the macrokinetics of the chemical transformation is strongly activated. It is shown that the activation can depend on the gasification energy and, consequently, the evaporation of additive oxides influences the stability of a steady-state combustion regime. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 77–83, May–June 1998.     

Effect of convective motion on the flame structure in combustion waves propagating in heterogeneous systems under natural and artificial gravity conditions

Convective motion in combustion waves arises from natural or artificial gravity (centrifugal effect) for a wide range of heterogeneous systems, such as ammonium perchlorate containing solid additives, metal and nonmetal powders with a gas reagent, thermite mixtures, and hybrid layered structures. The paper summarizes the results obtained by the author and coworkers in studies of the effect of convective motion in combustion of heterogeneous systems for the period from the early 1970s to the present. It is shown that convective processes in combustion waves of heterogeneous systems may determine the structure and propagation mechanism of the combustion waves and the concomitant heat and mass transfer. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 86–92, July–August, 2009.     

Condensed combustion products of aluminized propellants. II. evolution of particles with distance from the burning surface

Condensed combustion products of a model propellant on the basis of ammonium perchlorate and aluminum were studied using the sampling technique. The granulometric composition of combustion products and the content of metal aluminum in particles of size from 1.2 μm to maximum were determined within the pressure range of 0.6–7.5 MPa at a distance from the burning surface up to 190 mm. A multimode structure of mass distributions of oxide particles within the size range of 1.2–40 μm was found. An empirical dependence of burnout of metal aluminum from agglomerates on the residence time of particles in the plume of combustion products of the propellant sample was obtained. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4. pp. 66–78, July–August, 2000.     

Microstructural aspects of gasless combustion of mechanically activated mixtures. I. High-speed microvideorecording of the Ni-Al composition

Gasless combustion of a mechanically activated Ni-Al mixture is experimentally studied; the process is compared with the characteristics of combustion of a nonactivated Ni-Al mixture. Mechanical activation of the Ni-Al mixture is shown to form layered conglomerates consisting of numerous layers of the initial components. As a result, the structure of the medium becomes very similar to a cellular structure, which forms the basis for advanced microheterogeneous models of gasless combustion. A relation between the local (microscopic) and global (macroscopic) parameters of gasless combustion is found. Modification of the microstructure of the initial medium by means of mechanical activation allows obtaining products whose microstructure differs significantly from the microstructure of products obtained from nonactivated mixtures. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 61–70, July–August, 2006.     

Formation of metal oxide nanoparticles in combustion of titanium and aluminum droplets

A study was performed of the formation of metal oxide nanoparticles during combustion of aluminum and titanium drops which moved in air at a velocity of up to 3 m/sec. The source of the burning particles was a pyrotechnic mixture which contained an oxidizer, a binder, and metal particles of size 4–350 μm. Transmission electron microscopic studies showed that the combustion products were 1–10 μm aggregates of fractal structure consisting of primary particles (spherules) of Al₂O₃/TiO₂ 5–150 nm in diameter. The Brownian diffusion of the aggregates and their motion in electric and gravitational fields were observed using videomicroscopic recording. The charge distribution of TiO₂ aggregates and the equivalent radius of Brownian mobility were determined. In Al combustion, the zone of nanoparticle formation is separated from the particle surface by a distance approximately equal to the particle radius, and in Ti combustion, this zone is located directly near the surface. Coagulation of the oxide aerosol in the track of a burning particle leads to aerogelation with the formation of huge aggregates. Analytical expressions for approximate calculation of the parameters of the oxide particles and zones of their formation are proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 33–47, November–December, 2006.     

Detonation Velocity of Emulsion Explosives Containing Cenospheres

The detonation velocity of an emulsion explosive containing hollow alumosilicate microspheres (cenospheres) as the sensitizer is measured. The size of the microspheres is 50–250 μm. The relations between the detonation velocity and the charge density and diameter are compared for emulsion explosives containing cenospheres or glass microballoons as the sensitizer. It is shown that for a 55 mm diameter charge, the maximum detonation velocity of the composition with cenospheres of size 70–100 μm is 5.5–5.6 km/sec, as well as for 3M glass microballoons. The critical diameter for the emulsion explosive with cenosphere is 1.5–2 times larger than that for the emulsion explosive with glass microballoons and is 35–40 mm. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 119–127, September–October, 2005.     

Stable and critical heat- and mass-transfer regimes of a traveling carbon particle

We consider the effect of the relative velocity of a carbon particle on the specific features of the time dependences of the temperature and dimater of the particle at given temperatures of air and the wall of a reaction device and at a specified oxygen concentration in the medium. The stable and critical values of the temperature and combustion rate of the particle versus the initial particle diameter at its given velocity and the relative velocity of a particle of given size are studied. The effect of the diameter and velocity of the particle on the critical values of its initial temperature, which determine ignition inside hysteresis loops, is analyzed. The dependences of the critical particle diameter at which the thermal regime changes (ignition and extinction) on the relative particle velocity are derived. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 25–30, January–February, 1998.     

Combustion of thermite systems with orthogonal orientation of the overload and combustion-velocity vectors

The mechanism of combustion of the Fe₂O₃+Cr₂O₃+Al+C system in a field of centrifugal forces with orthogonal orientation of the overload and combustion-velocity vectors is studied. It is shown that a centrifugal force has a pronounced effect on the combustion mechanism and velocity. It is found that the flow of gaseous combustion products strongly influences the dependence of the combustion velocity on the magnitude of overload. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 57–60, January–February, 1998     

Mechanism and laws of combustion of composite solid propellants with a coolant

The model of combustion of a composition consisting of a quasi-homogeneous composite propellant (matrix) and coolant particles is considered. The model is based on the leading role of exothermal decomposition of the matrix and on the cooling effect of the second component by virtue of transverse heat transfer between the components in the condensed and gas phases. Formulas for combustion characteristics (temperature, burning rate, its sensitivity to pressure, and initial temperature) are derived and analyzed. The calculated dependences of these characteristics on pressure, particle size, concentration, and thermal effects of decomposition of the components show that transitional regimes with a stronger dependence of the burning rate on pressure than that of the initial propellant are reached in a certain range of parameters. An algorithm is proposed, and a parametric identification of the model on the basis of experimental data is performed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 51–60, November–December, 2007.     

Phase composition and morphology of products of combustion of ferrosilicon in nitrogen

The results of a study of silicon nitride phase formation in combustion of ferrosilicon in gaseous nitrogen are reported. It was shown that formation of α-or β-modifications of silicon nitride is basically determined by the composition of the batch for self-propagating high-temperature synthesis. When ammonium chloride was added to the initial ferrosilicon, a combustion product with a high (up to 80%) α-Si₃N₄ content is formed, while dilution with the final product and magnesium fluoride results in predominant (more than 95%) formation of β-Si₃N₄. The particle size and shape are a function of the conditions of synthesis and are primarily determined by the temperature and the additives incorporated in the initial alloy. __________ Translated from Steklo i Keramika, No. 2, pp. 28–30, February, 2007.