Deformations inside burning specimens

The dynamics of particle motion inside burning specimens producing solid combustion products was studied by flash radiography. In the experiments, we used specimens of aTi+C+20% TiC mixture, inside which marks in the form of strips of a ofZr+WO ₃ mixture or a tantalum powder were placed. The specimens were burnt in a semiclosed rigid casing with exhaust of impurity gases through slags. It is established that, just behind the combustion front, particles of combustion products begin to move in the direction opposite to the direction of motion of the combustion front. In the central zone of specimens, the particle velocity reaches values comparable with the combustion velocity of the specimens (∼20 mm/sec), whereas, on the periphery, the particle velocity is close to zero. Institute of Experimental Physics, Sarov 607190. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 4, pp. 78–83, July–August, 1997.     

Combustion of thermite systems in thin layers with an open surface

SHS combustion of thermite compositions in the form of thin layers without formation of a continuous layer of cast products is studied. It is shown that combustion of thin layers of high-temperature compositions can be accompanied by almost complete dispersion of liquid-phase combustion products. The height of filling material and the caloric content of the mixture are effective parameters to control the size of the dispersed particles of cast synthesis products. It is established that the macrostructure of synthesis products is formed in the vicinity of the combustion wave through coalescence and capillary drift of metal droplets.     

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.     

Preparation and Properties of Boron‐Based Nano‐B/NiO Thermite

Boron particles have several major burning problems, such as incomplete combustion, poor ignitability, and a complex burning process in solid propellants. It is documented that the low ignitability and combustion efficiency of boron are caused by the oxidation of its surface. In order to improve the combustion efficiency of boron particles, a precipitation method was employed to prepare nanometer‐sized NiO and coat it on boron particles. The morphology and coating results of the B/NiO nanocomposite thermite were characterized using different approaches such as SEM, X‐ray Diffraction (XRD), and EDS. The results indicated that the boron particles were well distributed and coated completely by nanocomposite NiO. The B/NiO nanocomposite thermite reaction process was tested by TG‐DTA. The results showed that the reaction temperature of B/NiO particles is about 30 °C lower than that of boron particles. The B/NiO thermite and boron powder were added to Mg/PTFE propellant to be measured for their respective combustion performance. The results showed that the burning rate of the B/NiO‐Mg/PTFE propellant increased by 22.8–25.2 %, mass burning rate by 26.7–30.8 %, and combustion temperature increased by 8–56 °C compared to the B‐Mg/PTFE propellant. The above results indicate that NiO coating of boron particles has a significant effect on the combustion behavior and increases the combustion performance of the propellant compared with uncoated particles.     

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.     

Effect of heat release conditions on the phase composition of the combustion products of a Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al/C thermite mixture

An X-ray method was used to study the phase composition of the combustion products of a Fe₂O₃/TiO₂/Al/C thermite composite mixture, in particular, during combustion of samples under cooling by water. The data of the experiments are compared with the results of thermodynamic calculations. A probable explanation of the reasons of the angular displacement of the line of iron in the X-ray spectrum of the combustion products and a mechanism for the crystallization of an eutectic TiC + α-Fe mixture from the melt of the Ti/Fe/C ternary system. The composition of the combustion products was found to be different from the equilibrium one, which is manifested in the presence of defects in the carbon sublattice of titanium carbide. A probable explanation of the deficiency of carbon is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 39–43, July–August, 2008.     

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.     

Reaction of a Particle Suspension in a Rapidly‐Heated Oxidizing Gas

The reaction of a suspension of solid particles in a rapidly‐heated oxidizing gas is relevant to metalized explosives and propellants, as well as to combustion of solid fuel‐particle suspensions in premixed‐gaseous‐fuel clouds encountered in accidents within the mining and process industries. A simplified model is considered, using a constant‐volume approximation, which assumes that non‐volatile particles react heterogeneously via a one‐step surface reaction. The resulting unified particle reaction rate includes both kinetic and diffusive reaction resistances. It is shown that the onset of the chemical reaction in a rapidly heated particulate suspension may occur by two different physical mechanisms. The first mechanism, realized in a dilute suspension of particles, is defined by the ignition of a single particle, i.e., by the critical phenomenon associated with the rapid transition from a kinetically‐ to diffusively‐limited reaction regime. The second mechanism dominates the reaction onset in a dense particulate suspension and occurs in a similar manner to the reaction onset in a rapidly‐heated homogeneous gas mixture, where the highly‐activated reaction occurs in an explosion‐like manner after some time delay and preheating. Unlike the single particle ignition phenomenon, the second mechanism lacks criticality and is not limited to particles above a certain size. The interplay between these two reaction‐onset mechanisms leads to a nontrivial dependence of the total reaction time on the particle size and solid‐fuel concentration within the suspension.     

Experimental study of different regimes of low-gas compound combustion in argon at atmospheric pressure

To study the possibilities of occurrence of a low-temperature steady-state combustion regime and to determine the existence regions of other regimes in more detail, we performed experiments on combustion of low-gas compounds in argon at atmospheric pressure. As a model system, we used a diluted ferro—zirconium thermite with a gas-forming admixture of molybdenum trioxide. The absence of a low-temperature steady-state combustion regime and a change in the existence regions of the other combustion regimes for low-gas systems compared with those observed for combustion in air were revealed. Institute for Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka 142432. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 1, pp. 69–72, January–February, 1997.     

Modelling of mass loss of char particles during combustion in flow of inert material

The inspiration for this research was an attempt at the numerical simulation of the process of mass loss of char particles during its combustion in the upper zone of a CFB boiler furnace. In the first part of the research an experiment was carried out, during which singular char particles were burned in a flow of heated sand. During the course of this test manually polished spherical particles chosen from arbitrary real coal particles were used whose diameter amounted to 10 mm. The mass loss of burning particles was recorded with a tensometric branch scale. The measuring equipment applied enabled the observation of the mass loss both as a result of combustion and erosion resulting from the contact of inert material accelerated particles with the upper surface of the motionless particle being tested. The results of the experiment were compared with the results of numerical simulation, where for the first time the mass loss of a coal particle was presented as the superposition of two separate processes i.e. combustion and surface erosion. The values obtained indicated a good level of conformity which may illustrate the possibility of applying this model in real conditions.     

Guniting linings of oxygen converters in the course of melting without stopping the blow

Problems arising in guniting converter linings in the course of melting without stopping the oxygen blow are considered. Deposition of a mixture based on magnesite is conducted in an oxygen jet. As the powder particles move in the vessel, they are heated due to the heat of the converter and afterburning of carbon monoxide, which ensures their strong cohesion with the surface of the lining. The velocity and diameter of the gas-powder jet are calculated as a function of the concentration of the periclase powder, the distance from the tuyere nozzle, and the temperature of the magnesite particles heated in the vessel. An experimental installation is developed and experiments on guniting in the course of melting without interrupting the blow are conducted. It is established that the gunited coating is deposited on the lining only at the end of melting. The endurance of the coating is 1–4 heats. In guniting, the temperature of the metal increases by 20–70°C. The experiments showed the possibility in principle of guniting a converter lining in the course of melting without interrupting the blow. Translated from Ogneupory, No. 3, pp. 27–31, March, 1996.     

Percolation Phase Transition in Combustion of Heterogeneous Mixtures

Combustion of heterogeneous mixtures with a stepwise dependence of the reaction rate on temperature is considered. A twodimensional model of the process is proposed, which takes into account, in addition to other factors, the random distribution of fuel particles in the mixture and nonisothermality of the latter. The flammability limits are compared by methods of numerical simulation for two types of heterogeneous systems with an identical mean density of the fuel: with a uniform distribution of the fuel for all particles of the mixture and with its distribution based on random sampling of particles. It is shown that, as the degree of heterogeneity increases (dimensionless coefficient of heat transfer between the particles, Bi, decreases), the flammability limit for systems of both types becomes significantly higher than its thermodynamic value, and the upper boundary of the limit is twice as high. Differences in flammability limits and burning rates for heterogeneous systems of these types are found. A relationship between the flammability limit of a random system and a percolation phase transition, which occurs in a heterogeneous condensed mixture with scarce inclusions of the fuel, is demonstrated. An analytical approach for evaluation of the threshold concentration of the fuel in such systems, based on the problem of percolation on random nodes, is proposed.     

Experimental and theoretical investigation on unburned coal char burnout in a pilot-scale rotary kiln

Oxidation reactivity studies are imperative for improving carbon re-burn technologies and valuing the heat content of unburned carbon within coal combustion ashes. Non-isothermal, thermal gravimetric analysis (TGA) was used to examine the oxidation kinetics of unburned carbon in coal combustion fly ashes having different particle size distributions; TGA results were related to combustion efficiencies as measured in a bench-scale rotary kiln. The activation energy and pre-exponential factor were determined for the chemically-controlled reaction regime; the transition temperatures between chemically-controlled and partially diffusion-controlled combustion regimes were obtained for unburned carbon particles of different sizes. After the oxidation reaction rates were evaluated, the residence time distribution (RTD) of fly ashes in the rotary kiln were experimentally measured and the mean residence times related to process parameters, including the rotating velocity and kiln inclination. By comparing these results with an advective-dispersive model, the axial dispersion coefficient of fly ashes was determined. The reaction rates obtained by thermal analyses and the RTDs were used to predict combustion efficiencies within the kiln and oxidation conditions of unburned carbon using various processing options.     

The effect of the structure of the starting mixtures on the combustion of gas-generating compositions based on sodium azide

It is established that the combustion of mixtures of sodium azide with aluminum oxide and potassium dichromate at the lower limit for the dichromate content occurs in a chaotic self-oscillatory regime. Modification of the structure of the starting mixture by increasing the dichromate particle sizes from 40–60 K to 250–500 μm lead to significant stabilization of its combustion parameters. The amount impurities and aerosols in generated nitrogen is considerably decreased by reducing the energy content of sodium azide based composition. Using the relay-race model of combustion of heterogeneous systems, it is shown that, under the given conditions, the burning rate of such compositions is determined by the time of heating of potassium dichromate particles to the ignition temperature by neighboring burning particles. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 72–76, November–December, 2008.     

Effect of a heat-conducting element on the gasless combustion of cylindrical samples under nonadiabatic conditions

The effect of a heat-conducting element on the ignition and combustion of a cylindrical layer of a gasless mixture with a nonadiabatic lateral surface is studied. It is shown that the introduction of an element with high heat conductivity allows one to extend the region of ignition of the gasless composition by a heated surface and to increase burning rate under conditions of external heat release. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 66–71, January–February, 2007.     

Effect of the phase transition on three-dimensional unstable regimes of gasless combustion

The thermal effect of the phase transition on unstable regimes of gasless combustion is numerically studied within the framework of the model of solid flame combustion of a cylindrical sample. The closer the phase transition temperature to the burning temperature, the more pronounced the effect of the phase transition on the combustion character. In this case, the combustion front surface is noticeably smoothed owing to reduction of temperature gradient values. A relationship between the change in the combustion modes considered in the study and the change in the phase transition parameters is found. An unsteady periodic symmetric mode of combustion with ring-shaped trajectories of motion of combustion sites over the side surface is obtained.     

Numerical studies of pulverized coal combustion in a tubular coal combustor with slanted oxygen jet

A pure two-fluid model for turbulent reacting gas-particle flow of coal particles is developed using a unified Eulerian treatment of both the gas and particle phases. The particles' history caused by mass transfer due to moisture evaporation, devolatilization and char reaction is described. Both velocity and temperature of the coal particles and the gas phase are predicted by solving the momentum and energy equations of the gas and particle phases, respectively. A k-ε-k_k two-phase turbulence model, EBU-Arrhenius turbulent combustion model and four-flux radiation heat transfer model are incorporated into a comprehensive model. The above comprehensive mathematical model is used to simulate two-dimensional gas-particle flows and pulverized coal combustion in a newly designed tubular oxygen-coal combustor of blast furnace. Predicted results of isothermal gas-particle flows are in good agreement with those obtained by measurements. The results also show that the proposed tubular oxygen-coal combustor prolongs the coal particle residence time and enhances the mixing of coal and oxygen. Results indicate that smaller coal particles of 10 μm diameter are heated and devolatilized rapidly and have volatile combustion in the combustor, while larger coal particles of 40 and 70 μm in diameter are heated but not devolatilized, and combustion of such particles does not occur in the tubular combustor.     

Premixed filtration combustion of micron and sub-micron particles in inert porous media: A theoretical analysis

An analytical model for one-dimensional premixed filtration combustion of volatile fuel particles-air mixture is presented. It is presumed that fuel particles first vaporize and a gaseous fuel with definite chemical structure is formed, which is subsequently oxidized in the gas phase. Flame structure is considered in the three zones. In the preheating vaporization zone, the mixture is heated until it reaches ignition temperature. In the reaction zone, the combustible mixture burns and the post flame zone is occupied by the combustion products. The temperature and mass fraction profiles are obtained of gaseous fuel in these three zones at a semi-infinite inert porous media. Thereafter, the effects of various parameters such as gas velocity, porosity, fuel particles diameter, number density of fuel particles, and heat of chemical reaction on the temperature and mass fraction profiles are investigated.     

Flame propagation in mixtures of monogermane and oxygen. II. Peculiarities of flame propagation and the structure of combustion waves

Different flame propagation regimes, including a two-wave regime, are studied in a closed pipe with single ignition of GeH₄−O₂ mixtures. It is shown that, depending on the initial conditions and, in particular, on the composition of the initial mixtures, spatially separated chemical reaction waves for oxidation and decomposition of monogermane are observed in certain parts of the reaction vessel which lead to the formation of a two-layer deposit of GeO/Ge. The temporal sequence of the separated combustion wave and wave from which layers of solid products are deposited, as well as the location of the deposition zones in the reaction vessel, are determined by two interacting chain processes—the oxidation and decomposition of monogermane. The thermal relaxation kinetics of the reactive mixture after passage of the combustion waves is determined by the rate of conductive heat transfer from heated solid particles (reaction products) to the gaseous phase. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 1, pp. 77–84, January–February 1999.     

Macroscopic and microscopic aspects of product formation in the Fe2O3-TiO2-Al disperse system burning in the SHS regime

The macromorphology and micromorphology of the combustion products of a Fe₂O₃-TiO₂-Al powder mixture burning in the regime of self-propagating high-temperature synthesis were studied. Significant differences were found in the structure of the products with variation in the ratio of the mixture components. A mechanism for shape and phase formation was proposed that allows the observed differences to be explained from unified positions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 79–88, May–June, 2006.