Dynamic combustion regimes of the Ti-(Ti+0.5C) layered system in a concurrent nitrogen flow

The filtration combustion of a layered porous fill consisting of alternating layers of a mixture of Ti + 0.5C a titanium powder with forced concurrent filtration of nitrogen was studied for the first time. The gas flow through the sample was provided by a vacuum pump attached to the lower end of the fill. The presence of the concurrent gas flow radically changes the character of propagation of the combustion front and the structure and composition of the products obtained. The layers consisting of carbonitride and titanium nitride make a single unit. The experiments provided scientific bases for the production of new laminated and composite ceramic materials by dynamic filtration combustion. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 44–51, November–December, 2008.     

Experimental investigation of combustion of a gasless pelletized mixture of Ti + 0.5C in argon and nitrogen coflows

Combustion of a pelletized mixture of titanium and carbon black placed in a quartz tube and exposed to a flow of argon or nitrogen is studied. The gas flow (cocurrent filtration) is provided by a fixed pressure gradient at the inlet and outlet of the tube, which did not exceed 1 atm. The possible modes of combustion of pelletized mixtures related to the presence of a more complex hierarchy of scales (micro, macro, and meso) compared to that of powder mixtures (micro, macro) are analyzed. A comparison is made of the burning rates of powder and pelletized mixtures. An increase in the burning rate when using pelletized mixtures was found experimentally. It is shown that the gas coflow through the pelletized mixture of Ti + 0.5C leads to an increase in the burning rate. It is established that the propagation of the flame front of the pelletized mixture of Ti + 0.5C in flows of nitrogen and argon is controlled by different reactions. In contrast to combustion of powder mixtures of Ti + 0.5C, in combustion of pelletized mixtures of Ti + 0.5C in a nitrogen flow, only one front is observed. It is proved that radiation plays a significant role in the propagation of the combustion front in the pelletized mixture of Ti + 0.5C.     

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.     

Combustion of the gasless system Ti + 0.5C in a nitrogen coflow

Combustion of mixtures of titanium with carbon black of bulk density placed in a quartz tube with nitrogen purging (concurrent filtration) is examined. The nitrogen flow is provided by a fixed pressure difference not exceeding 1 atm at the ends of the mixture. The burning rate is determined as a function of the amount of titanium nitride added to the initial mixture and of the pressure at the ends of the sample. The tests show that a nitration front is formed during combustion of a Ti + 0.5C (carbon black) powder mixture in the nitrogen flow, in addition to a carbidization front. Various modes of propagation of the carbidization and nitration fronts are found and described. A classification of combustion modes of the Ti + 0.5C (carbon black) powder mixture in the nitrogen flow is proposed.     

Combustion behavior of a Ti + TiC mixture in a nitrogen coflow

Combustion of powder and granular mixtures of TiC + Ti in a quartz tube purged with nitrogen was studied. Mixtures based on fine and coarse-grained TiC were used. It is found that purging of a powder mixture of bulk density with fine titanium carbide with nitrogen coflow does not lead to the spread of the flame front, whereas granular mixtures burn at the same pressure difference. Mixtures based on coarse-grained titanium carbide powder burn in both powder and granular form. The burning rate of a granular mixture of TiC + Ti with coarse-grained titanium carbide is significantly higher than when using fine titanium carbide. It is shown that in the case of a coarse-grained TiC, granulation of the mixture of TiC + Ti significantly improves the degree of nitriding of the synthesis products compared to the powder mixture. During combustion of granular mixtures of TiC + Ti, in contrast to powder mixtures of the same composition, a singlephase product of approximate composition TiC_0.5N_0.44 is formed as a result of synthesis.     

Combustion of titanium plates

The lower combustion limit is determined depending on the pressure of plates made of OT-4 titanium alloy in an O₂-Ar medium. The values of this limit decrease monotonically as the concentration of oxygen in O₂-Ar mixtures grows and increase with the sample thickness. The velocity of combustion propagation over the surface of the samples in a 75% O₂+25% Ar mixture at a pressure of ≦3.1 MPa is found. It is shown that, for titanium, the dependence of the combustion rate on the sample thickness is much weaker and, in contrast, it is much stronger on the pressure compared to similar dependences for cylindrical soft-steel samples known from the literature. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 60–65, July–August, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 96-03-32039a).     

Combustion of Cr2O3 + Al powder mixtures in a coflow of inert gas: 5. Effect of green density

Combustion of Cr₂O₃ + Al powder mixtures in a coflow of inert gas (Ar) was investigated upon variation in green density in the presence/absence of blowing agents (borax, baking soda). The results were rationalized in terms of the convection-conduction theory for combustion in heterogeneous condensed systems. For Part 4, see Int. J. SHS, 2008, vol. 17, no. 3, pp. 199–205.     

Filtration combustion of porous metallic samples in a gas diluted by an inert component

Filtration combustion of a porous layer with natural filtration of the oxidizer diluted by an inert component is studied. The problem of ignition of a porous layer by a heated surface permeable for the gas is solved, and the time of ignition is determined as a function of pressure in the reactor and initial porosity of the sample. Formation of an inert gas “plug” preventing chemical interaction is examined. It is shown that the mean depth of conversion of the condensed reagent in the porous layer depends on the sample length, porosity, pressure of the gas mixture, and concentration of the inert component. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 73–79, January–February, 2008.     

Low-temperature combustion of energetic materials in filled polymer systems

In systems filled by inert additives, combustion of cellulose nitrate (CN) proceeds in a flameless low-temperature regime with a low linear burning rate. At a standard temperature, the exponent in the low of CN combustion in ballasted mixtures with inert additives in the pressure range of 0.1 to 10 MPa is several times lower than that of pure cellulose nitrate and amounts to 0.23. The qualitative and quantitative composition of gaseous products of flameless CN conversion is found. It is noted that this composition approximately corresponds to data available in the literature for the products of thermal decomposition of cellulose nitrate at comparatively low temperatures. Based on this fact and on a weak dependence of the CN burning rate on pressure in ballasted systems, the process under these conditions is assumed to be controlled by conversion of the energetic component predominantly in the condensed phase. In the case of a composite consisting of cellulose nitrate, silicon carbide, and polymer binder, for samples 10–25 mm in diameter, armoring exerts practically no effect on combustion parameters. Combustion of the same mixture with smaller diameters of non-armored samples is unstable. The presence of a liner establishes a clearly expressed critical combustion diameter in the examined systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 98–102, May–June, 2007.     

Reduced kinetic mechanism for combustion of synthesis gas at elevated temperatures and pressures

The key combustion reactions of synthesis gas at elevated initial temperatures (T ₀ = 500–700 K) and pressures (p = 10–30 atm) are identified by analyzing the kinetic mechanism. A reduced mechanism of the oxidation reactions of synthesis gas consisting of 14 elementary reactions involving 13 species is proposed which adequately describes the results of experimental data on the burning velocities of mixtures of synthesis gas with oxygen and inert diluents at T ₀ = 300–700 K, p = 10–30 atm, and ratios CO/H₂ = 0.05–0.95, and satisfactorily predicts the flame structure and the dependence of the flammability limits on the initial temperature at atmospheric pressure.     

Combustion of bulk density powder mixtures in a coflow of inert gas: 6*(Fe2O3 + 2Al) + 30% Al2O3 mixtures

Investigated was the combustion of bulk density (Fe₂O₃ + 2Al) + 30% Al₂O₃ mixtures in a coflow of inert gas (Ar) in the presence/absence of blowing agents (borax, baking soda) and applied pressure difference δP. The results have been rationalized in terms of the conduction-convection theory for combustion in heterogeneous systems.     

Effect of the volume content of particles on the parameters of a gaseous suspension behind shock waves

The equilibrium state of mixtures of gases with solid, incompressible, inert particles behind oblique shocks is analyzed theoretically, taking the finite volume of the dispersed phase into account. Equations are obtained for the limiting (maximum) angles of displacement of the flow of equilibrium mixtures behind strong and weak oblique shocks. The effect of the volume content of particles in the mixture on the limiting degree of compression of the gas when a plane shock is reflected from a rigid wall is studied. Translated fromFizika Goreniya i Vzryva Vol. 35, No. 3, pp. 74–80, May–June 1999.     

Special characteristics of the filtration combustion of the system Ti?C?N in an audio-frequency sound field

The influence of sound fields on the filtration combustion of mixtures of titanium and carbon in nitrogen on the kinetics of self-propagating high-temperature synthesis and on the chemical and phase composition of the synthesis products is investigated. It is shown that filtration hindrances in a wide layer of the sample vanish if the charge material has an initial relative density of 0.2. The carbonitride component is formed most completely when the atomic fraction of carbon in the initial charge has values of 0.3 and 0.5. The optimum acoustic frequency stimulating the filtration of nitrogen deep within the briquette and yielding the most complete nitriding process is the resonance frequency of the acoustic system. Fast-framing photography exhibits the transient nature of the filtration combustion of titanium—carbon mixtures in nitrogen. Institute of Steel and Alloys, Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 5, pp. 44–51, September–October, 1995.     

Nitride formation during combustion of Ti-TiO2 and Ti-Al powder mixtures in air under SHS conditions

Nitride formation during SHS combustion of a micron-size titanium powder and its mixtures with additives in air was studied. It was shown that the yield of TiN Ti powder was higher for SHS combustion in air than for SHS combustion of powders of the same degree of dispersion in nitrogen. The mechanism of formation of TiN is probably determined by the reaction of the intermediate product TiO with atmospheric nitrogen. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 131–135, September–October, 2008.     

Stabilized gas combustion wave in an inert porous medium

A stabilized gas combustion wave in an inert porous medium with intense internal interphase heat exchange (at low velocities) is considered to elucidate the mechanism underlying the increase in the burning velocity of the homogeneous gas mixture due to the porous medium. It is shown that the major factor of the increase in the burning velocity is conductive heat recuperation from the postflame zone to the region of the fresh mixture through the solid skeleton of the porous medium. Analytical dependences of the degree of increase in the burning velocity of the mixture in the stabilized wave on the determining parameters are obtained. The possibilities and restrictions of the use of the results to analyze the operation of porous burners are discussed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 8–15, September–October, 2008.     

Modeling of solid-state combustion in thermally and chemically heterogeneous media

A thermally and chemically heterogeneous medium is modeled by a set of cylinders pressed from a mixture of solid reagents with coaxially aligned cylindrical rods made from an inert material. The change in velocity of combustion waves propagating over one cylinder is studied by numerical methods under the assumption that there is no heat release from the cylinder surface. The mean velocity of the combustion front in the specimen is shown first to decrease and then to increase with increasing thermal conductivity of the inert rod. Spinning waves are obtained in the range of low velocities of the combustion front. The laws of variations of the maximum temperature in the combustion front are determined. It is shown that the inert rod may serve as a heat sink from the hot charge mixture, may be manifested as dilution by an inert component, enhance heat recuperation, and increase the combustion-wave velocity. Introduction of the inert rod may either destabilize combustion-wave propagation in the region of a stable plane front or stabilize the combustion wave in the range of parameters where the plane front is unstable. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 6, pp. 21–30, November–December, 2007.     

Combustion of mixtures of ultrafine powders of aluminum and boron in air

Results of investigation of the combustion of mixtures of ultrafine aluminum and boron powders (the oxidizer is air) are presented. It is shown that the combustion proceeds in two steps, which differ in temperature. The addition of boron influences the concentrations of AlN, residual Al, and α-Al₂O₃ in the end products of combustion of mixtures of ultrafine powders of Al and B in air. For a fixed sample weight of 4 g, the maximum AlN content is observed in the combustion of an Al+20% B mixture of ultrafine powders, and the combustion temperature is also maximum in this case. When the sample weight is smaller than a certain critical value, the combustion proceeds in one step. Increasing the sample weight of the starting mixture of ultrafine powders of Al and B leads to an increase in the AlN content in the combustion products with simultaneous rise in the combustion temperature. A considerable part of the combustion products stabilizes as acicular polycrystals of micron and submicron sizes formed with participation of a gas phase during combustion. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 61–64, November–December 1999.     

Dependence of the normal rate of combustion of methane-air and hydrogen-air mixtures on pressure and composition

Numerical methods were used to study the normal combustion rate of methane-air and hydrogen-air mixtures as a function of pressure in the range 0.005–2.0 MPa and composition. It was found that the closer the composition of the fuel mixture is to the limiting composition, the lower the pressure index of the normal combustion rate. It is shown that chemical reactions in the preflame zone have no appreciable effect on the normal combustion rate of a lean methane-air mixture. Balashikha. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 5, pp. 12–17, September–October, 1993.     

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.     

Operation of a gas generator controlled by supplying a gaseous oxidizer

The possibility of arranging a gas generator with the heat-release process being controlled by supplying a gaseous oxidizer is experimentally checked. Gaseous hydrogen, liquid gasoline, and solid hexamethylene tetramine (solidified alcohol) is used as a fuel. The gas generator with a proposed configuration is demonstrated to ensure stable operation during combustion of various fuels; the pressure in the gas-generator combustion chamber does not exceed the pressure of oxidizer supply and clearly correlates with variations of the oxidizer mass flow. Quasi-steady calculations allow determining all parameters of the process, including those that are not measured in the experiment. In particular, the temperature of combustion products is found to be 600–1900 K, and the gas generator forms a high-temperature mixture containing a non-reacted fuel (the air-to-fuel ratio is α = 0.55–2.30). __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 18–25, November–December, 2008.