Thermal interaction of sodium azide with aluminum and silicon oxides in a combustion regime

Thermodynamic analysis of the sodium azide-aluminum oxide system shows that aluminum in such mixtures can be reduced by a chain of successive conjugate reactions. It is shown experimentally that in the presence of small amounts of silicon dioxide, aluminum oxide reacts exothermically with sodium azide in a combustion regime to release nitrogen and form the corresponding nitrides and alumosilicates. In the three-component system containing sodium azide, aluminum oxide, and silicon oxide, the upper flammability limit (for the sodium azide content) under normal conditions is established. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 40–44, January–February, 2007.     

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.     

The evolution of 100-µm aluminum agglomerates and initially continuous aluminum particles in the flame of a model solid propellant. II. Results

A quenching/sampling method was used to study the evolution of monodisperse 100-μm aluminum agglomerates and continuous particles in the flame of combustion products — composite propellant at a pressure of 0.7–8 MPa. The particle residence time in the flame was varied in the range 6–170 msec, whereas the calculated time of their combustion was ≈25 msec. The evolution of a burning particle is related to the consumption of metallic aluminum and the deposition of oxide in the form of a cap, which, after complete combustion of aluminum, is transformed to the final spherical oxide particle. The density of the final oxide particles was measured. The ratio of the diameters and masses of the initial metal particle and the final oxide particles was determined. Data on particle fragmentation during combustion were obtained by comparing the numbers of the initial metal particles and the final oxide particles. Considerable differences in the combustion behavior of the agglomerates and continuous particles of size 100 μm were not found. It was established that the smaller the size of the burning particle the less oxide is deposited on the particle and the more oxide is carried away in the form of oxide smoke. For 100-μm particles, the fraction of deposited oxide was found to be ≈0.1 of the total mass of oxide formed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 61–71, November–December, 2008.     

Condensed combustion products of aluminized propellants. IV. Effect of the nature of nitramines on aluminum agglomeration and combustion efficiency

The condensed combustion products of two model propellants consisting of ammonium perchlorate, aluminum, nitramine, and an energetic binder were studied by a sampling method. One of the propellants contained HMX with a particle size D ₁₀ ≈ 490 μm, and the other RDX with a particle size _{D 10} ≈ 380 μm. The particle-size distribution and the content of metallic aluminum in particles of condensed combustion products with a particle size of 1.2 μm to the maximum particle size in the pressure range of 0.1–6.5 MPa were determined with variation in the particle quenching distance from the burning surface to 100 mm. For agglomerates, dependences of the incompleteness of aluminum combustion on the residence time in the propellant flame were obtained. The RDX-based propellant is characterized by more severe agglomeration than the HMX-based propellant — the agglomerate size and mass are larger and the aluminum burnout proceeds more slowly. The ratio of the mass of the oxide accumulated on the agglomerates to the total mass of the oxide formed is determined. The agglomerate size is shown to be the main physical factor that governs the accumulation of the oxide on the burning agglomerate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 78–92, July–August, 2006.     

Effect of flameout on Combustion of small particles in an acoustically oscillating flow

Combustion of magnesium and titanium particles in an acoustically oscillating flow is experimentally studied for the case where the particle dimensions are smaller than the amplitude of gas displacement in an acoustic wave. An increase in the combustion time of magnesium particles and a decrease in the combustion time of titanium particles upon application of acoustic vibrations is found. Characteristic features of fluctuations of the intensity of the luminous flux of a burning magnesium particle as a response to vapor-phase burning of a metal drop to an external acoustic action are revealed. An explanation of the shape of registered fluctuations is offered on the basis of an assumption about flame-front blowoff from the frontal point of the drop. Conditions necessary for flameout in an oscillating flow and effects that can be induced by flameout from the drop in the case of burning of sprayed fuels in intermittent burning devices are discussed. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 10–16, March–April, 2000.     

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.     

Problems in studying formation of smoke oxide particles in combustion of aluminized solid propellants

The present paper deals with properties of smoke oxide particles formed near the surface of burning aluminized solid propellants. The global physical picture of formation of this product is formulated. Based on this picture, it is demonstrated that smoke oxide particles are formed in two processes: combustion of non-agglomerating metal in the gas-phase layer above the surface and combustion of agglomerating metal in the surface layer of the propellant. Combustion mainly proceeds in the heterogeneous and gas-phase modes in these two cases, respectively. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 45–53, January–February, 2007. An erratum to this article is available at .     

Effect of the addition of ultrafine aluminum powders on the rheological properties and burning rate of energetic condensed systems

The effect of small additives (1.25–5.00%) of ultrafine aluminum powders (UFAP) on the rheology and combustion of model four-component energetic condensed systems is studied. It is found that the addition of UFAP decreases the temperature of HMX decomposition. Small additives of UFAP increase the burning rate of model energetic condensed systems and decrease the exponent ν in the burning rate law without deteriorating the rheological characteristics of the model propellants. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 54–59, January–February, 2007.     

Correlating Aluminum Burning Times

Characteristics of aluminum combustion are summarized in an overview of the subject, focusing on the burning time of individual particles. Combustion data from over ten different sources with almost 400 datum points have been cataloged and correlated. Available models have also been used to evaluate combustion trends with key environmental parameters. The fundamental concepts that control aluminum combustion are discussed, starting from a discussion of the D ⁿ law. The exponent in the D ⁿ law is shown to be lower than two, with nominal values of ≈1.5 to 1.8 being typical. The effect of the ambient medium on the burning time is considered, oxygen as an oxidizer being twice as effective as water and about five times more effective than carbon dioxide. The effect of pressure and initial temperature is minor. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 55–69, September–October, 2005.     

Critical diameter and transverse waves of powder combustion

A critical review is given of the problem of powder extinction with a decrease in the sample diameter due to heat release. The results of our experimental studies of the critical combustion diameter are presented. A comparison of the experimental data on the critical diameter as a function of burning rate is shown to be the most informative. These functions follow a power law with an exponent of −1.15 to −1.17. The relations between the sizes of the cells (hot spots) formed by the set of transverse waves on the burning surface and the burning rate follow the same law. The sizes of the cells are 2.1–2.3 times smaller than the critical combustion diameter. It is also found that the transverse wave decays if its curvature exceeds a critical value. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 72–78, March–April, 2007.     

Sodium molybdate—a possible alternate additive for sodium dichromate in the electrolytic production of sodium chlorate

Sodium dichromate is commonly used in sodium chlorate production to maintain high current efficiency; however, it is also a well documented carcinogen. To reduce the environmental impact, identification of a suitable alternative with similar buffering characteristics to dichromate and without adverse effect on the electrolytic performance of sodium chlorate production is important; sodium molybdate is a good candidate. Molybdate ion and its conjugated acid work as a buffer pair at pH 5–6, a lower and slightly narrower pH window than the typical buffer region of dichromate. Nonetheless, the molybdate buffer works effectively during the electrolytic process by maintaining pH at 5.9. Although the use of molybdate buffer will lower the overpotential of hydrogen evolution reaction (HER) by 100 mV, the average off-gas oxygen content is noticeably compromised at 3.6–4.6%, measured using a pilot cell operated at 3 kA m⁻²and 80 °C during a 3-day trial. The resulting current efficiency of 91 92% is significantly lower than when dichromate is employed as the process additive (> 96%). Mixtures of different dichromate and molybdate ratio were also investigated in terms of the resulting cathode surface potential.     

Problems and prospects of investigating the formation and evolution of agglomerates by the sampling method

Problems related to the interpretation of information obtained from an analysis of collected particles of condensed combustion products of aluminized propellants are considered. It is shown that the difficulties that arise are due to three main factors: the complex statistical character of the combustion of heterogeneous propellants, which results in formation of agglomerates with a substantially polydisperse distribution in size and different (even for identical size) structure, the features of burnout of the agglomerates related to the accumulation of oxide on the burning particle, and the specific character of the motion of burning agglomerates in the gas flow. An experimental approach is proposed that makes it possible to eliminate the polydispersity of the agglomerates and uncertainty in the parameters of the gaseous products flowing away from the burning surface. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 1, pp. 161–172, January-February, 2000. This work was supported by INTAS (Grant No. 93-2560 ext).     

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.     

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.     

Effect of constant and pulsed-periodic electric fields on combustion of a propane-air mixture

The effect of constant and pulsed periodic electric fields on combustion of a propane-air mixture is studied experimentally. The experiments are performed for the laminar and turbulent combustion modes with flame stabilization by a separation region behind a central body. The constant and pulsed-periodic electric fields are found to exert different effects on the burning rate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 22–25, January–February, 2008.     

Ignition and Combustion of Dust-Gas Suspensions

The ignition and combustion of dust-gas suspensions are considered. It is shown that the ability of these systems to accumulate heat is determined not only by their kinetic and thermal properties but also by the relation between their reaction surface and the heat-removal surface (f). Experimental information on flame temperatures, ignition delays, and flame propagation over gas suspensions is processed using the parameter f, and the postulate on the stimulating role of the developed reaction surface in activating these processes is validated. It is shown that during overall burning, diffusion combustion of particles occurs only for rather small values of f. The ambiguous effect of the parameter f on the ignition and combustion processes leads to the necessity of optimizing the fuel size distribution and concentration for the effective operation of the power devices. The role of the macroparameters of two-phase flames of refractory metals in the synthesis of combustion nanoproducts is analyzed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 3–14, November–December, 2005.     

Real-time control of solid-propellant combustion by a catalytic method

Principles of contact catalysis of combustion are described. The essence of this process implies that a block catalyst contacting the surface of the burning propellant specimen increases the burning rate and allows controlling combustion at a temperature of 400 to 600°C with formation of a large volume of versatile gaseous products, i.e., is the basis for creating low-temperatures gas generators. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 86–91, January–February, 2007.     

Effect of ultrafine aluminum on the combustion of composite solid propellants at subatmospheric pressures

This paper presents the results of an experimental study of the combustion of composite solid propellants with a double oxidizer (ammonium perchlorate/HMX) at pressures of 0.03–0.1 MPa. Systems containing a micron-sized aluminum powder (ASD-4) and the Alex ultrafine aluminum powder were investigated. It was shown that the replacement of ASD-4 by Alex in propellant systems led to an increase in the burning rate. The aluminum particle size and the oxidizer excess coefficient were found to affect the exponent in the power-law burning-rate dependence. The range of the oxidizer excess coefficient was determined that corresponded to the effective replacement of micron-sized aluminum by ultrafine aluminum for which the exponent in the power-law burning rate dependence decreases. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 47–55, January–February, 2009.