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.     

Critical diameter and critical thickness of an emulsion explosive

For a fine emulsion explosive, the dependence of the detonation velocity D on the cylindrical-charge diameter d and plane-layer thickness Δ is determined. It is shown that the emulsion explosive considered has low values of the critical diameter (d _cr ≈ 5 mm) and thickness (Δ_cr ≈ 2 mm). According to curved front theory, the relation D(d) ≈ (2Δ) is valid and the ratio d _cr/Δ_cr is 2.4–2.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 121–127, May–June, 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.     

Electromotive force for solid-flame combustion of heterogeneous systems in loose and pressed states

By an example of condensed 3Zr + 2WO₃ and Al + Ni systems, it is demonstrated that the electromotive force of solid-flame combustion measured by probing in loose systems is significantly higher than that in the same systems in a pressed state. An explanation for this phenomenon is offered, based on the difference in electrical conductivity in different zones of the combustion wave in loose and pressed condensed systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 71–77, July–August, 2006.     

Hot-spot combustion of heterogeneous condensed mixtures. Thermal percolation

A model of combustion of heterogeneous condensed mixtures composed of reactive particles separated by an inert heat-conducting substance is considered. Propagation of the reaction in a one-dimensional periodic system of point reaction cells connected by inert thermal bridges is examined. The burning rate is determined as a function of the basic parameters of the system, and stability of the steady combustion mode is studied. It is shown that there exists a range of parameters in which the reaction propagates in an unstable manner. Combustion of the system in the instability domain is examined. It is shown that the reaction propagation loses its stability many times as the adiabatic temperature of the system decreases; in this case, the existing unsteady mode is replaced by another, more complicated mode, and the alteration of the regimes in the examined systems always proceeds as a period-doubling bifurcation. Beginning from a certain value of adiabatic temperature, the reaction-propagation process becomes stochastic. In the systems examined, there exists an ultimate adiabatic temperature, below which self-sustained propagation of the reaction in the system becomes impossible.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 41–54, January–February, 2005.     

Effects of equivalent diameter on vertical PMMA combustion in natural convection environment

A series of combustion experiments of polymethyl methacrylate (PMMA) plates with two shapes, three thicknesses, and different sizes was performed to investigate the effects of equivalent diameter on combustion characteristics of vertical PMMA in a natural convection environment. Two digital video cameras were used to record the burning process. The temporal variation of mass was recorded by an electronic balance. The influence of equivalent diameter on mass loss rate and flame height was then analyzed theoretically. Convection and radiation heat transfer were calculated and modeled in the theoretical analysis and compared to experiment. The result shows that the theoretical prediction of mass loss rate for each equivalent diameter is in good agreement with the experimental measurements. In addition, the relationship between flame height and heat release rate was analyzed using dimensionless analysis method. It was found that the flame height for vertical PMMA burning in a nature convection environment is dominated by heat release rate and equivalent diameter.     

流化床煤燃烧过程不同气氛下的气态氮释放特征

利用微型流化床反应装置,结合快速过程质谱仪,在850~940℃操作温度下,研究了三种不同粒度分布烟煤和无烟煤在热解、气化和燃烧反应条件下四种主要气态氮产物HCN、NH₃、NO和NO₂的释放规律。结果表明,微型流化床可以实时检测挥发分氮和焦炭氮的动态释放序和类型,热解、气化和燃烧反应气氛的改变主要影响HCN和NH₃的释放量。热解产物的气态氮主要是来自于挥发分,燃烧反应的HCN和NH₃的释放量与温度有明显关系,而气化反应的各类气态氮释放量随温度变化波动不大。煤颗粒尺寸和温度变化对烟煤和无烟煤中各类气态氮释放量产生影响比较复杂,其中NH₃的释放特性是区分挥发分N释放和半焦N释放的重要特征。     

Autoignition Parameters and Detonation Critical Diameter of Hexanitromannite

The dependence of the ignition delay of hexanitromannite on the initial temperature was determined experimentally. The detonation critical diameter of cast hexanitromannite and its solutions in nitroglycol and nitroglycerine were measured.     

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.     

Video-chromatic measurements of temperature fields during combustion of low-temperature solid propellants

A method for measuring temperature fields in luminous zones of burning nonmetal condensed substances (sooty flames) was implemented in the form of an automated information-measuring system which provides computer processing of digital video images with separation of the video signal into monochromatic components. The system was used to study the combustion of a low-temperature solid-propellant composition with a coolant. The statistical parameters of the temperature fields were estimated by stochastic modeling of video-recorded information, providing a better insight into the hotspot-pulsating combustion mechanism of such compositions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 66–76, May–June, 2009.     

Properties of the surface layer and combustion behavior of metallized solid propellants

The properties of the surface layer of burning metallized solid propellants are analyzed on the basis of available experimental data. A general physical concept of processes in the surface layer is developed. The structure called a skeleton layer is shown to play a key role in combustion. The factors influencing the properties of the skeleton layer are determined, and the effect of these of properties on the properties of condensed combustion products and the propellant burning-rate law is studied. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 156–165, July–August, 2009.     

Energy aspects of initiation of domestic gases

Calculated and experimental data are presented on the most important parameters of combustion and detonation of ethane, propane, and butane in mixtures with oxygen and air, diluted by inert gases, with variations in the initial pressure. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 96–101, January–February, 2008.     

炉膛内燃烧振荡的实验

对锅炉炉膛内燃烧振荡现象进行了实验研究.结果表明,炉膛内烟气脉动强度与风机气流脉动、燃料空气比等外界扰动有关,且成比例增大;气流脉动频率主要集中在10 Hz以下,属低频振荡.实验证明当锅炉实测雷诺数值大于某临界值时即会发生燃烧振荡,由此提出了表征燃烧稳定特性的准则(临界雷诺数Recr),并给出了临界雷诺数的表达式.增大临界雷诺数可提高燃烧稳定性,从而消除或削弱燃烧振荡现象发生,该结论可供今后锅炉或炉窑燃烧室设计时参考.     

Solution combustion synthesis of metal nanopowders: Nickel—Reaction pathways

Nanopowders of pure nickel were directly synthesized for the first time by conventional solution combustion synthesis (SCS) method. In this article, a specific reaction pathway is suggested to describe the metallic phase formation during SCS. It is proposed that the exothermic reaction between NH₃ and HNO₃ species formed during the decomposition of glycine and nickel nitrate acts as the source of energy required to achieve the self‐sustained reaction regime. A thermodynamic analysis of the combustion synthesis reaction indicates that increasing glycine concentration leads to establishing a hydrogen rich reducing environment in the combustion wave that in turn results in the formation of pure metals and metal alloys. TGA of reaction systems and XRD analysis of products in the quenched combustion wave show that the formation of oxide phases occurs in the reaction front, followed by gradual reduction of oxide to pure metallic phases in the postcombustion zone. A methodology for SCS of pure metals and metal alloys nanoparticles can be inferred from the results presented. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Reduction of agglomeration effect by aluminum trihydride in solid propellant combustion

Aluminum trihydride (AlH₃) has gained considerable attention as a substitute fuel for aluminum in solid propellants. In this study, we conducted a systematic investigation to evaluate the effects of AlH₃ content, ranging from 0 % to 18 %, on propellant ignition, combustion, and agglomeration. Experimental methods such as thermogravimetry−differential scanning calorimetry (TG-DSC), laser ignition, high-speed photography, and collecting condensed combustion products (CCPs) were employed. The results indicate that AlH₃ significantly promotes the high-temperature decomposition of ammonium perchlorate (AP). Meanwhile, the addition of cyclotetramethylene tetranitramine (HMX) in propellant does not affect the hydrogen release reaction of AlH₃. As the AlH₃ content increases from 0 % to 18 %, the spectral emission intensity of the propellants decreases, and the ignition delay time initially increases from 253 ms to 321 ms, and then decreases to 258 ms. Furthermore, the burning rate increases with increasing the AlH₃ content, while the pressure exponent is reduced from 0.551 to 0.460. The inclusion of AlH₃ in propellants significantly inhibits aluminum agglomeration near the burning surface. Additionally, as the AlH₃ content increases, the mean particle size D₄₃ of the CCPs decreases from 50.95 μm to 8.28 μm at 1 MPa. The agglomeration degree of aluminum is very weak at 7 MPa, especially when the AlH₃ content exceeds 9 %. The conclusions drawn from this study can serve as valuable guidance for optimizing propellant formulations.     

High-porosity TiAl foam by volume combustion synthesis

Preparation of high-porous TiAl foam by volume combustion synthesis (VCS) was investigated. It is shown that the optimum conditions for pore formation are attained when gas evolution coincides in time with melting of the reactants.