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.     

Multi-spot modes of unsteady gasless combustion of a thick-walled cylinder under adiabatic conditions

Propagation of multi-spot spinning waves in a cylindrical specimen with a coaxial inner channel is studied by numerical methods under the assumption of the absence of heat removal from the outer surface of the specimen and from the channel surface. Variations of the spinning wave characteristics (longitudinal and tangential velocities, “pitch,” and maximum temperature and period) are demonstrated by an example of two-spot spinning waves for specimens of different sizes with a fixed channel radius, for specimens of a particular size with different channel radii, and for specimens with a constant wall thickness and varied inner and outer radii. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 3–14, January–February, 2007.     

Propagation of a one-spot spinning wave in a thick-walled cylinder under adiabatic conditions

Propagation of a one-spot spinning wave in a cylindrical specimen with a coaxial internal channel is studied by numerical methods under the assumption of the absence of heat removal from the outer surface of the specimen and from the channel surface. Variations of the spinning wave characteristics [maximum temperature, longitudinal and circumferential (tangential) velocities, “pitch,” and time of rotation of the hot spot around the specimen centerline] are tracked for a specimen of a particular size with a varied channel radius, for specimens of different sizes with a fixed channel radius, and for specimens with a constant wall thickness. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 41–49, July–August, 2006.     

External diffusion limitations in initiator decomposition in heterogeneous media

The process of initiator decomposition in media, where it has a limited solubility will be influenced by external diffusional limitations. A theoretical framework has been built up to analyse such a process. An experimental investigation of AIBN decomposition has been undertaken and it is shown how falsified kinetics may result in the presence of diffusional effects. A method for extracting intrinsic kinetic information for both single bond and multibond initiators has been demonstrated.     

Modeling of controlled combustion of high-energetic materials on structured catalysts

A mathematical model of combustion of high-energetic materials on structured catalysts is developed. Numerical studies are performed for combustion of a typical material: aminoguanidine nitrate. An increase in catalytic activity, as well as an increase in the thermal conductivity of the catalyst, is found to expand the range of real-time controlling of the burning rate of the condensed substance. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 45–53, September–October, 2008.     

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.     

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.     

Mathematical simulation of mass transfer processes under heterogeneous reactions in polydisperse porous media

Submitted is a theoretical study of mass transfer processes in polydisperse porous media in the presence of chemical reactions. Kinetic regime of methane pyrolysis in a porous carbon skeleton considering external and internal diffusion resistances for different initial distributions of particles forming the porous medium is investigated. Derived is a general analytical expression describing the influence of the inner reaction surface variation on the degree of the pore filling for an arbitrary initial particle size distribution. Expressions defining the time of pores filling by pyrocarbon based on approximate and exact solutions of the equation for the probability density function (PDF) of particle size distribution are received. Dependence of pore filling time on effective diffusion coefficient and initial particle size distribution using both solutions for PDF-equation is compared. It is shown, that the dominant factor influencing the pore filling time is the dispersion of particle size distribution.     

Spatial forms of a combustion wave in energetic heterogeneous systems

The effect of the content of combustible particles on the spatial forms of the combustion wave in a gasless thermite-inert material model system is studied. It is shown that the lower flammability concentration limit for this mixture is related to the formation of a three-dimensional binding percolation cluster or to a percolation phase transition. Near the lower concentration limit, the combustion of model mixtures enters a smoldering regime in which separate combustion sources move randomly in the volume of the unburned material. If the initial composition contains more than 47% thermite particles, a motionless heated surface forms, along which separate incandescent combustion sources move. The motion of this surface in the unburned material occurs only locally as a result of motion of the combustion sources on it. The formation of this unified heated surface corresponds to the occurrence of a second percolation phase transition. When the volumetric content of thermite particles in the initial mixture is higher than 70%, separate combustion sources merge into a unified surface of layer-by-layer combustion due to the occurrence of a third percolation phase transition. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 60–65, July–August, 2008.     

A method to calculate the multiphase flow properties of heterogeneous porous media by using network simulations

A method is suggested to compute the capillary pressure and relative permeability curves of heterogeneous porous media. The broad pore radius distribution (PRD) and throat radius distribution (TRD) are decomposed into relatively narrow component distribution functions which are used for the computer‐aided construction of pore‐and‐throat networks. The quasi‐static motion of menisci in pores and throats is tracked by accounting for capillary forces. The presence of fractal roughness along pore walls ensures the coexistence of both phases in pores. The calculation of the hydraulic conductance of each phase is based on the concept of constricted unit cell. Simulations in component pore networks constructed from narrow PRD and TRD produce a set of capillary pressure and relative permeability functions, the arithmetic averaging of which yields the corresponding functions for a heterogeneous synthetic pore network. This information is used by a dynamic simulator of drainage in permeability networks to predict experimental results of soil columns. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

洗涤塔内相变脱除燃煤超细颗粒的试验研究

为研究洗涤塔内相变对燃煤超细颗粒的脱除,在洗涤塔液相进口上方注入蒸汽使烟气达到过饱和,由高效除雾器脱除凝并长大的含尘液滴。系统研究了蒸汽添加量、洗涤塔入口气液温差、液气体积比、烟气在相变区域的停留时间等操作参数对燃煤超细颗粒脱除效率的影响规律。结果表明:在塔内添加少量蒸汽,可显著促进燃煤超细颗粒的脱除,蒸汽添加质量浓度为0.03 kg/m3时,数量浓度脱除效率由10%增至60%以上;液气体积比的提高有利于燃煤超细颗粒的脱除,特别是当洗涤塔入口气液温差较大时。研究结果表明利用洗涤塔内相变脱除燃煤细颗粒是一种有应用前景的新方法。     

Effect of protonation media on chemically and electrochemically synthesized polyaniline

Polyaniline films have been prepared both chemically and electrochemically using formic (‘Pani‐Formic’), boric (‘Pani‐Boric’) or acetic acid (‘Pani‐Acetic’) as protonation media. Among the three, formic acid seems to be a better protonating medium because it selectively yields the conducting phase of polyaniline. The variation of potential with the amount of oxidizing agent suggests one dimensional growth of polymer chain, which proceeds through a PG/EM mechanism as indicated by spectral data. The Pani‐Boric and Pani‐Acetic acid demonstrate three‐ and four‐step decomposition patterns, respectively, with complete decomposition at about 760 °C. Pani‐Formic acid, in contrast, reveals a three‐step decomposition pattern with 90% weight loss at about 900 °C and shows a tendency to react with the Pt sample holder. The films of polyaniline generated electrochemically also suggest formic acid to be a better protonating medium for achieving a relatively greater fraction of conducting emeraldine salt. However, boric acid produces excellent quality adherent films with very high deposition rate. Morphological features give strong support to this observation. The films of Pani‐Formic and Pani‐Acetic acids show a spongy network and globular morphology, respectively, while Pani‐Boric acid yields an extremely uniform surface coating. Furthermore, cyclic voltammetry reveals different patterns depending upon the formation of different oxidation states and the results are in accordance with optical absorption studies. © 2000 Society of Chemical Industry     

Reflection of a shock wave in a dusty cloud

The problem of shock-wave passage along a cloud of particles adjacent to a solid surface is studied numerically and analytically. The wave pattern of the flow near the shock wave reflected from this surface is analyzed within the framework of the equilibrium approximation of mechanics of heterogeneous media. The conditions of the transition from regular to irregular reflection from the substrate of the refracted shock wave inside the cloud are obtained analytically. The results of numerical simulations of a nonequilibrium flow in the two-velocity two-temperature approximation are compared with data obtained in the equilibrium approximation. Nonequilibrium and equilibrium flows are found to become more similar as the particle size decreases. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 1, pp. 121–131, January–February, 2007.     

Characteristics of RDX combustion zones at different pressures and initial temperatures

The RDX burning rates and temperature profiles in the combustion wave are measured with the pressure and initial temperature varied within wide limits. Parameters of combustion waves are found. The gas phase of RDX is demonstrated to have a single-zone structure. Two regimes of RDX combustion are found: a basic regime at p > 0.1 MPa and a special regime at p ≤ 0.1 MPa. The main characteristics of these regimes are obtained, and the wave regions responsible for the burning rate are determined. Both regimes are characterized by wide reaction zones in the gas phase. The processes of vaporization and decomposition of the condensed phase proceed simultaneously on the burning surface. The fraction of thermal expansion in the reaction layer of this phase is estimated. Laws of RDX gasiffication (pyrolysis) in the combustion wave are found, which turn out to be the same as those for HMX. Distributions of the heat-release rate in the combustion wave (in the reaction layer of the condensed phase and in the gas phase) are obtained. Sensitivity of the burning rate to the initial temperature is measured. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 5, pp. 32–45, September–October, 2006.     

Experimental investigation of porous media combustion in a planar micro-combustor

The micro-combustor (emitter) is a key component of the micro-thermophotovoltaic (TPV) system. In order to improve the system efficiency, higher wall temperature and uniform distribution along the combustor wall is desirable. Porous media combustion of premixed H₂-air in a planar micro-combustor with the channel width of 1 mm is experimentally studied. The wall temperature is measured by using an infrared thermometer under the flow conditions of Ф = 0.6-1.0 and U₀ = 2-3 m/s. The effects of flow conditions and position of the porous media on both the wall temperature distribution and the emitter efficiency are investigated. The experimental results indicate that the wall temperature increases with increasing mixture velocity, and higher emitter efficiency is achieved for mixtures with Ф ≈ 0.8. In addition, the flames are found to be effectively anchored by the inserted porous media, despite the change of flow conditions. The emitter efficiency was noted to be significantly influenced by the position of the porous media, for which the mixture preheating (by the combustor wall) is believed to be a main reason.     

Combustion of pressed specimens of heterogeneous systems subjected to a constant axial mechanical load

The growth dynamics of the height of burning pressed specimens with the specimen ends subjected to a compressive force of constant magnitude is studied using the heterogeneous Ti+C+20% TiC system as an example. It is found that in the examined range of compressive forces 0.1–2 kg/cm², the growth of the height of the burning specimens with time obeys a linear law; under a compressive force of ≈0.1 kg/cm², the specimen height increases by ≈100%, and under a force of ≈2 kg/cm², it increases by about 25%. The method used to measure the growth dynamics of the specimen height during combustion proved a useful tool for the experimental determination of both the burning time of the specimens and the delay in the transfer of combustion through an obstacle. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 103–109, May–June, 2007.     

Experimental investigation of agglomeration during combustion of aluminized solid propellants in an acceleration field

Agglomeration during combustion of solid rocket propellants in an acceleration field up to 60 was studied experimental. In the experiments, the acceleration direction and magnitude were varied. The amount, chemical composition, and particle size of the agglomerates and the structure of the surface layer were determined as functions of the acceleration magnitude and direction. The most significant feature of the effect of tensile accelerations is an increase in the agglomerate mass to a certain limiting value, and that of compressive accelerations is the formation of a layer of condensed products on the burning-propellant surface. The data obtained provide a deeper understanding of the general physical picture of agglomerate formation and aluminized-propellant combustion and highlight the range of engine operation problems under flight conditions due to the presence of condensed combustion products. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 38–46, January–February, 2009.     

Parametric investigations of premixed methane-air combustion in two-section porous media by numerical simulation

Motivated by detailed designs of industrial porous burners published in patents, the combustion of methane-air mixtures in a two-section porous burner has been studied numerically. The software FLUENT is used to solve a two-dimensional transient mathematical model of the combustion. In order to reveal the reality of the combustion in porous media, the user defined function (UDF) is used to extend the ability of FLUENT and enable two-dimensional distributions of temperature and velocity to be obtained. Some operating or property parameters, which mainly affect the functions and quality of the industrial burner design, such as the inlet velocity of the reactants, the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, have been investigated. The results show that the contours of temperature and velocity change considerably at the interface of the porous media and near the wall, the gas temperature at the low inlet velocity limit is higher than that for the high velocity limit, the thermal conductivity in the upstream section has more influence on the temperature than that in the downstream section and finally, the temperature profiles of both the gas and the porous skeleton vary considerably with changes of the radiative extinction coefficient of the large-pore porous media.     

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.