Convective Regime of Filtration Combustion of Energetic Materials in a Cocurrent Flow of Their Combustion Products

The convective regime of filtration combustion of energetic materials in a cocurrent flow of their combustion products is studied using a model with extremely simplified kinetics and heat transfer, which shows instability of the process. It is shown that the more accurate twotemperature model describes a steadystate regime. In this regime, the gas temperature on the hot boundary of the heating zone is well below the combustion temperature, and the solidphase temperature is well below the temperature proposed in recent studies on this topic. It is pointed out that the twotemperature approach is unjustified and intragranular nonisothermicity must be taken into account for convective regimes. It is shown that the threetemperature model, which takes into account this effect, does not give a stable steadystate solution.     

Effect of Kinetic Properties of a Mixture on Wave Macrocharacteristics of Filtration Combustion of Gases

On the basis of the latest experimental data using a detailed kinetic model, the influence of kinetic features of ultrarich methaneair mixtures (in comparison with ultralean ones) on the main characteristics of superadiabatic waves of filtration combustion of gases is considered. It is shown that substantially lower concentrations of O, OH, and H radicals are typical of ultrarich mixtures, which results from effective inhibition of atomic hydrogen participating in the chain branching reaction H + O₂ = OH + O by methane in the reaction H + CH₄ = CH₃ + H₂. Therefore, extension of the preheating zone and noticeable expansion of the heatrelease region are typical of rich compositions. A decrease in generation of the main radicals in ultrarich mixtures (as compared to ultralean compositions) leads to an increase in the maximum skeleton temperature by 300– 350 K and to a significant increase in velocity of wavefront propagation.     

Mechanism of Upper Temperature Limits in a Wave of Filtration Combustion of Gases

A physical mechanism is established, responsible for the experimentally observed strong deceleration of the growth rate of the maximum skeleton temperature in a wave of filtration combustion of gases with increasing flow rate. The maximum temperatures of the gas and skeleton become commensurable, and the length of the thermalrelaxation zone becomes much shorter. A classification of regimes based on the temperatureheterogeneity criterion ₁ is proposed. Explicit analytical solutions are obtained for the wave for ₁1 and ₁1. A correction to reverse reactions in combustion products is considered. The effect of composition on wave behavior is studied by means of numerical calculations with a detailed kinetic scheme. The activation energy for ultrarich and ultralean methane–air mixtures is evaluated. It is concluded that the limiting efficiency of the heatrecuperation cycle in the wave is reached as ₁1; methods for maximizing the efficiency are suggested.     

Microheterogeneous Mechanism of Gasless Combustion

A model for gasless combustion is considered, based on the assumption that the medium consists of reaction cells whose exchange with heat proceeds much more slowly than heat transfer in each cell. For various reaction rates and kinetic laws, temperature dependences of the burning rate are calculated and compared with available experimental data. Thermal and concentration structures of the combustion wave are revealed. Existence domains and stability boundaries of microheterogeneous and quasihomogeneous modes of gasless combustion are established, depending on the preexponent and activation energy of the chemical reaction. The effect of abrupt acceleration of the reaction at a critical point (for instance, at a phasetransition point) on the combustion pattern is analyzed.     

Combustion of Composite Metal Systems

Characteristic features of combustion of Ni–Al, Co–Al, and Pd–Al composite metal systems forming intermetallic compounds are studied. The effect of the inertgas pressure, initial temperature, diameter, and initial porosity of compacts on the rates, maximum temperatures, and phase composition of the combustion products are studied. It is inferred that selfpropagating hightemperature synthesis of composite powders is more adaptable to manufacture and effective than that of elementary powder mixtures.     

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.     

Aluminum Combustion in Nitrogen

The process of nitration of aluminum and aluminumcontaining mixtures in the regime of selfpropagating hightemperature synthesis with a high pressure (up to 300 MPa) of the reacting gas (nitrogen) is considered. The dependences of ignition temperatures and also burning temperatures and burning rates of these initial mixtures on test conditions (nitrogen pressure and composition of the initial mixture) are studied. The dependence of the burning rate of initial mixtures on factors affecting spreading of the liquid component (melt containing aluminum and nitrogen) over the surface of the second component (aluminum nitride or titanium diboride), such as the equilibrium wetting angle, interaction at the interface, and melting of the second component, is studied. The microstructure and some properties of materials obtained are examined. Based on these studies, the combustion mechanism is determined, a mechanism of phase formation in combustion of these mixtures is suggested, and the structure of the combustion wave is determined.     

Characteristics of Microscale Combustion in a Narrow Heated Channel

The characteristics of microscale combustion were investigated by using a microchannel heated by an external source. The inner diameter of the channel was 2 mm, which was slightly smaller than the quenching distance of the stoichiometric methane–air mixture under normal conditions. The effects of the equivalence ratio and the averaged flow velocity on the characteristics of combustion in the microchannel were examined. At a channelwall temperature of 1000°C, flames could be stabilized at equivalence ratios of 0.05–1.9 and mixture velocities up to 150 cm/sec in a Ushaped quartzglass channel. At moderate equivalence ratios and lower velocity conditions within the flammability region, oscillatory combustion was observed. A simple analytical model predicting flame oscillations on the basis of the linear analysis of steady solutions is proposed.     

Impact of Structural Factors on Unsteady Combustion Modes of Gasless Systems

Based on the twotemperature, twovelocity timedependent model of gasless combustion, taking into account structural transformations related to the force action of the gas filtering in the pores and vitrification and volume variation of the condensed phase during the chemical transformation, selfoscillatory combustion modes are studied. Structural transformations are shown to have a pronounced effect on the propagation pattern of combustion waves and can either stabilize or destabilize combustion. The major structural parameters appreciably affecting combustionwave stability are the initial porosity, particle size, and pressure.     

Condensed Combustion Products of Aluminized Propellants. III. Effect of an Inert Gaseous Combustion Environment

The effect of gaseous combustion environment on particle size distribution and chemical compositions of condensed combustion products of a model propellant containing ammonium perchlorate, binder, and 23.4% aluminum was studied. Experiments were conducted at pressures of 0.6, 4.0, and 7.5MPa. Oxide particles with sizes of 1.2–60 m and agglomerates with sizes from 60 m to maximum were investigated. In experiments with nitrogen and helium, the difference in the mean sizes of the sampled agglomerates does not exceed the experimental error. The difference in the amount of unreacted (metallic) aluminum in the agglomerates sampled in nitrogen and helium is also negligible. Replacement of nitrogen by helium affects the size distribution of the oxide particles by increasing the mass fraction of particles with sizes of 1.2–10 m, and this effect is enhanced with pressure.     

Impact charging of particulate materials

When a particle impacts on a wall, electrostatic charges may be generated. This is called contact/impact/frictional electrification. One question to be answered is how and what process dominate the amount of the charge. In this respect, the separating process, rather than the contacting one, is important. It is shown that the potential difference between the surfaces can increase so rapidly in the separation process that it can cause gas discharge. Once such a gas discharge takes place, the charge on each surface can relax on the path: the amount of charge after the separation is a kind of residual charge after the charge relaxation. This process has been modeled and the model is called the “charge relaxation model.” It can estimate the amount of the impact charge without any empirical parameters, although the gas discharge itself is only an assumption so far which has not been directly observed or confirmed experimentally. In the experimental aspect, an “impact charging experiment” with single particles was carried out to provide a basis for the fundamental discussions. Polymer as well as metal particles about 3 mm in diameter ones were initially used, and the results showed good agreement with the “charge relaxation model.” The sensitivity of the instrument was enhanced to allow measurements with micrometer particles. These results are also reviewed here.     

Kinetic Analysis of the Chemical Structure of Waves of Filtration Combustion of Gases in Fuel-Rich Compositions

The structure of heat release and the dynamics of formation of radicals and methane oxidation in a wave of filtration combustion of gases in fuel-rich methane–air compositions is studied with the use of skeleton diagrams and sensitivity analysis. Depending on heat release, the wave is divided into a preheating zone, an exothermic zone characterized by partial oxidation of methane in the reaction CH₄ + 0.5O₂ = CO + 2H₂, and an endothermic zone with the conversion processes CO + H₂O = CO₂ + H₂ and CH₄ + H₂O = CO + 3H₂. It is shown that the composition of products in the wave front is essentially nonequilibrium. Several typical regions are also identified from the viewpoint of prevailing reactions of formation of the basic radicals in the wave. Thus, the dominating mechanism of chain branching is the reaction CH₃ + O₂ = CH₃O + O in the low-temperature region, H₂O₂(+M) = 2OH(+M) and HO₂ + CH₃ = CH₃O + OH in the transitional region, and H + O₂ = O + OH in the high-temperature region. Two former regions correspond to the preheating zone and the latter region corresponds to the exothermic peak of the wave of filtration combustion of gases.     

Preparation and microbial fuel cell application of sponge-structured hierarchical polyaniline-texture bioanode with an integration of electricity generation and energy storage

The choice of anode materials and structure has an important influence on the performance of microbial fuel cells (MFCs). In this paper, a flexible and compressible bioanode with the features of integration of electricity generation and energy storage in MFCs was reported. With sponge skeleton as the substrate, this bioanode has been coated with carbon nanotubes and graphene, and then aniline has been polymerized on it. Compared with a carbon nanotube-sponge electrode, the charge transfer impedance of the polyaniline composite bioanode decreases from 26.6 to 4.67 ohm, and the maximum power density increases from 259.7 to 571.5 mW m^?2; meanwhile, with the charge–discharge time of 60–60 min, the stored charge increases by 4.7 times, and the steady current density increases by 6.2 times. These results can be ascribed to a synergistic effect of some factors including the great specific surface area and distinct macroporous architecture of the sponge substrate, the effect of two carbon nanomaterials on decreasing the bioanode resistance, the energy storage characteristic of polyaniline, and the good biocompatibility of coating materials. The MFC with the PANI/rGO/CNTs/S capacitive bioanode has a very good ability to synchronously produce electricity and store energy and can release the stored charge in short bursts, which is expected to meet the demand of electric equipment.

Graphical abstract

Schematic illustrations of the fabrication process of PANI/rGO/CNTs/S electrode.

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Mathematical Simulation of Three-Dimensional Spin Regimes of Gasless Combustion

A three-dimensional mathematical model is constructed for the gasless combustion of a solid circular cylindrical specimen. The steady-stated spin regimes obtained were studied by numerical methods. The structure and mechanism of spin combustion are illustrated and discussed. It is shown how the space–time pattern of spin-wave propagation is complicated as the radius of the cylinder increases. Spin propagation of the front can proceed in a regime in which the structure of the front does not change (for small radii of the sample) or in an unsteady regime, in which the structure of the front undergoes numerous changes over a period. In the second case, a synchronous or alternate flicker of the sites is observed on the surface of the cylinder. The nonuniqueness of the combustion regimes is detected. It is shown that the average velocity of propagation of the spin-combustion front is of the order of the velocity of steady front propagation under adiabatic conditions.     

Combustion of Ultrafine Aluminum in Air

The paper reports results of studying the combustion of ultrafine aluminum (surface average diameter of particles is 0.1 m) in a sealed bomb at an initial air pressure of 1 atm. The combustion proceeds in two stages, similarly to combustion in air. It is shown that during the twostage combustion of ultrafine aluminum powder in the bomb, the mass concentration of chemically bound nitrogen in the final products increases by 20% in terms of aluminum nitride. An increase in nitrogen content in confined combustion validates the previously proposed mechanism of binding air nitrogen with participation of the gas phase during aluminum combustion.     

Nonequilibrium Thermodynamics of Autowaves of Laminar Combustion

Within the framework of the Zel'dovich–FrankKamenetskii theory of thermal propagation of flame, thermodynamic properties of an open nonlinear system are considered, and nonequilibrium entropy of a steady combustion wave is constructed. The nonequilibrium dynamic system is analyzed qualitatively and quantitatively, and the distribution functions of local entropy production in terms of the spatial variable are constructed. It is shown that total entropy production in the system is a functional on integral curves that possess extreme properties, and its minimum corresponds to a unique physically sustainable solution of the problem. The procedure of cutting (vanishing) of the reaction rate is justified by methods of nonequilibrium thermodynamics. A variational formulation of the problem is presented for calculation of a steady combustion wave.     

Unsteady Combustion of Layered Condensed Systems. Parallel Burning of Components

A model of unsteady combustion of a sandwich:type layered condensed system consisting of parallel layers of simultaneously burning components capable of selfsustaining combustion is considered. The response function of the mass burning rate of the system to periodic pressure oscillations with allowance for the interaction between the components caused by the difference between their burning rates is found. In the linear approximation, combustion of such systems under a rapid change in pressure is analyzed. It is shown that the pattern and duration of these processes depend on the ratio of the layer thickness of the slowly burning component and the thickness of its heated layer. It is found that bending of the burning surface of components in the course of unsteady combustion of the layered condensed system can substantially affect the pattern of its unsteady combustion as a whole, in particular, enhance or reduce its stability.