Showing papers in "Combustion, Explosion, and Shock Waves in 2015"

Combustion of Nano Aluminum Particles (Review)

Abstract: Nano aluminum particles have received considerable attention in the combustion community; their physicochemical properties are quite favorable as compared with those of their micron-sized counterparts. The present work provides a comprehensive review of recent advances in the field of combustion of nano aluminum particles. The effect of the Knudsen number on heat and mass transfer properties of particles is first examined. Deficiencies of the currently available continuum models for combustion of nano aluminum particles are highlighted. Key physicochemical processes of particle combustion are identified and their respective time scales are compared to determine the combustion mechanisms for different particle sizes and pressures. Experimental data from several sources are gathered to elucidate the effect of the particle size on the flame temperature of aluminum particles. The flame structure and the combustion modes of aluminum particles are examined for wide ranges of pressures, particle sizes, and oxidizers. Key mechanisms that dictate the combustion behaviors are discussed. Measured burning times of nano aluminum particles are surveyed. The effects of the pressure, temperature, particle size, and type and concentration of the oxidizer on the burning time are discussed. A new correlation for the burning time of nano aluminum particles is established. Major outstanding issues to be addressed in the future work are identified.

Current status of research of continuous detonation in fuel-air mixtures (Review)

Abstract: The current status of experimental research of continuous detonation of fuel-air mixtures in flow-type annular combustors is described. Experimental data for C2H2-air, H2-air, and CO/H2-air mixtures are analyzed, systematized, and generalized. Basic specific features of continuous spin detonation and the influence of geometric parameters of flow-type combustors are described. The effect of physical and chemical parameters on the domain of continuous detonation formation is analyzed. It is concluded that the fundamental knowledge of continuous detonation processes in hydrogen and hydrocarbon fuels gained up to now allows one to consider the possibility of using them in air-breathing detonation engines.

Numerical simulation of ignition of particles of a coal–water fuel

Abstract: The problem of particle ignition of coal–water fuel has been solved. The simultaneous processes of water vaporization and thermal decomposition of the solid fuel are taken into account. The conditions and characteristics of particle ignition of coal–water fuel under typical furnace conditions were determined by numerical simulation. The obtained values of the ignition delay time are in good agreement with published experimental data.

Thermal explosion in mechanically activated low-calorific-value compositions

Abstract: The effect of preliminary mechanical activation of low-calorific-value powdered formulations in a planetary ball mill on the main parameters of the subsequent thermal explosion has been studied. It has been found that in mechanically activated compositions, the initiation temperature of thermal explosion is reduced by hundreds of degrees. The maximum decrease (1300°C) is observed for the Ti + 4 wt.% C system. Regimes of preliminary mechanical activation of reaction mixtures and the subsequent thermal explosion conditions producing Ti3Al and Ni3Al single-phase intermetallic compounds with nanometer grain size were determined. For the 3Ni + Al composition, the energy accumulated during mechanical activation was evaluated. It is shown that the initiation temperature of thermal explosion in the MA compositions studied can be used to estimate the temperature that develops in the mill drums.

Experimental verification of discrete models for combustion of microheterogeneous compositions forming condensed combustion products (Review)

Abstract: This paper presents an analytical review of modern quasihomogeneous and discrete models of gasless combustion. Particular attention is given to experiments that make it possible to distinguish between homogeneous and microheterogeneous regimes of this process. It is shown that in the cases where different theoretical models predict different behavior of the combustion wave at the macroscopic or microscopic level, experiments provide data in support of discrete models. The development of these models allows for a fresh look at the problem of controlling the propagation parameters of gasless combustion waves and the development of reaction compositions with specified strictly reproducible characteristics.

Catalytic combustion: Achievements and problems

Abstract: A catalytic method of combustion of a solid fuel in a fluidized bed is compared with a noncatalytic method. It is shown that the use of catalysts reduces the fuel consumption and sizes of heat generators approximately by an order of magnitude, while the specific load on the reactor volume increases by more than a factor of 20. Emission of toxic substances with fuel combustion products drastically decreases. Comparative stability of oxide non-platinum catalysts is estimated in the course of catalytic burning of the fuel with addition of an inert material. In burning fuels with a large content of sulphur, the maximum deactivation is found to occur within the first several tens of hours; this process is accompanied by sulphur accumulation in catalysts. Later on, the catalyst activity remains almost unchanged. It is found that a critical factor of catalyst stability is attrition resistance. The prospects of fuel burning in a layer of cermet honeycomb catalysts are demonstrated.

Application of detailed and reduced kinetic schemes for the description of detonation of diluted hydrogen–air mixtures

Abstract: A possibility of using some kinetic models for the description of detonation of a gaseous hydrogen–air mixture is justified. A hierarchy of mathematical models from the simplest model of combustion under static conditions to the model of unsteady nonequilibrium gas dynamics is numerically constructed. Verification is performed on the basis of experimental data on the ignition delay time as a function of temperature and on the detonation wave velocity as a function of dilution of the mixture by argon or nitrogen. A mathematical technology for the description of cellular detonation propagation in channels of various engineering devices is developed on the basis of detailed and reduced kinetic mechanisms of nonequilibrium chemical transformations within the framework of the ANSYS Fluent commercial software system. It is demonstrated that the cell size in a mixture diluted by argon by 92% in a channel 30 mm wide is in good agreement with experimental data.

Mechanically activated oxidizer-fuel energetic composites

Abstract: Preparation of mechanically activated energetic composites (MAECs) based on solid fuels (Al, Mg, and Si) and oxidizers (S, MoO3, (-C2F4-) n , KClO4, NH4 ClO4, etc.) is considered. Compared to conventional mechanical mixtures, the burning rate of MAECs is significantly increased, and in some cases high-velocity detonation can be obtained. The propagation of the reaction in MAECs is accompanied by high energy release comparable to the heat of explosion of powerful aluminized explosive materials. The composites are highly sensitive to heat treatment and are capable of rapid transition from combustion to detonation. The results obtained in this work show that MAEC based formulations are promising energetic materials for a wide range of applications, from igniting and initiating compositions to components to small charges in microsystem devices.

Specific features of ignition and combustion of composite fuels containing aluminum nanoparticles (Review)

Abstract: Modern research of properties of aluminum nanoparticles and specific features of their combustion in various media is reviewed. Particular attention is paid to the mechanism of combustion of Al nanoparticles in air and water vapor. Results of experimental and theoretical investigations are presented, which testify to the validity of the gas-phase concept of combustion of Al nanoparticles. A large part of the paper deals with numerical studies of combustion of a composite fuel consisting of a hydrocarbon (methane) and Al nanoparticles in air. The kinetic mechanism of Al combustion in the gas phase is presented. The mechanisms of ignition of this fuel are analyzed. The methodology of simulating the formation of the condensed phase consisting of Al2O3 particles is discussed. Results of experimental investigations of ignition and combustion of liquid and vaporized hydrocarbon fuels containing a moderate amount of Al nanoparticles are presented.

Interaction of a shock wave with a closed cell aluminum metal foam

Abstract: The present investigation examines the interaction of shock waves with closed cell aluminum foam samples in a conventional shock tube The effect of the sample thickness on shock wave attenuation and/or enhancement and the use of the foam in the sandwich structure is studied Results in terms of incident and reflected shock pressures are obtained, and the effectiveness of the samples with and without the foam is compared It is demonstrated that the foam density and thickness, as well as the placement of cover plates of the same material in front of and behind the foam have the most significant effect on the reflected shock pressure It is concluded that the closed cell aluminum metal foam can be effectively used as a sacrificial layer in blast protection of structures

Energy efficiency of a continuous-detonation combustion chamber

Abstract: Systematic experimental and computational studies of the energy efficiency of continuous-detonation combustors (CDCs) have been performed. A small-size and a large-size CDCs using hydrogen as fuel and oxygen or air as oxidizer have been developed and tested. It was first experimentally proved that the Zel’dovich thermodynamic cycle with continuous-detonation combustion of a hydrogen-oxygen mixture in an annular combustor is more efficient than the Brayton thermodynamic cycle with continuous combustion of the mixture, other things being equal. The specific impulse of a small-size bench-scale rocket engine with a 50 mm diameter CDC operating in the continuous-detonation mode was 6–7% higher than that in the continuous combustion mode of operation. The measured fuel-based specific impulse for the large-size CDC of 406 mm diameter running on a hydrogen-air mixture was at a level of 3000 s. Three-dimensional calculations to optimize the structure and operation mode of the large-size CDC have shown that when running on a combustible mixture with a nearly stoichiometric overall composition, the specific impulse can be increased to ≈4200 s.

Aluminum/HMX nanocomposites: Synthesis, microstructure, and combustion

Abstract: Aluminum particles with a diameter of ≈50 nm were synthesized by means of the Gen-Miller flow-levitation method with alumina or trimethylsiloxane coatings formed on the surface of these particles. Aluminum/HMX nanocomposites manufactured by suspension atomization drying or dry mechanical mixing were investigated by x-ray diffraction analysis, scanning electron microscopy, and local x-ray analysis. The combustion of these mixtures with changing particle size of the components and composition of the coating on the metal particles was studied. It was found that, when the composites produced by atomization drying were stored as loose powder, HMX crystals grew, which increased the burning rate of compressed samples from 19 to 55 mm/s in the pressure range 3–10 MPa, and the pressure exponent varied from 0.34 to 0.84, depending on how the burning rate correlates with the pressure.

Numerical analysis of the effect of the geometric parameters of a combined shaped-charge liner on the mass and velocity of explosively formed compact elements

Abstract: The formation of high-velocity compact elements of shaped charges with a liner of a combined hemisphere-cylinder shape has been analyzed by numerical simulations of a two-dimensional axisymmetric problem of continuum mechanics. The influence of the radius and thickness of the hemispherical portion of the combined liner on the parameters of the compact elements has been studied. It is shown that in implosion of hemispherical liners whose thickness decreases from top to bottom, there is an increase in the velocity of the resulting jet flow. When using a hemispherical portion of variable thickness (to increase the velocity of the compact elements formed), it is necessary to solve two additional problems—to implement timely separation of the head part of the jet flow with increased velocity and increase its mass. For this, it is proposed to use the jet-forming part of the combined liner in the form of a truncated sphere or a slightly elongated ellipsoid of revolution. Combined shaped-charge liners in the form of a truncated ellipsoid of variable thickness were developed, allowing the formation of compact elements having a velocity of 7.5–10 km/s.

Gas-disperse synthesis of metal oxide particles

Abstract: The main results of years of research of metal dust flames aimed at the development of the scientific basis for the method of gas-disperse synthesis of metal oxide nanopowders are discussed. Methods of burning metal dust in oxide-containing media, the key problems of gas-disperse synthesis, and possible ways to solve these problems are considered. The ways of controlling the disperse composition of vapor-phase and gas-phase combustion products of metal particles by variation of the macroparameters of the dust flame and ionization of condensed and gaseous phases in the combustion zone with the help of adding easily ionized atoms to the fuel are analyzed. It is shown that an adequate description of condensation in a flame is impossible without consideration for the influence of electrophysical processes on nucleation and coagulation in the flame. It is established that ionization of the condensed phase is the most significant factor during coagulation of nano-oxide particles in a dust flame. This allows expecting that the influence on particle ionization may turn out to be an effective method of controlling the dispersion of the target products of gas-disperse synthesis.

Effect of the time of mechanical activation of a Ti + 2B mixture on combustion of cylindrical samples and thin foils

Abstract: The effect of the activation time of the Ti + 2B mixture on the burning rate of cylindrical samples and thin foils is studied. For cylindrical samples, combustion of samples activated in argon is compared with combustion of samples activated in air. The burning rates are almost identical in these two cases. It is demonstrated that the burning rate of cylindrical samples continuously increases with increasing activation time. The burning rate of thin foils remains almost unchanged as the activation time increases up to 4 min and then drastically increases and reaches a value twice greater than the burning rate of cylindrical samples. For the titanium and boron powders used in this study, the time needed to reach the maximum burning rate is 7 min in the case of activation in air and 5 min in the case of activation in argon; if the activation time is longer, then the product of combustion is formed. The features of combustion observed in this study can be explained from the viewpoint of convective–conductive model of combustion wave propagation.

Physicomathematical modeling of ignition and combustion of silane in transient and reflected shock waves

Abstract: A model of nonequilibrium gas dynamics is proposed to describe ignition and combustion of a mixture of silane, hydrogen, oxygen, and an inert gas (nitrogen or argon). The model is based on detailed chemical kinetics of nonequilibrium chemical reactions. The model adequately describes the behavior of experimental data on the ignition delay time for this mixture versus the temperature behind the reflected shock wave in accordance with three criteria of ignition. The detonation wave velocity and equilibrium parameters of the mixture (pressure and temperature) are calculated as functions of the fuel–oxidizer equivalence ratio. Based on the dependences of the ignition delay time on the temperature behind the reflected shock wave calculated by this model, an approximation formula for the silane–oxygen–nitrogen/argon is derived.

Deformation models under intense dynamic loading (Review)

Abstract: This paper considers currently available models of irreversible deformation processes of materials under dynamic, in particular shock-wave, loading. The models can be divided into three groups: (1) macroscopic (continuum) models—traditional models of continuum mechanics, primarily classical models of elastic-plastic deformation, their various generalizations to the case of dynamic processes and models of viscoelastic relaxation media; (2) microstructural models based on the description of microstructural mechanisms of irreversible deformation (usually, the concept of the kinetics of a dislocation ensemble); (3) atomistic molecular dynamics models and calculations. A special category includes the most promising (from the point of view of the author) multilevel models which combine the advantages of each of these approaches and consider deformation mechanisms of various levels. Examples of calculations using such models are presented.

Cellular structures of a multifront detonation wave and initiation (Review)

Abstract: The state of the art in the field of initiation of combustible mixtures is overviewed. Recommendations on using various formulas for estimating the critical energy (of both ignition of the mixture and initiation of detonation) are given. The importance of the spatial and temporal components in the energy release law is noted, especially for initiation optimization. Unsolved and new problems of the theory of combustion and detonation are formulated.

Air explosion characteristics of a novel TiH2/RDX composite explosive

Abstract: An explosive mixture of cyclotrimethylenetrinitramine (RDX) and titanium hydride (TiH2) is introduced. To investigate the explosion characteristics of the composite explosive, charges with various contents of the TiH2 powder are prepared and tested in air explosion experiments. Results show that the peak overpressure, positive duration, and positive specific impulse increase as the content of TiH2 increases from 10 to 20%, as compared to passivated RDX. The peak overpressure, duration, and specific impulse have the largest increase of 6, 9, and 23%, respectively, as compared to passivated RDX, when the TiH2 content is 20%. The effect of the TiH2 particle size is also considered. The charge containing the TiH2 powder with a mean particle size of 4.6 μm shows higher values of the three parameters than that containing 45-μm TiH2 particles under the condition of the same content of TiH2. However, the relationship between the detonation velocity and TiH2 content is a linear inverse proportion, and the particle size of TiH2 has a minor effect on it. Solid explosion products of the TiH2/RDX composite explosive are analyzed by x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDX). TiO2 is found in explosion products, which is believed to form due to TiH2 oxidation.

Filtration combustion of carbon in the presence of endothermic oxidizers

Abstract: This paper presents a model for the steady-state filtration combustion of carbon mixtures with a solid incombustible material in opposed gas flow containing an endothermic oxidizer such as steam and/or carbon dioxide. A computation scheme for calculating the characteristics of the process (combustion temperature and composition of the products) is proposed for the case where the product composition is determined by the thermodynamic equilibrium in the hightemperature zone. For stoichiometric regimes, finite analytical expressions are obtained that relate the combustion temperature and the composition of the products with the oxidizer gas composition and the proportion of carbon in the carbon/solid inert material mixture. Predictions of the model are in qualitative agreement with published experimental data.

Effect of combustor geometry on continuous spin detonation in syngas–air mixtures

Abstract: Regimes of continuous detonation burning of syngas–air mixtures in transverse (spinning) detonation waves in a flow-type annular cylindrical combustor are considered. Mixtures of carbon oxide and hydrogen in proportions of 1/1, 1/2, and 1/3 are used. The varied parameters are the combustor geometry and the fuel injection system, as well as the flow rates of air and syngas. The influence of additional supply of air to the products on the detonation wave parameters, pressure in the combustor, and specific impulse is determined. The range of realization of continuous spin detonation of the syngas–air mixture in terms of specific flow rates of the mixture is expanded from 25 to 786 kg/(s ·m2). It is shown that additional supply of air increases the pressure in the combustor, the thrust, and the number of detonation waves, but decreases the detonation wave velocity. The flow structure in the domain of detonation waves is studied. For some values of the combustor expansion coefficient, a chart of detonation regimes in the coordinates of the fuel-to-air equivalence ratio and specific flow rate of air is constructed, and the specific impulse of the thrust force is calculated.

Study of the combustion behavior and kinetics of different types of coal gangue

Abstract: Three types of coal gangue are compared systematically. The burnout index and the comprehensive combustibility index are found to increase significantly as the ash content decreases. The Arrhenius activation energy is obtained by using the Ozawa-Flynn-Wall and Vyazovkin models.

Calculation of the characteristics of agglomerates during combustion of high-energy composite solid propellants

Abstract: The problem of calculating the characteristics of the agglomerates formed during combustion of high-energy composite solid propellants is considered. It is shown that the mathematical models developed by the authors can be used for different propellant formulations to evaluate not only the dispersion of agglomerates, but also their quantity, chemical composition, and structure. The rules (algorithm) of using the developed models for a wide range of propellant formulations are determined. Modeling results for a number of propellant formulations based on various components are analyzed.

Correction of PIV data for reconstruction of the gas velocity in a supersonic underexpanded jet

Abstract: A method of correction of particle image velocimetry (PIV) data for reconstruction of the gas velocity based on the particle velocity in supersonic underexpanded jets is considered. The method is based on estimating the velocity lag of tracer particles on the basis of their velocity relaxation parameter as a correction to PIV data in the Newton approximation of interphase interaction. It is shown that the velocity relaxation parameter of tracer particles in flows with velocity jumps can be determined from the initial PIV data. Correction with the found parameter of velocity relaxation of the phases provides good accuracy.

Superadiabatic temperature phenomenon in the combustion processes due to a competition between chemical reactions

Abstract: The existence of a new type of superadiabatic temperature phenomenon in flames and during autoignition due to a competition between chemical reactions is inferred from literature data and the results of mathematical modeling of chemical kinetics and numerical experiments. The mechanisms, conditions for the occurrence, and nature of the phenomenon are discussed. It is noted that this phenomenon may have promising academic and practical applications.

Smoothed particle hydrodynamics simulation of the submarine structure subjected to a contact underwater explosion

Abstract: In this paper, a modified smoothed particle hydrodynamics (SPH) formula is deduced to solve the problem of interfaces with a high density ratio. Simplified SPH models for single and double cylindrical shells (abbreviated as single-hull and double-hull models, respectively) are established to study shock wave propagation and to conduct the damage analysis. The SPH results for the single-hull model are verified by AUTODYN. In addition, the damage analysis indicates that the single-hull model is damaged more severely than the double-hull model. The inner shell in the double-hull model is protected by a water interlayer.

Nanostructured explosives produced by vapor deposition: Structure and explosive properties

Abstract: This paper generalizes the experimental data of the authors on the production and properties of thin-layer nanostructured explosives obtained by thermal vacuum sublimation. The method involves sublimation of explosive under heating in high vacuum, followed by deposition (condensation) of the explosive vapor on the substrate. Under these conditions, it has been shown that nanostructured polycrystalline layers of explosives containing a large number of micro-defects (pores and dislocations) are formed. In the explosive transformation in the deposited explosive layer, nano- and submicron-sized defects of the structure act as hot spots. The result is a significant reduction in the critical detonation dimensions. The nanostructured explosives studied by the authors are able to detonate at a layer thickness of 20–100 µm. Furthermore, their detonation velocity is substantially less dependent on the layer thickness than that of charges of the same explosives made by traditional technologies. Nanostructured explosives can also be used as components of explosive compositions with improved detonability.

Temperature of the detonation front of an emulsion explosive

Abstract: This paper presents a new look at the structure of the radiance signal recorded by an optical pyrometer in measuring the brightness temperature of the detonation front of an emulsion explosive with glass microballoons as a sensitizer. The structure of the optical signal is typical of heterogeneous explosives: first a short temperature spike of up to 2500–3400 K occurs related to the formation of a layer of hot spots igniting the matrix capable of releasing energy, after which the radiance decreases to the quasi-equilibrium level corresponding to a temperature of 1880–2370 K at a detonation pressure of 0.7–11 GPa. There is satisfactory agreement between the experimental data and the results of independent calculations.

Combustion in vortex chambers with a fluidized particle bed

Abstract: This paper is devoted to methods of estimating the combustion regimes of gaseous fuel in vortex chambers of various designs in the presence of a bed of inert particles in the chamber, which rotate with the gas flow. A simple geometric criterion for determining the possibility of fuel combustion in the particle bed without the formation of a flame above the bed is proposed.

Hydrogen and air detonation (deflagration) synthesis of carbon-encapsulated iron nanoparticles

Abstract: With ferrocene as a precursor, carbon-encapsulated iron nanoparticles are synthesized through detonation of a gas mixture of hydrogen and air in a titanium detonation tube. XRD and TEM characterization shows that a downward trend in the size of particles can be observed with increasing amounts of the precursor. However, no further decrease occurs when the size of nanoparticles reaches approximately ≈40 nm, after which they remain in the range of 30–50 nm. The initial temperature of the detonation tube at 353 K is the optimal initial temperature for the synthesis. The average grain size of the synthesized products becomes larger as the temperature of detonation increases.