N. I. Poletaev

Degree of dispersion of metal combustion products in a laminar dust flame

This paper presents the results of experimental and theoretical studies of the effect of parameters of laminar dust flames of metal particles (Al, Fe, Ti, and Zr) on the degree of dispersion of the combustion products of these metals in oxygen-containing media. Extensive experiments with Al powders showed that with variation in the mass concentrations of fuel and oxidizer, fuel particle size, type of carrier gas, and conditions of dust flame production, the most probable particle diameter varied in the range of 50–70 nm. Similar results were also obtained for other metals. The results of the experiments agree with numerical calculations. The experiments showed that the particle size of metal combustion products in laminar dust flames can be substantially increasing. The proposed method for controlling the particle size is based on the ionization of the gas phase by adding impurities to the initial fuel to affect nucleation conditions in the flame.

Gas-disperse synthesis of metal oxide particles

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 addition of potassium carbonate to aluminum powder on the grain size of Al2O3 nanoparticles formed in the laminar dusty flame

Results of studying the effect of K2CO3 additives on the grain size of the products of combustion of a gas suspension of Al particles (with the mean particle diameter of 4.8 µm) in a laminar diffusion flame are reported. An extreme character of the dependence of the mean size of Al2O3 particles on the additive concentration is experimentally observed. For the concentration of the K2CO3 additive equal to 0.5%, the mean diameter of Al2O3 particles is 30 nm; for the additive concentration of 5%, the mean particle size increases to 67 nm. It is demonstrated that the change in the mean size of Al2O3 particles as a function of the concentration of the readily ionized additive is caused by interaction of the dusty and ionic subsystems of the plasma of the combustion products in the reaction zone in the flame. At a high concentration of ions (above 1020 m−3), this interaction increases the rate of coagulation of Al2O3 particles.

Nanoparticle Formation by Combustion Techniques

This chapter reports physical and technical aspects of a production method for synthesis of spherical nanoparticles of oxides of refractory metals by means of combustion of metal dust clouds in oxygen containing gaseous oxidizers. We describe the reactor, the conditions for stabilization of different flames of dispersed metals, and methods for carrying out experiments on nanooxide synthesis. The mechanism for combustion of particles of different metals is briefly considered, and results are given from spectral measurements of combustion characteristics (metal particle, ambient gas, and condensed nanooxide temperature). Data from dispersion analyses of the combustion products are reported as well. The influence of macroparameters of fuels (particle size and concentration) and oxidizer (oxygen concentration and consumption) on the properties of the condensed combustion products is considered. It is shown that a change of macroparameters for the fuel-oxidizer system, within the range permissible for a given reactor, practically does not alter the nanoproduct properties. This is connected with the fact that the zones for chemical condensation of oxides in microflames of individual particles serve as nanooxide generators during the metal dust cloud combustion. This explains the high reproducibility of the properties of nanooxides synthesized by this method. A simplified physical model of nanooxide formation during metal dust cloud combustion is proposed. It provides a foundation for the mechanism of chemical condensation. The model allows experimental data to be described with sufficient accuracy. Possible ways for synthesis control are suggested.

Heat Transfer of Submicron MgO Particles in the Combustion Zone of Single Magnesium Particles

The mechanism of heat transfer of submicron MgO particles in the combustion zone during their growth is studied at pressures of 104 – 105 Pa using opticospectral measurements and numerical simulation. It is shown that at the early stage of particle growth, heat transfer is caused exclusively by collisions with gas molecules with an energy‐accommodation coefficient of ∼ 0.01 – 0.02. During particle growth and with decrease in pressure, the role of radiation increases, and, ultimately, radiation becomes a leading mechanism of heat transfer.

Formation of condensed combustion products in metal dust flames: Nucleation stage

The nucleation of the ionized combustion products of small (d10 ≈5 µm) particles of Al, Mg, Zr, Fe, and Ti under laminar dust flame conditions at atmospheric pressure is considered in an isothermal approximation. It is shown that under conditions close to the experimental ones, the condensation of the products of gas-phase combustion of these metals is “rapid.” Description of the “rapid” nucleation regime requires a nonstationary approach and knowledge of the kinetics of nucleation of the condensed phase and does not need a detailed analysis of the influence of environmental parameters on the free energy of formation of small nuclei. It is shown that the characteristic nucleation time of the gas-phase combustion products of metal particles is several orders of magnitude smaller than the residence time of the products in the combustion zone of the flame dust. This allows coagulation to be considered as the basic process which determines the degree of dispersion of primary particles of the metal combustion products.

Electrical oscillations in combustion of magnesium particles in a constant electric field

For a magnesium particle (6 mm in diameter)burning in air, electrical oscillations in the measuring circuit were found at a threshold intensity of the external constant electric field of 30 kV/m. Spectral analysis of the oscillations revealed low-frequency (5 kHz) and high-frequency (25 kHz) oscillation modes. It is shown that low-frequency electrical oscillations are due to variations in the density of the positive space charge of submicron MgO particles between the metal particle surface and the negatively charged plate of the capacitor. High-frequency oscillations are caused by modulation of the dielectric permittivity of the medium in the chemical condensation zone of the magnesium particle. The cause of the modulation is instability in the condensation zone of the burning particle, which is manifested in the excitation of longitudinal oscillations of the MgO particles (dust acoustic waves) propagating in the thermionic plasma of magnesium combustion products. The occurrence of the dust acoustic waves is due to streaming instability caused by differences in the drift velocities of charge carriers in the electric field.

Using Unsteadiness of the Processes in Diagnostics of Burning Objects

A technique for studying burning processes based on their unsteadiness is proposed. The temperature and spectral dependences of emitting and absorbing characteristics of MgO particles and their mixtures with gases at high temperatures are studied. The rates obtained allow one to solve problems of radiative heat transfer. The problem of existence of equilibrium between the gaseous and condensed magnesium oxide in the combustion zone is discussed.

Relationship between the dust flame propagation velocity and the combustion mode of fuel particles

A possibility of determining the regime of combustion of individual fuel particles on the basis of the dependence of the flame velocity on the fuel and oxidizer concentrations is considered by an example of a dust flame of microsized metal particles with diameters d10 < 15 μm and particle concentrations from ≈1010 to 1011 m−3 in oxygen-containing media at atmospheric pressure. The combustion mode (kinetic or diffusion) is responsible for the qualitative difference in the character of the normal velocity of the flame as a function of the basic parameters of the gas suspension. The analysis of such experimental dependences for fuel-rich mixtures shows that combustion of zirconium particles (d10 = 4 μm) in a laminar dust flame is controlled by oxidizer diffusion toward the particle surface, whereas combustion of iron particles of a similar size is controlled by kinetics of heterogeneous reactions. For aluminum particles with d10 = 5–15 μm, there are no clearly expressed features of either kinetic or diffusion mode of combustion. To obtain more information about the processes responsible for combustion of fine aluminum particles, the flame velocity is studied as a function of the particle size and initial temperature of the gas suspension. It is demonstrated that aluminum particles under the experimental conditions considered in this study burn in the transitional mode.

Firmation of condensed combustion products in dust flames of metals: Coagulation stage

The coagulation dynamics of condensed products of vapor-phase or gas-phase combustion of gas mixtures of microdispersed metal particles in dust laminar flame considered taking into account the ionization of the combustion zone due to additives of electronegative and electropositive atoms and due to thermionic emission. The influence of the degree of ionization of a monodisperse coagulating aerosol and the charge of the particles on the coagulation rate constant is studied. It is shown that the rate of coagulation of the aerosol is most significantly affected by the Coulomb interaction of like-charged condensed-phase particles, which, under certain conditions, leads to an early stop of this stage in the condensation of combustion products. Ionization of the coagulating particles gives rise to a dependence of the size of primary particles of combustion products on the environmental parameters affecting their electric charge, and can be used for targeted control of the degree of dispersion of the combustion products.