A. N. Zolotko

Synthesis of nanooxides in two-phase laminar flames

This paper deals with a method for production of nanopowders of high-melting metal oxides by burning the starting metal powders in a laminar disperse flame. The method is called gas-dispersed synthesis. The combustion-zone structure for a laminar diffusion dispersed flame and for a laminar flame of premixed fuel and oxidizer was studied experimentally. Information was obtained on the temperature of combustion gases and its spatial distribution, the temperatures of the burning particles and of the condensed combustion products, and the combustion regimes of metal particles. The dependence of the properties of the resulting oxides on the flame and combustion-zone parameters and the mechanism of particle combustion is studied. Based on the results, an attempt is undertaken to reproduce the mechanisms of formation and growth of the condensed phase under conditions of laminar diffusion flames. The mass-averaged particle size of the oxide powders is estimated.

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.

Mechanism of transport of condensed combustion products to the surface of a burning magnesium particle

Numerous investigations into the combustion of metal particles have shown that the formation of condensed particles has an important influence on the burning rate. To explore the effect of the electrophysical processes on the mass transport mechanism in such systems, the authors start out from the vapor phase model of magnesium particle combustion. Together with the transport of condensed magnesium combustion products by convective flow from the droplet, charged condensed particles are found to be transported toward the droplet as a result of drift in the intrinsic electric field associated with combustion. Taking this process into account may improve the agreement between the calculated and experimentally observed magnesium particle burning times.

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.

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.

Extinction of dispersed heterogeneous systems

We consider extinction of various dispersed systems. Isolated boron particles and boron particles in gases are studied. Stability analysis of steady-state thermal regimes of reacting heterogeneous systems for the case of two parallel reactions on the reaction surface using the Frank-Kamenetskii method gives extinction conditions in oxygen-containing media. Curves of the extinction particle size versus the ambient temperature, oxidizer concentration, and, for particles in gases, also versus the oxidizer-to-fuel ratio are plotted. Approximate analytical calculations showed that the extinction process can be most actively controlled by varying the combustion temperature: a decrease in the latter increases the extinction particle size and decreases the completeness of fuel combustion. It is shown that at low ambient temperatures the extinction particle size for suspensions is larger than that for isolated particles. This effect is caused by a decrease in the oxidizer concentration during combustion of suspensions. At high temperatures, the role of this factor weakens.

IGNITION OF HIGH ASH COAL PARTICLES

This article examines the combustion kinetics of high ash coals in oxidizing media. Microcine photographs of individual coal particles were taken inside a quartz microfurnace tube, with a weak oxygen flow supplied. The photographs and the physicochemical characteristics of the coals indicate that both the ash content of the fuel and the ratio of the thermal equilibrium loss temperature and the ash fusion point determine the differences in ignition. The porous mineral residue has no practical effect on the interaction between carbon and oxidizer for coals with a low ash content or fusible ash. Two limiting ignition mechanisms, thermal and thermokinetic, are identified. It is demonstrated that for coals of various brands, the ignition mechanism is determined primarily by the coal ash content and the ratio of the critical dispersed system ignition temperature to the fusion point of the fuel mineral component.

Combustion of a suspension of biofuel droplets in air

This paper presents the results of theoretical and experimental studies of the combustion of rapeseed methyl ester droplets in comparison with mineral diesel fuel. Droplets of 0.7–1.2 mm diameter were investigated. The effect of the oxidizer equivalence ratio in the range of 1.7–4.0 on the duration of droplet combustion was analyzed using the model of combustion of droplets in an adiabatic shell taking into account Stefan flow. It is shown that the burning time of rapeseed methyl ester droplets is smaller than that of diesel fuel droplets, all other things being equal.

Wave regimes of dust combustion

Experimental studies were performed to investigate the dependence of the laminar flame velocity in dust clouds of Al, Mg, Zr, Fe, and B particles on the physicochemical parameters (fuel concentration and composition, particle size distribution) and hydrodynamic conditions of the combustion process (semi-open tubes, free clouds of particle-air mixtures). Heat conduction was found to make a predominant contribution to the overall heat transfer in the combustion wave. The main causes of instability of laminar flames (acoustic disturbances, interfacial exchange, forced and natural convections), transient phenomena, and vibrational and turbulent combustion of dust were studied experimentally.

Ignition of Disperse Systems During Stage Processes on Their Surface

The possibility of moderating the conditions for ignition of a solid combustible that has a difficultly penetrable oxide coating on its surface is illustrated by solution of the problem of ignition of a gas suspension of boron particles in an oxygen—water vapor mixture as an example. The decrease in the ignition temperature of this combustible is reached owing to an active gaseous reagent capable of reacting with an oxide film and transforming it to the products which having no blocking properties. The calculation has shown that if the B2O3 film which hinders particle ignition is removed only as a result of its evaporation, the ignition temperatures of the particles are high. Gasification of the oxide significantly accelerates the removal of the film, which causes a decrease in ignition temperatures compared to a dry medium. The experiment has supported the validity of the calculation. The effect is manifested the stronger the higher the reaction surface of the reacting system. The critical condition for ignition of a gas mixture of particles is obtained from analysis of the stationary solutions of a system of equations on the stability to small perturbations. It is shown that the optimum ratio of the oxidizing components at which the minimum ignition temperature of the suspension occurs exists.