Ya. I. Vovchuk

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.

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.

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.

Thermal regime of the vapor-state combustion of a magnesium particle

An analysis of the relationship between the heat fluxes in the combustion of a single magnesium particle in an oxygen-containing medium shows that vapor-state combustion is possible only in the case where an appreciable portion of magnesium oxide condenses on the droplet surface.

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.