Phase changes in metal combustion

Condensed-phase modifications in magnesium particle combustion in air

An experimental study on the nonsteady ignition process of magnesium and 50-50 magnesium-aluminum alloy was made by using the stagnation region of an impinging hot air stream. The study has revealed that the ignition of magnesium was caused to occur through a four-stage surface oxidation process and finally through a homogeneous exothermic reaction in the gaseous phase. The ignition mechanism of the magnesium-aluminum alloy was essentially identical with that of magnesium, although the observed ignition behavior was more complicated. The difference could be explained in terms of the aluminum ingredient contained in the oxide film formed on the sample surface.

Ignition of Magnesium and Magnesium-Aluminum Alloy by Impinging Hot-Air Stream

Constant pressure combustion of aerosol of coarse magnesium particles in microgravity

In the prospect of using metals as fuel of breathing combustion engines in carbon dioxide rich planet atmospheres without oxygen such as those of Mars and Venus, a fundamental study was performed experimentally on the ignition and combustion of metals in an impinging pure CO2 gas stream. Metals selected were lithium, magnesium, boron and aluminum, because of their high heats of reaction with CO2. To help the understanding of combustion processes of the metals in CO2, the chemical equilibrium compositions of reaction products and flame temperatures were calculated, and compared with the experimental results. Li and Mg could ignite spontaneously in the CO2 stream. The ignition process of Li in the CO2 stream had two distinct steps, first a surface reaction and then a gas phase reaction. This process was found to be similar to those of Mg and Na in air streams, but was different from that of Li in an air stream. The spontaneous ignition temperature of Li in the CO2 stream was much lower than that in the air stream. It was also found that Li burned in vapor-phase at first and then on the metal surface. At the ignition in the CO2 stream, the Mg surface was covered with a protective film against further oxidation, resulting a higher spontaneous ignition temperature than that in the air stream. Mg could burn in vapor-phase. B could not ignite in the CO2 stream, but the reactions occurred appreciably on the surface. The reaction rate was quite slow, compared with that in an oxygen stream, and the molten boron oxide appeared on its surface. Al could not ignite within the temperature limits of our apparatus. However, the possibility of its ignition and combustion in the CO2 stream was demonstrated in a different experiment allowing the attainment of somewhat higher temperatures.

Ignition and combustion of metals in a carbon dioxide stream

This paper describes an experimental investigation of the flame structure of metal vapor-phase diffusion flames, in particular the low-pressure flames of Mg, Ca, and Sr. Two types of experiments were employed. In the first type, metal wire samples were mounted between electrodes and heated electrically. In the second type, a two-dimensional diffusion flame burner was adapted to low-pressure metal combustion studies. Space-resolved spectroscopy was used to define the location of emitters in both experiments. A new model for the flame structure is presented. The model is characterized by a broad reaction zone in the inner region of the luminous envelope. The radiation is attributed mainly to a thermal excitation mechanism. As a result of the reaction (which is thought to be predominantly homogeneous), a large supersaturation of oxide vapor is generated in the inner zone leading to rapid nucleation of particles. Various possible kinetic steps for the reaction are compared. It is concluded that the t...

I. Glassman

Papers

Vapor-Phase Diffusion Flames in the Combustion of Magnesium, Calcium and Strontium