Vapor-phase diffusion flames in the combustion of magnesium and aluminum. i - analytical developments

Abstract

Abstract High heats of vaporization or dissociation of metal oxides upon vaporization generally limit the temperatures of metal-oxygen flames to the boiling points of the respective oxides. Aluminum and magnesium can burn in the vapor phase because their boiling points are lower than those of their oxides. The combustion of single magnesium and aluminum droplets is described by an extension of the quasi-steady vapor-phase diffusion flame theory previously used to describe the combustion of hydrocarbon droplets. The theory is modified to consider flame radiation, transport of condensed oxide products, and evaporation of the metal. The fraction of oxide vaporized in the reaction zone varies with the ambient oxygen content. High flame emissivities, resulting from the presence of condensed oxide in the reaction zone, render the metal flames nonadiabatic. Droplet surface temperatures can be hundreds of degrees lower than the metal boiling points. The flame zones lie very close to the droplets. Calculated evaporation constants of magnesium and aluminum droplets are in the range of values observed with hydrocarbons. Good results for the burning rate and flame radius are obtained with very little computation.