Combustion of Aluminum Suspensions in Hydrocarbon Flame Products

Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane-air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used to determine the flame temperature via both the continuous and aluminum monoxide spectra. The flame burning velocity is measured and the condensed flame products are collected and analyzed for unburned metallic aluminum content. It was observed that, below a critical concentration of about 120 g/m(3), aluminum demonstrates incomplete oxidation with a flame temperature close to the methane-air flame. Below the critical concentration, the flame burning velocity also decreases similar to a flame seeded with inert silicon carbide particles. In contrast, at aluminum concentrations above the critical value, an aluminum flame front rapidly forms and is coupled to the methane flame. The flame temperature of the coupled methane-aluminum flame is close to equilibrium values with aluminum as a reactant, and the flame burning velocity remains flat for increasing aluminum concentrations. A simple theoretical estimation, which assumes that the aluminum reaction rate is controlled by the kinetic evaporation of aluminum, adequately predicts the critical concentration range at which the aluminum flame front can be coupled with the methane flame.