Flames structure measurement of single, isolated aluminum particles burning in air

Abstract

Spatially resolved measurements of the flame structure about single, isolated aluminum particles burning in air were made using planar laser-induced fluorescence (PLIF) and electron probe microanalysis (EPMA). Spherical aluminum particles of 210 μ m diameter were generated continuously by mechanical chopping of wire strands and laser heating, which also ignited the particles. After ignition, combustion was found to occur in two stages consisting of an initial steady burning phase, characterized by a spherical flame positioned off the surface, and a second violent, unsteady burning phase, during which gaseous ejections occurred from the particle surface. PLIF was used to measure the radial profiles of gaseous AlO and AlO vibrational temperature during the steady burning stage of particle free fall. The radial distribution of condensed-phase Al 2 O 3 was measured by impaction and rapid quenching of individual particles and their associated oxide clouds on silicon plates. Analysis of the impacted cloud distribution was performed off-line with EPMA and Abel transform techniques. Results indicated a flame structure of finite thickness, wherein AlO is a gas-phase intermediate. Relative peak concentrations of AlO and Al 2 O 3 were measured at r/r s =2.8 and r/r s =3.5, respectively. The temperature rose from ∼2350 K at the particle surface to a nearly constant value of ∼3800 K between 5< r/r s <6. The surface temperature agrees well with previously reported ignition temperatures, while the plateau temperature is ∼300 K higher than the theoretically predicted adiabatic flame temperature for a stoichiometric Al-air mixture. The species and temperature measurements support previously proposed models where gasification can occur by both vaporization and heterogeneous surface reactions. Transport of Al outward and reaction with inward-diffusing molecular oxygen yields AlO. Subsequent heterogeneous reactions involving AlO yield condensed-phase Al 2 O 3 at even greater radii.