Ignition and combustion phenomena of laser-ignited aluminum agglomerates at elevated pressures

An experimental investigation of the ignition and combustion phenomena of laser-ignited aluminum agglomerates at elevated pressures is presented. Aluminum agglomerates of nominal diameter of 535 μm were ignited by a focused CO₂ laser pulse in a quiescent environment of O₂/N₂ (20/80) and at different ambient pressures ranging between 0.1 and 4.3 MPa. A two-color pyrometer and a broad band radiometer were employed to measure the surface temperature and the radiation emitted by the burning aluminum agglomerate, respectively. From the outputs of the two-color pyrometer and the broad band radiometer, the ignition temperature, the ignition delay time and the burning time were obtained. A high speed camera was utilized to observe the phenomenological events during the burning of the aluminum agglomerate, and also to correlate with the results obtained from the pyrometer and radiometer. In the range of pressures (0.1 to 4.3 MPa) investigated in the present study, aluminum agglomerates burn primarily in the vapor phase, characterized by a detached flame surrounding the burning droplets. At pressures below 2.2 MPa, the burning time was found to decrease with increasing ambient pressure. At high pressures (>2.2 MPa) the burning time is not sensitive to the ambient pressure. As the pressure increases, the flame moves closer to the aluminum droplet surface. In this study, the ignition of aluminum agglomerates occurs in the neighborhood of the melting temperature of the aluminum oxide (2315 K) which indicates that the break-off of the aluminum oxide layer is the mechanism of aluminum ignition. Qualitative studies of flame structure from high-speed camera showed that the flame, surrounding the aluminum burning droplet, consists of three different zones: a luminous white zone surrounded by a green thick envelope (AlO) which is in turn adjoined by a thin blue-violet zone. The luminosity of the inner zone increases with increasing pressure.