Effects of initial diameter and ambient pressure on the ignition and combustion of laser-ignited aluminum agglomerates

An experimental investigation was conducted to determine the effects of initial diameter and ambient pressure on the ignition and burning characteristics of aluminum agglomerates. Agglomerates with initial diameters of 535, 835, and 1000 µm were ignited by a CO*sub2; laser pulse in an oxygen/nitrogen (20/80) environment at pressures ranging from 0.1 to 4.3 MPa. The combustion event was recorded using high-speed cinematography. A two-color pyrometer and broad-band radiometer were used to measure the surface temperature of and the radiation emitted by the burning droplet. Data from these three sources was used to determine the ignition delay time and burning time of the aluminum agglomerates.

Analysis of the combustion residue showed that the combustion process at atmospheric pressure varies greatly from that at elevated pressures. The products from experiments conducted at atmospheric pressure consisted of hollow spheres of oxide. Experiments conducted at elevated pressures resulted in products consisting of a small core of unburned aluminum surrounded by an opaque layer of oxide.

The ignition delay time and burning time were found to be dependent upon ambient pressure. The ignition delay time and burning time initially decrease with increasing pressure. However, a pressure limit is eventually reached beyond which increasing pressure no longer has an effect. Power law analysis showed the ignition delay time to be proportional to the initial agglomerate diameter raised to the power of 1.28. The burning time was found to be directly proportional to the diameter.