Laser ignition of aluminum particles in water

This study presents experimental results of ignition of Al particles with a CO₂ laser in water vapor environment. Al powder, with nominal particle sizes in the range of 4.5 μm - 7 μm, is aerosolized using a parallel plate capacitor by charging particles contacting the electrodes. A thin, laminar aerosol jet is carried out by nitrogen heated to -150°C through a small opening in the top electrode and is fed into a focused CO ₂ laser beam. A shroud flow of superheated steam, also at ∼150°C, is maintained around the central aerosol jet. The velocities of particles in the jet can be varied in the range of 0.1 - 3 m/s. The particle velocity is controlled by the inert gas jet velocity while keeping the shroud steam flow velocity constant. Numerical simulation using Fluent CFD code was used to determine the gas composition at the laser focal spot. For each selected central flow velocity, the laser power was increased until the particles were observed to ignite. The ignition was detected optically using a photomultiplier. The ignition thresholds for spherical aluminum powder were measured at varied particle velocities resulting in varied heating rates and vapor concentration. Similar experiments were conducted by replacing superheated steam with hot air. Laser ignition threshold from air was found to be lower than that for superheated steam.