Heating and ignition of metallic particles by a CO₂ laser

This paper presents an experimental technique and respective heat transfer model for studying ignition of micron-sized metallic particles at varied heating rates in the range of 10⁶ K/s. The experimental setup uses a CO ₂, laser as a heating source. The interaction of the laser beam with particles is particle size dependent and a narrow range of particle sizes (around 3.37 μm) is heated most effectively. Therefore, the heat transfer model needs to be analyzed only for the particles with this specific size, which greatly simplifies the interpretation of experiments. The powder is aerosolized inside a plate capacitor by charging particles contacting the capacitor's electrodes. A thin, laminar aerosol jet is carried out by an oxidizing gas through a small opening in the top electrode and is fed into a laser beam. The velocities of particles in the jet are in the range of 0.1-3 m/s. For each selected jet velocity, the laser power is increased until the particles are observed to ignite. The ignition is detected optically using a digital camera and a photomultiplier. The ignition thresholds for spherical aluminum powder were measured at three different particle jet velocities resulting in three different heating rates. The experimental data for aluminum, for which the ignition kinetics parameters are known, were acquired at a specific heating rate and used to calibrate the detailed heat transfer model. The experiments with different heating rates were performed to validate the model. The developed experimental technique and the heat transfer model can now be used to determine the ignition kinetics of different metallic powders in various gas environments.