Minimum ignition energy of a laser-ignited single micron-sized iron particle

Abstract

In this work, we develop a novel method to accurately measure the minimum ignition energy (MIE) of a micron-sized flying iron particle. A stable feeding of single iron particles is achieved using a newly developed particle supply system. Ignition is initiated by a 915 nm infrared continuous-wave laser with a maximum power of 65 W. A criterion for determining successful ignition has been established. The MIE is calculated based on the critical laser power density, residence time of iron particles in the laser spot, and laser absorption efficiency. Using this approach, we measured the critical laser power density and the MIE for spherical iron particles with different sizes (62 μm-98 μm) and in a wide range of oxygen concentrations (16%-100%). The results demonstrate that both particle size and oxygen concentration have a significant impact on the laser ignition process. The MIE increases according to a power law with the particle size and decreases with the oxygen concentration. As oxygen concentration rises from 16% to 100%, the MIE decreases dramatically and then gradually approaches a limit. The ignition process can be divided into oxygen-sensitive and oxygen-insensitive zones, indicating that successful ignition of iron particles is influenced by the combined effects of energy input and oxygen concentration. The significant dependence of the MIE on oxygen concentration in laser ignition is quite different from that of the ignition temperature in hot atmosphere ignition reported in previous work. This would possibly be attributed to the fact that the heating rate of laser ignition is much higher. It significantly enhances the internal kinetic consumption rate of iron particles, promoting the transition from kinetics-controlled to external diffusion-controlled regimes. Consequently, the external oxygen diffusion becomes the rate-limiting step for laser ignition, whereas for hot atmosphere ignition, the internal iron ions diffusion dominates. Novelty and significance statement A novel method to determine the minimum ignition energy of a micron-sized flying iron particle is developed. The minimum ignition energy of a single micron-sized iron particle is measured experimentally for the first time to the best of authors’ knowledge. The significant oxygen concentration dependence of the minimum ignition energy is observed in laser ignition, which is quite different from the ignition temperature in hot atmosphere ignition. It would be attributed to the different heating rates between the laser ignition and the hot atmosphere ignition. The power-law correlations of the minimum ignition energy with the particle size and the oxygen concentration are proposed based on the experimental results. The experimental data have a high potential to be used in validating ignition models of individual iron particles.