Size-resolved ignition temperatures of isolated iron microparticles

Ignition temperatures of metal particles play an essential role in not only the fundamental theories of non-volatile dust flames but also the robust operation of practical metal fuel burners. The present paper introduces a novel approach to accurately measure the ignition temperature of an isolated particle. Micron-sized single particles are injected downwards into a quartz tube heated externally by a premixed flame near the bottom end. During free fall, a particle, if sufficiently small, closely follows the gas-phase temperature that increases gradually from top to bottom. By measuring the ignition position of the particles, the ignition temperature is determined from the gas-phase temperature profile that is quantified a priori. Applying the approach together with high-speed imagining and diffuse backlight-illumination techniques, the ignition temperature of approximately 30–60 $\mu$m iron particles in O₂/N₂ mixtures are comprehensively measured at oxygen mole fractions of 10%–50%. The experimental results reveal that the measured ignition temperatures is in the range of 1030–1130 K that are independent of the oxygen mole fraction and the particle size. In contrast, the particle size significantly influences the ignition probability. Smaller particles have lower probabilities to ignite. At the oxygen mole fraction of 10%, ignition is only observed for iron particles larger than approximately 45 $\mu$m. For all other cases, ignition is detected for all particle diameters. Possible mechanisms underlying the experimental observations are discussed.

Novelty and significance statement

A novel approach to measure the ignition temperature of an isolated, micron-sized particle is developed. Compared to existing methods, the new approach provides a convenient way to determine the ignition temperature accurately and examine the size dependence of the ignition characteristics. For the first time, size-resolved ignition temperatures of isolated iron particles are reported. For a fixed particle diameter, the existence of an ignition probability is revealed. The quantitative experimental results have a high potential to be widely used to validate models of iron particle ignition.