Visualizing Particle Melting and Nanoparticle Formation During Single Iron Particle Oxidation with Multi-Parameter Optical Diagnostics

High-temperature oxidation of single iron particles is experimentally investigated for laminar flow condi-tions using multi-parameter optical diagnostics. Iron particles with initially porous structures are studied with average diameters of 28 mu m (A) and 87 mu m (B). Simultaneous diffuse backlight-illumination (DBI) and luminosity imaging (LU) measurements at 10 kHz are used to evaluate single iron particles burning in a hot gas environment with 20 vol% oxygen. Particle dynamics, particle melting, and nanoparticle forma-tion are evaluated. The particle velocity is assessed by temporally tracking the particle position, showing different accelerations correlated to the particle sizes. Based on the velocity profile, the particle character-istic time tau prt, slip velocity Vslip, and particle Reynolds number Reprt are determined experimentally. Parti-cle melting processes are visualized by temporally tracking the aspect ratio beta prt of particle shape and the average melting time tm is statistically analyzed, increasing from 10 ms to 30 ms with particle diameters A to B. Correspondingly, the heating rate reduces from 1.5 x 10 5 K/s to 0.5 x 10 5 K/s. Nanoparticle clouds in DBI images are observed in the vicinity of the molten iron core showing a correlation between the sig-nal topology and iron particle size. Combining DBI with simultaneous luminosity imaging, the formation of nanoparticle clouds is observed to be temporally synchronized with increasing luminosity intensities indicating surface temperature rise and the appearance of radiative nanoparticles downstream of the par -ent iron particles. The nanoparticle formation terminates at the peak luminostiy signal of the parent iron particles. Further analysis of appearance probability and spectrally integrated intensity of nanoparticle luminosity reveals a temporal correlation between the onset of nanoparticle formation and melting of parent iron particles. Based on the experimental observations and in accordance with previous studies by others, for the present conditions it is concluded, that oxidation of micrometer-sized iron particles occurs partly in the gas phase. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.