Observation of an unmixed particle surface during single-particle experiments of iron combustion

Abstract

International audienceThis study explores the combustion of single iron particles, emphasizing the phenomenon of an unmixed surface during the liquid-phase combustion regime. While previous single-particle combustion experiments have advanced the understanding of the combustion process, the exact configuration of the liquid phases remains unclear. In addition, insights derived from ex situ microstructure analysis are limited by uncertainties introduced during particle cooling, which can alter the internal structure. To address this, we utilized an electrodynamic levitator and laser ignition to study suspended iron particles heated to a high initial temperature (between 1820 K and 2092 K). The combined use of a high-speed color camera and a luminance acquisition system enables high resolution in situ imaging and luminance tracking. A distinct “unmixed surface period” is observed, during which two immiscible liquid phases – pure iron (L1) and iron oxide (L2) – coexist at the particle surface. Initially, the surface is fully covered with L2, followed by the appearance of moving L1 spots, likely driven by a Marangoni flow. This period concludes with the formation of a core–shell structure. These findings provide new insights into the dynamics of liquid-phase oxidation in iron combustion, as the unmixed surface configuration might influence both the rate-limiting mechanism and evaporation dynamics. Such observations contribute to improving numerical models, particularly in capturing the initial stages of the liquid-state combustion. Moreover, particle size analysis indicates that smaller particles deviate further from a fully external-diffusion-limited regime, underscoring the role of alternative rate-limiting mechanisms in their combustion behavior