Multi-stage oxidation of iron particles in a flame-generated hot laminar flow

The paper presents multi-parameter high-speed optical diagnostics of single iron particle oxidation in a hot laminar flow consisting of O2, H2O, CO2, and N2. Prior to luminous combustion, micron-sized particles are visualized using diffuse-backlight-illumination with high temporal and spatial resolutions, enabling in situ particle sizing. The incandescent burning particles are then monitored by direct high-speed imaging. This approach allows for a successful determination of the multiple oxidation stages of individual iron particles, including the solid phase, melting, and liquid phase. Two characteristic timescales are quantified as a function of the resolved particle diameter at various O2 volume fractions, namely the solid-phase oxidation time (SOT, defined as the time between the onset of rapid particle heating and melting) and the liquid-phase combustion time (LCT, defined as the duration between melting initiation and peak luminosity). SOT shows a linear dependence on the inverse O2 mass fractions at relatively high O2 levels but it tends to plateau when the O2 content is below 20vol.%. LCT is inversely proportional to the O2 mass fraction, suggesting that liquid-phase oxidation of iron particles is limited by external oxygen diffusion. The experimental LCT quantitatively agrees with a theoretical mode developed for nonvolatile particle combustion in the diffusion-limited regime, considering O2 as the only oxidant. This also implies that at the presence of O2, the oxidizing effect of H2O and CO2 on diffusion-limited iron particle combustion is negligible.