Abstract
The combustion behavior of single laser-ignited iron particles is investigated. Transient particle radiant intensities at 850 nm and 950 nm are measured by post-processing recorded high-speed camera images using an in-house developed particle tracking program. Then, the time-resolved particle temperature is obtained based on two-color pyrometry. A plateau-like stage shortly after the ignition is repeatedly observed, and identified as iron particle melting by the measured temperature and the estimated melting time. Besides, an abrupt brightness jump near the end of combustion is observed for most burning particles, while a small portion of the particles (< 10%) show a second plateau-like stage instead. The particle temperature right after the brightness jump (1880±70 K) is almost identical to that during the second plateau-like stage. This temperature corresponds to two phase-change temperatures in the Fe-O phase diagram: i) L2 <=> Fe3O4 (s) at 1869 K (congruent melting) and ii) L2 <=> Fe3O4 (s) + O2 (g) at 1855 K (eutectic reaction), where L2 represents a liquid iron oxide. Based on this, the presence of the brightness jump (spear point) is explained by a sudden solidification of supercooled iron oxide droplets with an atomic O/Fe ratio larger than or close to 4/3. Particles’ near-peak temperatures are also measured based on time-integrated spectra. The results indicate that the near-peak temperature increases first fast and then slowly with an increase of oxygen concentration. At higher oxygen concentrations, smaller particles have a slightly lower temperature. The effect of particle size on the near-peak temperature is negligible at lower oxygen concentrations due to weaker radiation. The morphology of combusted particles is examined by micrography. Some burned particles appear as hollow thin-shell spheres at all adopted oxygen concentrations. Additionally, nano oxides are found at 13–51% oxygen concentrations. Less traces of nano oxides were observed at reduced oxygen concentrations. The nano-oxide formation mechanisms are analyzed based on thermochemical equilibrium calculations.
Highlights
To reduce greenhouse gas emissions, new combustion technologies and fuels are needed
Time-resolved temperature measurements are presented for single laser ignited iron particles, as well as near-peak temperatures deduced from time-integrated spectra for various conditions
The estimated melting duration of an iron particle based on diffusion-limited combustion assumption agrees well with measurements at two oxygen concentrations, which is another strong evidence for the iron-melting hypothesis
Summary
To reduce greenhouse gas emissions, new combustion technologies and fuels are needed. In our previous work [14], which aimed at systematically measuring laser-ignited particle burn time at room temperature in nitrogen/oxygen mixtures with various oxygen concentrations, two phenomena similar to that reported in the two above-mentioned works were observed: (i) a plateau-like stage on the particle brightness profile shortly after ignition, which was hypothesized as melting; (ii) an abrupt brightness jump near the end of a burning particle streak (cf Fig. 8), which was speculated as sudden solidification of the supercooled iron oxide droplet or one of other phase transitions of the iron-oxygen binary system. Phase transitions are important for understanding metal particle combustion [15] It is thought as the reason for some unique phenomena observed during metal particles combustion, such as the temperature jump occurring for many metals including steel, by Dreizin et al [16].
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