Temperature of burning iron microparticles with in-situ resolved initial sizes

Simultaneous, in-situ, optical diagnostics are performed to measure initial diameters and the temporal temperature evolution of iron microparticles burning in hot laminar oxidizing atmospheres with 10–30 vol% O${}_2$. The pre-ignition particle diameter and temperature evolution during combustion are monitored using synchronized high-speed diffuse backlight-illumination and two-color pyrometry techniques, respectively. Average temperature histories are obtained for particles sorted into three size fractions centered at 40, 45, and 50  $\mu$m with a span of 5  $\mu$m. Increasing the oxygen level from 10 to 30 vol%, particles burn faster and reach a higher peak temperature that increases approximately from 2400 to 3200 K. From their temperature trajectories, the peak temperatures of individual particles are extracted and correlated with their initial diameters. It is observed that the maximum particle temperature decreases with the increasing particle diameter, attributed to the enlarged radiative and evaporative heat losses relative to the chemical heat release of the larger particles that burn in the diffusion-limited regime. In addition, the size dependence of the maximum particle temperature enhances considerably when the particle peak temperature increases from approximately $2400$K to around $2800$K, but its further variation is small as the particle peak temperature continues approaching the boiling point of the particles. This observation does not align with previous non-size-resolved measurements that have lower temporal resolutions and wider particle size distributions. Possible reasons for this inconsistency are discussed. A theoretical analysis is performed to quantitatively reveal the role of surface radiation and evaporation in the size dependence of the particle peak temperature. The results suggest that at relatively low particle temperatures the size dependence is determined mainly by radiation and that the effect of evaporation becomes more dominant at higher particle temperatures. Moreover, with increasing particle temperature, radiation strengthens the size dependence of the particle peak temperature. On the contrary, evaporation weakens the size dependence at higher temperatures because of the increasing sensitivity of vapor pressure (evaporative heat loss) to the temperature according to Clausius–Clapeyron relation.

Novelty and significance statement

This work presents, for the first time, the simultaneous, in-situ measurements of the initial sizes and time-resolved temperatures of micrometer-sized iron particles burning at elevated gas temperatures. Using current size-resolved measurements, particle-temperature evolutions for several particle diameters are statistically obtained with significantly increased precision. This high-fidelity experimental database over a wide range of operating conditions is a very valuable reference for the iron fuel community to validate and improve numerical models of iron particle combustion. Due to the novel design of the experiment, in this work negative correlations between the initial diameters and the maximum temperatures of isolated burning iron particles are observed and theoretically explained. This result does not align with previous non-size-resolved measurements that have lower temporal resolutions and wider particle size distributions.