Micro-explosion of burning iron particles with carbon impurity

Micron-sized iron particles are promising carbon-free fuels, which can provide stable and clear energy by combustion. Micro-explosion, a phenomenon that the burning particle breaks into several fragments, can change the burn rates and size distribution of combustion products. However, the underlying mechanisms are still unclear. In this work, iron particles with carbon concentrations of 0.117 wt% and 0.037 wt% are burnt. Through online observation as well as offline element analysis, two kinds of micro-explosion, i.e. during the diffusion-limited burn time and at the spear point, are observed. Micro-explosions during the diffusion-limited burn time are very likely to be caused by carbon oxidation at the interface of L1(liquid iron)-L2 (liquid iron oxide) when the iron droplet is totally covered by liquid iron oxide. Micro-explosion at the spear point is due to the release of O2 from the supersaturated liquid oxide droplet. Both kinds of micro-explosion are facilitated when the carbon impurity increases. Further experiments indicate that an increasing oxygen concentration (from 10% to 40%) facilitates particle expansion and the occurrence of strong micro-explosion. A high-speed micro-shadowgraphy system of 60k fps is adopted to resolve the particle expansion rate and the intensity of micro-explosion. Both weak and strong micro-explosions are defined in terms of the number of fragments formed during the micro-explosion. Meanwhile, liquid film of varying thickness has been observed before the micro-explosion. Based on these results, the mechanisms of strong and weak micro-explosions are hypothesized. The strong micro-explosion is caused by a violently accelerated motion of the bubble induced by a high differential pressure (Pint−Pint_e) (∼104Pa ), triggering the instability of the liquid shell and developing thickness modulations ultimately piercing it in a number of places. The weak micro-explosion is a result of limited holes formed due to continuing bubble increase as well as the surface flow.