Resolved simulations of single iron particle combustion and the release of nano-particles

Abstract

We present a numerical study of the combustion of single iron particles in an O2–N2 atmosphere. By resolving the full boundary layer, mass and heat transfer are accurately modeled, including Stefan flow. Only the conversion of Fe to FeO is taken into account and evaporation is implemented to investigate the formation of nano-sized iron-oxides products. Temperature- and composition-dependent heat capacity and density are used and phase transitions from solid to liquid (and vice-versa) are accounted for by the apparent heat capacity method. The model is validated by comparing the time to maximum temperature (tmax) and the maximum temperature (Tmax) of a 40 and 50 µm particle in an O2–N2 atmosphere with experiments. The current model, which assumes infinitely fast internal transport, can well estimate the maximum particle temperature and the time to reach this maximum temperature, but it does not capture the particle size effect on the maximum temperature. Even though the particle temperature stays below its boiling temperature, a small but non-negligible amount of mass is lost due to evaporation of the particle. Evaporation of the particle and oxidization of the gaseous Fe-containing species in the boundary layer limit the maximum temperature of the particle when increasing the oxygen concentration. By means of a sectional model, the formation and growth of the iron oxide nano-particles is numerically investigated. It is shown that somewhat further downstream of the particle, the volume fraction of nano-particles attains a maximum. An average nano-particle diameter of 40 nm is found at t/tmax=1 in the wake of a 50 µm iron particle burning at 21% oxygen concentration.