LES of iron-powder combustion in a jet-in-hot-coflow burner - Insights on flame structure and ignition characteristics

Abstract: Micron-sized iron powders are a rapidly advancing novel energy storage technology. In order to improve the design of real-world iron-powder combustors, adequate understanding of the ignition behavior in such settings is necessary. In this work, a jet-in-hot-coflow (JHC) burner designed by Hameete et al. (2024) to test ignition of iron particles in lab-scale turbulent flames is modeled numerically using LES and Lagrangian point-particles. The JHC burner is simulated in two different modes--an open flame and an enclosed flame--similar to the experimental reference. Two iron-oxidation-rate models--the first-order model and oxide-layer model--are used to examine the effect on capturing the ignition behavior. For the radiative heat transfer between the two phases, a simplified Stefan-Boltzmann approximation model and the P1 model are considered, similar to Ramaekers et al. (2025). Analysis of flame structure indicates ignition in the circumference of the jet, aided by the break-up of the coflow. At higher $T_\mathrm{coflow}$, ignition onset and oxidation completion is earlier prior to jet break-up. Minimum coflow temperature for particle ignition with the oxide layer model is $1125\mathrm{K}$ with complete oxidation at $1250\mathrm{K}$, and for the first-order model at $800\mathrm{K}$ and $900\mathrm{K}$, respectively. Both of these results do not match the experimental results of Hameete. Oxidation degree is predictably higher for enclosed flames. For the chosen particle distribution, the P1 model exhibits higher radiative heat loss and results in a slightly lower oxidation degree. Analysis on particle ensembles with partial oxidation shows that the overall oxidation degree at a sufficient height above the nozzle reflects particle ignition probability. Further analysis in regards to particle size shows ignition failure is more prevalent in larger particles.