Numerical study probing the effects of preferential concentration on the combustion of iron particles in a mixing layer

The iron power cycle is a novel carbon-free energy storage technology that has seen considerable advancement over the past few years. The design of large-scale industrial iron powder combustors relies on a good understanding of not only the combustion process of Fe particles but also the interaction of the burning particles with complex turbulent flow structures. Preferential concentration is one such effect observed in turbulent particle-laden flows, that clusters particles into regions of high particle concentrations. To simulate such phenomena of particle-flow interactions for reacting Fe particles, we first establish a numerical framework based on a coupled Eulerian–Lagrangian approach and a switch-type Fe combustion model. A mixing layer is chosen as the canonical flow scenario to simulate particle-flow interactions. In the present work, the effects of preferential concentration on different particle sizes d_p=14,20,28,39,55µm are captured and examined. The smaller particles with d_p≤20µm retain the structure of the mixing layer whereas the larger particles d_p≥28µm perturb the mixing layer and significantly alter the imposed flow structure. In the present work, we use minimum spacing δ_min, normalized by the mean interparticle distance δ̄, to quantify particle clustering through preferential concentration. In the cases with sufficiently larger particles (d_p≥28µm), particles with longer burn times τ_B statistically exhibit lower values of minimum spacing, indicating particle clustering which results in the localized depletion of O₂. A comparison of minimum spacing with simulations of inert particles shows a deviation in the mean and mode of minimum spacing for 39µm particles that coincide with the overall combustion burnout times. This deviation is attributed qualitatively to the modification of the particle relaxation and flow timescales as a consequence of particle combustion. Further analysis to quantify the timescales involved in Fe combustion might be beneficial in achieving deeper insight into this deviation.