Critical nanoparticle formation in iron combustion: single particle experiments with in-situ multi-parameter diagnostics aided by multi-scale simulations

In practical iron combustion systems, the formation of iron oxide nanoparticles (NP) presents challenges such as efficiency penalties and fine dust emissions, necessitating a deeper understanding of the underlying formation mechanisms and critical thermochemical conditions. This study, utilizing both experiments and multi-scale simulation tools, investigates the NP clouds generated by single iron particles burning in high-temperature oxidizing environments. The ambient gas conditions were provided by a laminar flat flame burner, where the post-flame oxygen mole fraction was varied between 20, 30, and 40 vol%, with a constant gas temperature of approximately 1800 K. In the experiments, high-speed in-situ diagnostics were employed to simultaneously measure particle size, NP release onset, NP cloud evolution, and the surface temperature history of the microparticles. The setup involved three 10 kHz imaging systems: one for two-color pyrometry and two for diffuse backlight-illumination (DBI), specifically targeting microparticle size and nanoparticle cloud measurements. The study demonstrates the powerful capabilities of these multi-physics diagnostics, allowing for precise quantification of NP release onset time and temperature, which were found to depend on both particle size and ambient oxygen mole fractions. Detailed CFD simulations revealed that the enhanced convection velocity, driven by increased Stefan flow, transports NPs towards the parent iron particles, particularly under high-oxygen conditions. This phenomenon delayed the appearance of detectable NP clouds, leading to higher apparent microparticle temperatures at the onset of NP release. This insight complemented the experimental observations, providing a more comprehensive understanding of the observed NP-cloud release temperature that increases with higher oxygen levels. Further analysis through molecular dynamics (MD) simulations uncovered potential reaction pathways of precursor formation and nanoparticle agglomeration. The MD simulations showed that the initial temperature significantly influenced the amount of gas-phase precursors and subsequently the composition of the resulting nanoclusters, with Fe(II) predominating at higher temperatures and Fe(III) at lower temperatures. This integrated approach combining experiments and numerical analysis not only advances our understanding of NP formation in iron combustion but also provides valuable insights into the thermochemical conditions that dominate nanoparticle characteristics.