In situ X-ray absorption spectroscopy and multispectral pyrometry for spatially resolved probing of iron oxide phase evolution and temperature in iron dust flames

This study reports spatially resolved, coupled quantification of iron (oxide) phase evolution and particle temperature in laminar iron dust flames of Bunsen-type using synchrotron-based in situ X-ray absorption spectroscopy (XAS) and multispectral pyrometry. Line-of-sight (LOS) XAS measurements at the Fe K-edge were evaluated by linear combination analysis (LCA) to obtain LOS molar fractions of Fe, FeO, and Fe$_3$O$_4$, which were subsequently reconstructed into local radial distributions using inverse Abel transformation. Simultaneously, multispectral pyrometry provided spatially resolved particle ensemble temperatures, enabling direct correlation between local chemical and thermal states. The coupled measurements reveal a two-step oxidation sequence: conversion of Fe to FeO coincides with the high-temperature reaction front, followed by subsequent formation of Fe$_3$O$_4$ in the post-flame region. With increasing oxygen concentration in the oxidizer from 19 vol.% to 25 vol.% O$_2$, particle temperatures in the reaction front increase from about 2140 K to 2350 K, resulting in nearly complete conversion of Fe within the investigated region. ... mehrVariations in equivalence ratio induce minor changes in iron oxide phase evolution, likely associated with particle loading,
whereas the temperature field shows limited sensitivity within the investigated range. Beyond resolving the spatial coupling of particle temperature and oxidation state, the multimodal diagnostic approach establishes a transferable framework for quantitative, non-intrusive probing of iron dust flames and, combined with previously reported nanoparticle size distributions, provides a data set linking particle temperature, phase composition, and nanoparticle formation for validation and development of predictive models.