In-situ measurement of spectral emissivity of aluminum flame combustion products using saturated atomic lines of alkali metals

Aluminum combustion occurs at extremely high temperatures, exceeding those of typical hydrocarbon flames by over 1000 K. Due to high temperatures and the condensed nature of metal combustion products, radiation heat transfer is often expected to play a dominant role in aluminum flames, as predicted by many theoretical models. Paradoxically, however, no definitive experimental evidence has confirmed these predictions. To reconcile theory with reality, accurate radiation heat transfer models require reliable radiative properties of combustion products at high temperatures and for sub-micron particle sizes. However, direct measurement of the emissivity of high-temperature aluminum combustion products in flames is particularly challenging and requires complex calibration procedures. This paper introduces a novel method for measuring the absolute spectral emissivity of high-temperature combustion products in-situ . The method is calibration-free and instrumentally simple, and is based on the comparison of the intensity of the continuum spectra emitted by the condensed combustion products and the intensity of saturated atomic lines of alkali metals. The intensity of the saturated atomic lines reaches the intensity of a blackbody, and depends only on temperature. Line saturation is achieved by adding less than 2% of the corresponding alkali metal chloride (LiCl, NaCl, KCl, RbCl, CsCl) to powdered aluminum fuel. Measurements of the emissivity of the condensed products from aluminum Bunsen dust flames within the 589–852 nm spectral range reveal strikingly low spectral emissivity values on the order of ϵ λ ∼ 0.02–0.03, which is more than an order of magnitude below values typically assumed in most theoretical models. The obtained results align with Mie theory predictions for an optically thin aluminum oxide nanoparticle cloud. The potential applications of the proposed diagnostic technique extend well beyond metal combustion—for example, to non-reactive particles seeded to hydrocarbon flames, flame-synthesized nanoparticles, and dusty plasmas—and can be leveraged not only to determine spectral emissivity of condensed-phase species, but also the temperature of the gas medium. Novelty and significance statement This paper presents a novel calibration-free technique to measure the absolute spectral emissivity of the condensed-phase species in flames. The new technique is used to measure, for the first time, the absolute spectral emissivity of high-temperature aluminum oxide combustion products. These measurements are needed for theoretical modeling to properly quantify the role of radiation heat transfer in metal flames, and rectify the apparent disconnect between theoretical predictions and available experimental data. The derived insights elucidate many common misconceptions regarding the optical properties of the condensed emitters in aluminum flames, which have far reaching implications in metal flame modeling and pyrometric measurements.