Measuring the thermal conductivity of high-temperature liquids is essential for advanced nuclear energy systems, where accurate thermophysical property data support reactor design, safety analysis, and model validation. In this study, we develop and demonstrate a fiber-optic frequency-domain thermoreflectance (fiber-FDTR) technique tailored for in situ measurements in high-temperature liquid environments. We report relative thermal conductivity measurements for liquid gallium up to 350 °C, as well as absolute values for Ga at 45.5 °C, Bi at 280 °C, and Hg at 25 °C. Absolute conductivities were obtained by fitting experimental data to a heat-transfer model, yielding uncertainties of ∼20%, dominated by the precision of the mode-field diameter. Steady-state thermoreflectance measurements provided relative thermal conductivity curves for gallium with excellent repeatability and low uncertainty (3.8%). These results establish fiber-FDTR as a flexible, high-throughput approach for characterizing thermophysical properties of high-temperature liquids, particularly in geometries where optical access through windows is impractical. The technique shows strong potential for extension to molten salts and other harsh liquid systems.
Rizzuto et al. (Sun,) studied this question.