Use of a Needle Probe to Measure the Thermal Conductivity of Electrically Conductive Liquids at High Temperatures

This paper considers the necessary conditions for using a derivative of a transient hot-wire (termed a needle probe) approach to measure the thermal conductivity of electrically conducting fluids at high temperatures, especially molten halide salts. The focus is on the development of a new theory based on a multi-layer system necessary to ensure electrical isolation of electrical wires from the surrounding fluid. This includes the use of a thin annulus of fluid to minimize convective heat transfer modes within the fluid of interest, which was inspired by the concentric cylinder method. Good measurements require the following considerations: concentricity of the probe and surrounding crucible to ensure a consistent fluid gap, accounting for corrections for deviation of the model at early times, and modeling radiation heat transfer through transparent fluids. Uncertainties are larger than transient hot-wire methods because of the deviations from experimental conditions that can easily match an analytical approximation. An appropriate estimation of the measurement uncertainty can be obtained through careful design of the instrumentation, thorough uncertainty analysis, and limiting the measurements to only the applicable thermal property ranges of the approach. The 1D model used to interpret measured temperature data has been shown to be reliable for thermal conductivity measurements ranging from at least 0.39 W (mK⁻¹) to 0.92 W (mK⁻¹) and for temperatures from 293 K to 1023 K. The approach is used to present thermal conductivity data of the molten salts NaCl–KCl (51–49 mol%) and LiCl–NaCl (72–28 mol%).