Design and performance of a variable gap system for thermal conductivity measurements of high temperature, corrosive, and reactive fluids

•Techniques used for the measurement of thermal conductivity are reviewed.•A steady state instrument designed to measure the thermal conductivity is presented along with the measurement theory.•Tested on helium and KNO3NaNO3 molten salt from 300 °C to 500 °C.•Effects of convection and radiation were mitigated using this technique.•Conductive heat losses will need to be minimized to extend operability above 500 °C. High-temperature fluids such as molten salts, liquid metals, and gasses are being proposed for many advanced energy systems including thermal energy storage devices, concentrating solar plants, and advanced nuclear reactor designs. However, the chemical behavior and thermophysical properties of many of these fluids have not been well characterized, which hinders the design, modeling, safety analysis, and deployment of these systems. Thermal conductivity is a property that is especially limited by existing measurement capabilities, which are subject to errors caused by convection, material interaction, radiative heat transfer, and instrument degradation. Therefore, there is a lack of standard, systematic measurement techniques for high-temperature, reactive, and corrosive fluids. In this work, the development of a variable gap thermal conductivity measurement system is detailed. The system is designed to measure the thermal conductivity of highly corrosive and reactive fluids, and survive operation between 100 °C and 800 °C. The effects of convection are minimized by limiting the thickness of the specimen to thin sizes (