Frequency-Domain Hot-Wire Measurements of Molten Nitrate Salt Thermal Conductivity

Abstract

The thermal conductivity of liquids has traditionally been determined by measuring either the steady-state temperature gradient or time-dependent temperature rise in a sample due to an applied heat flux. The transient hot-wire method has become the standard for measuring liquid thermal conductivity; in this method, direct current Joule heats a long, thin wire immersed in a liquid sample, and the measured temperature rise over a particular time interval is fit to a heat transfer model to extract the thermal conductivity of the liquid. However, in the high-temperature environments of molten salts, long metal wires are vulnerable to corrosion, and convection errors are magnified in larger volumes of molten salts. Here, we utilize a new frequency-domain, corrosion-resistant, short hot-wire technique (sensor and model) to measure the thermal conductivity of molten NaNO3 (2.78% standard uncertainty), KNO3 (2.38% standard uncertainty), and a solar salt mixture (2.66% standard uncertainty). Operating in the frequency domain and utilizing a full 3D model allows the use of short sensor wires, reducing the total contact area to minimize corrosion, and the use of small sample volumes, mitigating convection errors. An alternating current at varying frequencies Joule heats a short platinum wire sensor (6.5 mm), which is protected by a 1.6 μm-thick Al2O3 coating, submerged in molten salt. The frequency-dependent temperature response of the sensor surrounded by salt is measured and fit to a 3D thermal model to obtain the thermal conductivity of the molten salt. Together, the new sensor design and 3D model keep the probed molten salt volume below 1 μL. Our frequency-domain measurements show an ∼11 to 15% higher thermal conductivity of molten nitrate salts than the current reference correlations made from steady-state and time-domain measurements.

Topics & Concepts