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Developing predictive thermal property models for liquids based on microscopic principles has been elusive. The difficulty is that liquids have gas-like and solid-like attributes that are at odds when considering the frameworks of microscopic models: Models for gases are simple due to randomness and low density, whereas models for crystalline solids rely on symmetry and long-range order for easier calculation. The short-range order in liquids does, however, provide structure to neighboring molecules similar to amorphous solids, and there have been recent advances indicating that collective vibrational modes store heat in liquids. Models combining Debye approximations from solid-state physics and Frenkel's theory of liquids can accurately predict the heat capacity of liquids. Phonon-like dispersions in liquids have also been widely observed in neutron scattering experiments. These developments motivate us to propose a model where high-frequency vibrational modes, which travel at the speed of sound and have a mean free path on the order of the average intermolecular distance, conduct heat in liquids. We use this liquid phonon gas model to calculate the thermal conductivity of liquids with varying intermolecular interaction energies from strongest to weakest—Coulomb, hydrogen-bonding, Keesom, and London dispersion energy. Generally, the model is more accurate as the intermolecular interaction energy and density of liquids increase. The calculated thermal conductivity of Coulombic-bound molten sodium nitrate and hydrogen-bonded water is within 1.46% and 2.98% of the experimentally measured values, respectively, across their entire temperature ranges. Further modal analysis of the velocity and the mean free path of collective vibrations could establish the liquid phonon gas model as an accurate model for weakly interacting liquids as well. [ABSTRACT FROM AUTHOR]

High temperature heat transfer fluids like molten salts and molten metals will unlock the higher efficiency and lower cost of next generation grid scale energy sources such as concentrated solar power and advanced nuclear power plants. Their thermal conductivity will help determine how much heat power can be extracted from high temperature systems to do useful work. However, there is a large spread in liquid thermal conductivity data at high temperatures, and well-established, general models of liquid thermal conductivity across liquid classes and temperature ranges are lacking. In this work, we review experimental techniques used to measure liquid thermal conductivity – various steady-state, time-domain, and frequency-domain techniques – and we discuss strategies to minimize errors from convection, radiation, and corrosion that are amplified at high temperature. We classify liquids based on their dominant intermolecular interaction (simple, molecular, coulombic, or metallic) and examine their resulting short-range order that will inform models of heat conduction in liquids. Through the lens of intermolecular interactions and short-range order in liquids, we review previous analytical models of liquid thermal conductivity – modified kinetic gas, quasi-crystalline, and electron dominated models – and we compare their results with reliable experimental measurements of various types of liquids. The results suggest that modified kinetic gas models do not match experimental data for liquids. Quasi-crystalline models can accurately match some available experimental results of molten salts. We explore underlying similarities between various quasi-crystalline models that may be explained by frequency dependent vibrational modes in liquids. Electron transport is the dominant mechanism for thermal conductivity in molten metals. However electrical conductivity measurements cannot be used directly for molten metal thermal conductivity measurement using the Wiedemann-Franz law because the Lorentz number varies with pressure, temperature and metal composition. In addition to analytical models we review molecular dynamics simulations, using equilibrium and non-equilibrium methods. The results show that MD simulations for molten salt thermal conductivity slightly overpredict experimentally measured reference values. These simulations can provide insights into the frequency-dependent behavior of vibrational modes in liquids. Lastly, we discuss future research directions of high temperature liquid thermal conductivity research and provide an outlook for applications for high temperature heat transfer fluids including use in power generation. [ABSTRACT FROM AUTHOR]

• Non-contact technique to handle high-temperature corrosive liquids. • Frequency controlled thermal penetration depth eliminates natural convection. • Front-side sensing and vertical holder orientation minimize salt creeping effect. • Accurate thermal conductivity data obtained for nitrate and chloride salts. • MPR is applicable for in-situ diagnostics of high-temperature thermal system. Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts (NaNO 3 and KNO 3), solar salt (NaNO 3 –KNO 3 mixture), and chloride salt (NaCl–KCl–MgCl 2). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems. [ABSTRACT FROM AUTHOR]