UC San Diego

The thermal conductivity of liquids is an integral part of the thermal design for future clean energy sources that can provide higher temperature heat and higher thermodynamic efficiencies. Molten salts and molten metals are leading candidates for heat-transfer fluids in next-generation concentrated solar power and nuclear plants, which can provide both clean electricity and industrial heat. However, the thermal conductivity of liquids at high temperature is not well understood since there is no well-established model for liquid thermal conductivity and because errors from convection, radiation, and corrosion have created a large spread in experimental data at high temperatures. In Chapter 1, I review the various steady-state, time-domain, and frequency-domain experimental techniques used to measure liquid thermal conductivity at high temperature, as well as the various modified-gas and quasi-crystalline models of liquid thermal conductivity – rather than providing exhaustive lists of all previous methods, I provide frameworks for understanding the diverse approaches for measuring and modeling liquid thermal conductivity. In Chapter 2, I formulate a new phonon gas model for liquid thermal conductivity, which can accurately calculate the thermal conductivity of dense, strongly-interacting liquids like water and molten nitrate salts. The model is able to match reliable experimental results, explain the mechanism of heat conduction in certain liquids, and encapsulate previous quasi-crystalline models that took slightly different forms depending on which liquids they were formulated for. In Chapter 3, I present a newly developed frequency-domain hot-wire measurement technique – designed specifically to measure the thermal conductivity of high temperature liquids and minimize errors from convection, radiation, and corrosion. Using frequency-domain measurements, I show that the thermal conductivity of molten nitrate salts is ~15% higher than the current reference value. In Chapter 4, I use frequency-domain measurements and various models to better understand ionic liquid thermal conductivity, which have elements of both molten salts and long molecular chained liquids. And in Chapter 5, I provide a brief outlook on future research directions for enhancing the effective thermal conductivity of high-temperature liquids for clean energy applications.