Evaluating Fluoride Molten Salt Thermophysical Properties with Transient Grating Spectroscopy

Abstract

Accurate molten salt thermophysical properties are required to optimize the efficiency and safety of molten salt-based energy technologies. Unfortunately, existing thermal conductivity data for molten fluorides are plagued by large uncertainties. In addition, the data displays a curious positive temperature coefficient. Understanding of heat transfer in simple fluids suggests thermal conductivity should decrease as a function of temperature. Unaccounted-for contributions from convection and radiation may result in the observation of erroneous positive temperature coefficients. Interestingly, negative temperature coefficients have been observed in non-fluoride salts when measured using Transient Grating Spectroscopy (TGS). This work has shown that convection and radiation are insignificant for most TGS regimes. Therefore, it was hypothesized that TGS measurements of fluorides could finally resolve the discrepancies between theory and experiments.

The design and validation of a first-of-a-kind fluoride salt compatible TGS setup is presented. Demonstration of system performance is achieved through the acquisition and comparison of sound speed and thermal diffusivity data in lithium chloride (LiCl). The system has subsequently been used to measure the thermal conductivity of fluorides (FLiNaK). Results show a flat to slightly increasing thermal conductivity as a function of temperature (0.7086 + 0.0002 ⋅ T(oC) +/- 0.08 W/m-K). In addition to thermal conductivity, sound speed data as a function of temperature (2998 – 1.24 ⋅ T(oC) +/- 27 m/s) has also been obtained for the first time in FLiNaK. The use of accurate sound speed data in theoretical models of thermal conductivity provides better, but not complete agreement with the results from TGS.

The continued existence of a positive temperature coefficient poses more questions than it answers. Three new hypotheses are presented, namely, the influence of vapor pressure in multi-constituent salts, uncertainty surrounding existing heat capacity data, and the validity of neglecting diffusive contributions from theoretical models. Ultimately, this work suggests that fluoride molten salt thermal conductivity is weakly temperature-dependent. Reliable property data has far-reaching consequences for the size of heat exchangers, response to accident scenarios, or more broadly, the safe and efficient deployment of molten salt-based energy technologies.