Thermal conductivity prediction of molten salt-based nanofluids for energy storage applications

Molten salt-based nanofluids (nano-salts) form an important class of thermal fluids that can act as both heat transfer and thermal energy storage media for high temperature applications, including solar thermal systems. Among these, the class of binary nitrate salts (60:40 wt. % NaNO3:KNO3) and their mixtures with metal oxide nanoparticles are the most prominent. This study evaluates the stability of nano-salts using a computational technique based on Lagrangian particle tracking, with the model considering the motion of solid nanoparticles suspended in a molten fluid. The technique enables various multiscale forces, with different characteristics, to be established. The system considered consists of 25-71 nm Al2O3 ceramic nanoparticles at volume fractions ranging from 1.0 to 5.0% suspended in fluids of different density ratios, with homogeneous temperature distributions from 250-600 °C. The simulation results demonstrate the effectiveness of the technique, with predictions elucidating the role of oscillatory structural, Brownian motion and particle collision forces, and their influence on the enhancement of thermal conductivity. The liquid structuring of salt melts around the embedded nanoparticles is found to play a key role in the nano-salts' thermal behaviour, with predictions in agreement with previous theoretical and experimental studies. The outcome of this research forms the basis for the potential use of nano-salts in solar thermal systems.