Influence of hybrid nanoparticle additives on the thermal conductivity and dispersion stability of molten salts

Hybrid nanofluid, a new fluid engineered by dispersing two or more different nanoadditives into conventional heat transfer fluid, possesses many interesting properties and has been considered as a promising next-generation heat transfer medium in engineering applications. The thermal conductivity and dispersion stability of molten salt-based hybrid nanofluids containing SiO2, MgO and Al2O3 nanoparticles were studied using molecular dynamics simulations. The heat conduction mechanisms were analyzed from the aspects of aggregation and non-aggregation. The results show that Al2O3-SiO2 and Al2O3-MgO nanofluids in the non-aggregated states show a decrease in thermal conductivity in the temperature range from 525 to 725 K, up to approximately 9.2 % and 23.3 % lower than that of Al2O3 nanofluid. In contrast to the hypothesis proposed in experiments, the lower thermal conductivity of hybrid nanofluids cannot be explained by the solid-liquid interfacial thermal resistance and the averaging of thermal conductivity values of different nanoadditives. Based on the analysis of contributions of different heat flux fluctuation modes, and energy fluctuations of ions, the correlation between the potential energy and the reduced thermal conductivity of hybrid nanofluids in the non-aggregated states is highlighted. Moreover, the dispersion stability of Al2O3-SiO2 and Al2O3-MgO nanofluids is higher than the Al2O3 nanofluid. The improved dispersion stability is related to the lower potential energy, the lager adsorption layer around the nanoparticles and the weaker Brownian motion of nanoparticles. The contradictory conclusions on heat conduction of hybrid nanofluids are attributed to the opposite effects of the enhanced dispersion stability based on the aggregated states and the reduced heat conduction based on the non-aggregated states. This study provides guidance on the selection of nanoadditives and the design of molten salt based nanofluids.