Research Progress on Thermophysical Properties and Convection Heat Transfer Enhancement of Molten Salts

Molten salts are essential heat transfer and storage media in high-temperature applications such as Concentrated Solar Power (CSP), owing to their high boiling points, low vapor pressures, and excellent thermal stability. The overall performance of such systems is largely governed by the convective heat transfer characteristics of molten salt fluids. This review systematically synthesizes recent advances over the past five years in enhancing the thermophysical properties and convective heat transfer of molten salts, focusing on two primary strategies: improving the intrinsic properties of molten salts through nanoparticle doping, and optimizing the structural design of heat exchangers. The enhancement of thermophysical properties is mainly achieved by preparing molten salt-based nanofluids. Dispersing low concentrations (typically 0.1–1.0 wt.%) of nanoparticles such as SiO2, Al2O3, and carbon nanotubes (CNTs) can yield significant improvements—thermal conductivity increases of up to ~100% (e.g., 0.5 wt% SiO2 in NaNO3-KNO3) and specific heat capacity enhancements of 20–30% (e.g., 1.0 wt% Al2O3 in carbonates). Multiscale simulations, particularly molecular dynamics (MD), have revealed key enhancement mechanisms, including the formation of ordered ionic layers on nanoparticle surfaces that create efficient nanoscale heat conduction pathways, and the modulation of ion–ion interactions. Concurrently, significant heat transfer enhancement can be achieved through structural optimization. Single-method technologies, such as enhanced heat transfer tubes, improve performance by disrupting the thermal boundary layer. For instance, spirally grooved tubes can increase the Nusselt number (Nu) by 19% for Re > 25,000, while twisted tape inserts can enhance laminar flow heat transfer by up to 8.6 times. Composite strategies that couple nanofluids with enhanced geometries demonstrate superior overall performance, with Performance Evaluation Criterion (PEC) values reaching up to 1.48 for converging–diverging tubes with SiO2 nanofluids and 1.21 for trefoil-shaped U-tubes with Cu-based nanofluids. Compact heat exchangers (CHEs) offer high efficiency, achieving PEC values of 1.07–1.4 in optimized designs, but face challenges such as clogging risks in large-scale applications. Future research directions include the development of advanced composite molten salts, the application of artificial intelligence and multiscale simulations for mechanistic analysis and design optimization, the fabrication of novel heat exchanger structures via additive manufacturing, and cross-disciplinary integration for full-chain system optimization. These concerted efforts are essential for realizing efficient, cost-effective, and reliable molten salt-based energy systems.