A new numerical study published in Engineering investigates the thermal–hydraulic performance of molten salt-based nanofluids flowing inside a novel twisted cloverleaf U-tube, aiming to improve heat transfer uniformity and efficiency in thermal energy storage systems. Molten salt is widely used in thermal energy storage owing to its favorable thermophysical properties and low cost, yet its high viscosity and density often lead to non-uniform flow and local overheating in conventional U-tube heat exchangers. To address these issues, researchers from Southeast University and Henan University of Science and Technology combined a twisted cloverleaf U-tube structure with molten salt-based nanofluids and carried out systematic numerical simulations to evaluate the effects of tube geometry, operating parameters, and nanoparticle types on flow and heat transfer characteristics.

The study employed Hitec salt as the base fluid and tested SiO₂, Al₂O₃, and Cu nanoparticles to form molten salt-based nanofluids. Numerical simulations used the realizable k–ε turbulence model and considered incompressible Newtonian flow under uniform heat flux boundary conditions. Grid independence tests and model validations were conducted against published correlations and experimental data to ensure computational reliability. Results show that the twisted structure generates intensified secondary flow in the U-bend region, promoting mixing between core and near-wall fluids and effectively reducing wall temperature peaks and circumferential temperature differences. Compared with smooth U-tubes, the twisted cloverleaf configuration enhances turbulence kinetic energy and improves the synergy between velocity and temperature gradient fields, which favors convective heat transfer.

Among the three nanoparticle types, Cu-based molten salt nanofluids exhibit the best overall performance, providing considerable heat transfer enhancement with relatively low pressure drop penalties. The team found that inlet velocity and inlet temperature exert stronger influences on flow and thermal performance than heat flux. Higher nanoparticle volume fractions within a reasonable range help increase convective heat transfer coefficients while maintaining acceptable pressure loss levels. Local heat transfer analyses reveal that the local convective heat transfer coefficient rises notably when fluid passes through the U-bend section, and this enhancement becomes more significant with increasing Reynolds number (Re) and nanoparticle loading.

To balance heat transfer enhancement and flow resistance, the research team adopted the response surface method and the non-dominated sorting genetic algorithm II for multi-objective optimization. Optimization results yield a set of Pareto optimal solutions, from which a balanced operating condition is selected for the twisted cloverleaf U-tube with 5% Cu nanoparticles. Under this condition, the device achieves a peak performance evaluation criterion value of 1.21, with controlled maximum wall temperature difference and pressure drop while maintaining a high average convective heat transfer coefficient.

This work provides quantitative references for the design and operational optimization of heat exchange tubes using molten salt-based nanofluids. The combined application of twisted cloverleaf U-tubes and suitable nanofluids offers a practical approach to alleviate flow maldistribution and local overheating in molten salt heat exchangers, supporting more stable and efficient operation in thermal energy storage systems for renewable energy integration and conventional power plant flexibility upgrades.

The paper “Numerical Study of Heat Transfer Performance of Molten Salt-Based Nanofluid in the Novel Twisted Cloverleaf U-Tube,” is authored by Yifan Gui, Yuanqiang Duan, Shuo Zhang, Yu Huang, Minmin Zhou, Lunbo Duan. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.10.010. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.