Investigation on thermal–hydraulic performance of nitrate molten salt nanofluids in enhanced tube with converging–diverging bulge

Molten salt is popularly applied in solar thermal power generation as high-temperature heat transfer medium. The thermophysical properties of nitrate molten salt nanofluids can be improved compared with that of pure molten salt. In order to improve the heat transfer performance of molten salt nanofluids, a new heat transfer enhanced tube with converging–diverging bulge is proposed. In this study, the convective heat transfer characteristics of molten salt nanofluids in the enhanced tube with converging–diverging bulge are studied numerically. The effects of the height-width ratio ( J ) and length–width ratio ( K ) on heat transfer and flow resistance are investigated. The results show that Nussel number (Nu) increases with the increase of J value, and Nu decreases with the increase of K value. The range of Reynolds number (Re) is 8000–24,000, the range of PEC in enhanced tube is 1.15–1.48. When J = 0.27, K = 2.08, and Re = 8000, the PEC of the enhanced tube is the largest 1.48. The mechanism of heat transfer enhancement is analyzed based on the field synergy principle, and the simulation results are analyzed based on the improved entropy theory. The change trend of heat transfer efficiency and heat transfer entropy production is opposite. With the increase of J , the heat transfer exergy loss number ( N HT ) decrease, the heat transfer effectiveness ( ε ), flow resistance entropy number ( N p ) and pressure drop (Δ p ) increase. With the increase of K , the heat transfer exergy loss number ( N HT ) increase, the heat transfer effectiveness ( ε ), flow resistance entropy number ( N p ) and pressure drop (Δ p ) decrease. The new enhanced tube could be used to promote the further development of molten salt heat exchanger.