Thermal-Hydraulic Performance and Entropy Generation Analysis of a Square Molten Salt Absorber Tube with Wavy-Tape Insert at Low Reynolds Number

Abstract

During periods of weak sunlight, to maintain the molten salt temperature in a square absorber tube (SAT), the flow is at low Reynolds number (Re), accompanied by low heat transfer (HR) performance. To enhance the HR performance of a square molten salt absorber tube (AE) at low Re, a wavy-tape insert (WET) is first introduced. The performance of square molten salt AE with WET is evaluated under various operational conditions and structural parameters. Results show that the WET generates a substantial swirling flow in the square molten salt AE, which promotes the fluid mixing and then enhances HR. Compared to a smooth SAT, the friction factor (f) for square molten salt AE with WET increases by at least 2.55 times, whereas the Nusselt number (Nu) increases by 1.11–5.77 times. Additionally, the entropy generation rate (EGR) and exergy loss (EL) decrease by 35.54%–82.65% and 43.43%–84.61%, respectively. Furthermore, the transport efficiency of thermal energy (TETE) and exergy efficiency (EE) increase by 6.58%–26.24% and 10.26%–46.13%, respectively. This study may have the potential to offer valuable insights for the design of square molten salt AE at low Re, particularly in terms of improving efficiency.