Numerical and experimental investigation of hydrothermal performance enhancement of the tube for solar thermal receiver

• Main Author: Hashem Faisal Hashem Shatnawi, Dr.
• Format: text::Thesis
• Language: English
• Published: 2023
• Subjects: Numerical and experimental investigation

In the concentrated solar power plant, the molten salt tube receiver is a key component of the central receiver system in harvesting solar energy. Half of the tube’s circumference is heated with non-uniform heat flux, whereas the other is insulated. The performance of the molten salt tube receiver depends crucially on the design of the receiver, heat transfer medium used, tube material and irradiate heat flux distribution. This research work adopts the hydrothermal approaches for investigating the effect of different receiver designs and heat transfer mediums on the heat transfer performance in the tube flow of the cylindrical solar receiver. Detail parametric studies have also been carried out on the tube of the cylindrical solar receiver which includes tube material, molten salt, tube thickness and heat flux distributions. The present work designs a novel high-temperature and high-efficient solar tower receiver tube using small finned tubes. The new configurations developed are numerically investigated and experimentally validated to assess the effects of various geometrical parameters. Results show that the new design of receiver’s tube enhances the overall thermal performance and strengthens the tubes’ structure. Molten salt is used as the heat transfer fluid (HTF) with the Reynolds number ranging between 14,000 and 38,000. Insertion of square, rectangular, circular and triangular longitudinal fins with various heights (1, 2, 4 and 6 mm) improves the overall efficiency of the heat transfer in the tube flow. An increase in the number of fins improves the heat transfer. The use of four square fins delivers the highest heat transfer enhancement. In using a singular fin, a triangular fin with a height of 1 mm delivers the best heat transfer performance. The triangular fins exhibit a better heat transfer performance for a similar flow rate and hydraulic area than the square, circular and rectangular fins. The results are verified experimentally by inserting longitudinal fins of triangular, circular and square shapes. The experimental study is performed with Reynolds numbers ranging from 28,000 to 78,000. A good agreement is observed between the numerical and the experimental results. In agreement with the numerical results, triangular fins demonstrate the best improvement for heat transfer. The Nusselt number is greater by 3.5% and 7.5%, respectively, compared to the circular and square tube fins for Reynolds numbers near 43,000, but varies up to 6.5% towards Re = 61000. The lowest friction factor is seen in a triangular fin receiver, where it deviates from circular fins by 4.6% and square fin tubes by 3.2%. The peak difference between the Nusselt numbers is ±4%. The maximum difference in Nusselt number measured experimentally and benchmarked against Dittus–Boelter correlation reaches 8%. Finally, the new receiver tube is optimised in terms of the number of fins, heat flux aiming point, HTF, the influence of Al2O3 nanoparticle with molten salt as the base fluid and the type of receiver material. The cosine distribution of heat flux has a higher maximum temperature than the Gaussian distribution by 29% and has a higher receiver efficiency by 102%. The use of Al2O3 nanoparticles with a concentration of 0.5% increases the thermal efficiency by 14% more than that when using pure molten salt. Moreover, adding triangular fins decreases displacement and thermal stress by 6.5% compared with a bare tube.