Enhancement of supercritical heat transfer in U-bend by partial curvature

• Published in: The International journal of heat and fluid flow Vol. 107; p. 109401
• Main Authors: Gao, Yumiao, Yang, Genxian, Zhai, Yuling, Wang, Hua, Li, Zhouhang
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.07.2024
• Subjects: Organic Rankine cycle; Partial curvature; Supercritical heat transfer; U-bend; Supercritical heat transfer; U-bend; Partial curvature; Organic Rankine cycle
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2024.109401

•We introduced partially curved, composite U-bend (CU) into supercritical vapor generator of ORC.•Larger curvature in the bending section of CU dramatically improves heat transfer.•The straight section of CU can thermally outperform the corresponding curved section of typical U-bend (TU).•The overall performance of CU is superior to TU when Grq/Grth ≥ 25.•The critical factor determining the overall enhancement of CU is the thermal redevelopment. This study explores the introduction of partially curved, composite U-bends into a supercritical vapor generator of Organic Rankine Cycle. The curved section of composite U-bends is decomposed into two bends with larger curvature, and its thermal performance was numerically studied over a wide range of heat flux, mass flux, and inlet temperature. Results reveal a dual-peak heat transfer pattern that notably improves local heat transfer. Meanwhile in the straight section between two bends, the strong secondary flows induced completely disrupt the original buoyancy-stratified flow, which have almost an identical effect as the thermal development at the tube entrance. The enhancement of heat transfer resulting from the ‘thermal re-development’ is more significant than that caused by the relatively small curvature in typical U-bends. Consequently, the straight section of composite U-bends can thermally outperform the corresponding curved section of typical U-bends, ultimately determining the overall enhancement effect of composite U-bends. Moreover, as buoyancy effect becomes stronger, the re-development effect is also more remarkable. When buoyancy parameter Grq/Grth ≥ 25, composite U-bends demonstrates superior overall performance compared to typical U-bends. Within the scope of this research, the heat transfer enhancement of composite U-bends can reach up to 20 % without increasing material costs or assembly space.