Heat transfer performance of supercritical R134a in a U-bend vapor generator for transcritical ORC system

Jiangtao Wang, Yuling Zhai, Hua Wang and Zhouhang Li

Energy, 2023, vol. 276, issue C

Abstract: Sufficient knowledge about the bend influence on heat transfer is of great significance to the design of U-bend vapor generator in transcritical Organic Rankine Cycles. In this work, horizontal flow of supercritical R134a in U-bend tubes was numerically investigated. Results show that two typical vortices appeared in the flow field downstream the U-bend as a result of competition between centrifugal and buoyancy forces. With the decrease in mass flux, or the increase in heat flux and tube diameter, the influence of bend on the downstream region becomes more significant. The affecting distance could be more than 70 times the tube diameter in strongly mixed convection with Grq/Grth > 200, with the average enhancement rate of heat transfer soaring up to 42%. As the bend was located at positions with different extents of buoyancy, the rate of enhancement changed from 29% to 49%. Meanwhile, an increase in the bend curvature caused only 7% enhancement. Further details of flow and temperature fields reveal that bend-induced enhanced heat transfer was essentially a synergistic effect of inlet temperature and fading secondary flow. The former was reflected in the mixing effect of the centrifugal force on the temperature field, and the latter was due to the reconfiguration of downstream flow field. Finally, some suggestions were made for the design of vapor generators based on the bend enhancement mechanism.

Keywords: Transcritical ORC; Vapor generator; Supercritical heat transfer; Bend effect; Buoyancy (search for similar items in EconPapers)
Date: 2023
References: View references in EconPapers View complete reference list from CitEc
Citations: View citations in EconPapers (2)

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Persistent link: https://EconPapers.repec.org/RePEc:eee:energy:v:276:y:2023:i:c:s0360544223009143

DOI: 10.1016/j.energy.2023.127520

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