Experimental and computational assessment of the heat transfer exchange in evacuated tube receiver using helical external fins

Abstract

This study examines the performance of solar thermal systems with a longitudinal finned tube, utilizing both experimental and computational fluid dynamics (CFD) methods. The University of Technology in Baghdad manufactured two copper tubes, one with a smooth surface and the other with a longitudinal fin, for the experiments. The research examines thermal efficiency and useful energy extraction across varying flow rates of 2, 3, 5, and 6 LPM. The experimental results show significant differences in the performance of the smooth-surfaced tube compared to the helically finned tube at different flow rates. At 2 LPM, the smooth-surfaced tube shows a gradual increase in useful power from 248.7 W at 8:00 to 1506 W at 13:00, which corresponds to available solar energy to the collectors ranging from 2400 W to 3084 W, resulting in a thermal efficiency between 17 % and 20 %.In contrast, the finned tube has higher Qu values, starting at 442.15 W at 8:00 and 538.7 W at 13:00, while Qs range from 2415 W to 3087 W. Thus, the thermal efficiency of this tube ranges from 10 % to 52 %. Conversely, at higher flow rates, both systems show comparable thermal efficiencies. Comparative analysis between the experimental results and the ANSYS results indicates agreement within a reasonable margin of error, confirming the validity of the computational simulation procedure. The CFD results were consistent with the experimental data, showing a similar trend in thermal efficiency and useful energy extraction for both smooth tubes and finned tubes. A higher temperature difference typically indicates greater heat absorption and heat transfer efficiency, reflecting a more efficient use of solar radiation. The error rate between the experimental and ANSYS results was no >12 %. The study confirms the role of longitudinal fins in improving the efficiency of vacuum tube solar collectors.

Keywords

• Energy conversion
• Helical fins
• Solar thermal systems
• Solar energy
• Thermal performance enhancement