Performance evaluation of a 3D-printed regularly spaced 4-lobed swirl tube in a solar collector

This study experimentally investigates the thermal performance of an evacuated U-tube solar collector retrofitted with regularly spaced 4-lobed swirl tubes manufactured via 3D printing. We modified two sets of evacuated tube collectors (ETCs): one with 3D-printed 4-lobed swirl tubes and the other with plain circular tubes for comparison. To reduce leakage risks, the wall thickness of the 3D-printed tubes was increased from 0.6 mm to 1 mm. Results show that at high flow rates, the swirl ETC enhances thermal performance relative to the circular one. While the swirl ETC achieves a 2 °C higher outlet temperature under turbulent flow, its enhancement is negligible at low flow rates. This discrepancy indicates a need for further research on the flow-rate dependency. At the highest tested flow rate (9 L/min), the swirl ETC exhibits a 15 % higher pressure drop than the plain tube but 3 % higher average thermal efficiency, demonstrating a moderate balance between performance and energy consumption. These findings highlight the potential of regularly spaced 4-lobed swirl tubes to improve ETC efficiency in turbulent flow regimes.