Effects of Fins Base Rounding on Heat Transfer Characteristics of Absorber Tube of Parabolic Trough Collector

In this study, the heat transfer characteristics of an improved absorber tube of parabolic trough solar collector LS-2 are investigated using ANSYS software. Oil syltherm 800 type is used as a heat transfer fluid. Three types of absorber tubes are tested; the first is a smooth tube and the others are finned tubes. One of the rough cases is rectangular cross-section fins with rounding at fin base and the other does not have rounding at fin base. Simulations are performed with fin thickness variations of 2, 4 and 6 mm. The fin lengths change at 5, 10, 15, 20 and 25 mm. The radii of rounding are 2, 3 and 4 mm and angles between fins are 45° and 90°. Wide range of operating parameters is considered, such as inlet fluid temperatures (300:600 K), flow rate (6:24 m3/h) and direct normal irradiance (500:1000 W/m2). The thermal efficiency, Nusselt number and thermal enhancement index are calculated under different operating conditions. The results show that thermal enhancement index of the fin with round edge radius of 4 mm is higher than that of the fin with the sharp edge (R = 0 mm) by about 10.74% under the considered conditions. The thermal enhancement index of the round edge fins with a length of 25 mm is nearly 25.6% higher than that of the round edge fins with a length of 5 mm. At a fin thickness of 6 mm, the thermal enhancement index of the round edge fin is nearly 7.8% higher than that of the sharp edge fin. At 45° angle and 25-mm fin length, the thermal enhancement index for round and sharp-edged fins is 1.644 and 1.532, respectively. When the inlet fluid temperature increased from 300 to 600 K, the heat enhancement index value increased by 14.57%; as the flow rate increased from 6 to 12 m3/h, the heat enhancement index value decreased by 11.63%. The thermal enhancement index increased from 1.265 to 1.359 as the direct normal irradiance varied from 500 to 700 W/m2.