Experimental Investigation of Triangular Transversely Corrugated Tubes for Solar Air Collectors under Low Solar Irradiance

Solar thermal energy is widely applied in domestic and industrial systems, including solar air-heating units for drying food, raw materials, and finished products. However, efficient utilisation requires collector designs capable of operating effectively under low solar irradiance conditions associated with cloud cover, mist, fog, or precipitation. This study experimentally investigated triangular transversely corrugated tubes for solar hot-air generation under low solar radiation conditions. Comparative experiments were conducted under controlled laboratory conditions using both corrugated and smooth tubes over a Reynolds number range of 500–5500, covering laminar to turbulent flow regimes. All test tubes had a diameter of 50 mm, a groove pitch of 150 mm, and a total length of 1000 mm, while the corrugation depths were 2.5, 5.0, and 7.5 mm. The test section was inclined at 45°, and artificial solar radiation was supplied by a tungsten–halogen lamp at an intensity of 110 W/m². The results showed that, at Re = 1335, corresponding to laminar flow conditions, the tube with e = 5.0 mm and e/p = 0.033 achieved the best thermo-hydraulic performance, yielding an air temperature rise of 3.71 K, a Nusselt number of 4.5, and a Thermal Enhancement Factor of 1.26. This represented a 16.67% improvement over the smooth tube under the same flow condition. These findings demonstrate that triangular transverse corrugation can enhance convective heat transfer under low irradiance and has strong potential for solar drying applications requiring stable and moderate thermal conditions.