Enhancing Solar Air Heater Heat Transfer Performance: The Impact of Hexagonal 90° and 120° Inline Ribs with Varying Blockage Ratios in Trapezoidal Ducts

An experimental investigation was carried out to evaluate the thermo-hydraulic performance of a solar air heater (SAH) equipped with a hexagonal ribbed absorber plate. The study examines the effects of geometric parameters—blockage ratio (e/Dh = 0.1109, 0.1479, and 0.1849), rib pitch (P = 60 mm, 80 mm, and 100 mm), and angle of attack (α = 90° and 120°) on heat transfer and flow dynamics within a trapezoidal duct, over a Reynolds number range of 5000 to 30,000. Trapezoidal ducts were selected for their ability to enhance surface area and flow interaction. For Reynolds numbers ranging from 5000 to 30,000, the Nusselt number for α = 90° inline ribs increased by 20–40%, while the α = 120° inline ribs exhibited a higher enhancement of 40–60%. This improvement in heat transfer was accompanied by an increase in flow resistance. The friction factor for α = 90° ribs was about 1.5–2.0 times higher than the smooth duct, whereas for α = 120° ribs it increased by nearly 2.5–3.0 times. Considering both parameters, the thermo-hydraulic performance factor (TPF) was found to be superior for the α = 120° configuration, ranging from 1.09 to 1.29, compared to 1.01 to 1.11 for the α = 90° case. This suggests that higher angles and blockage ratios enhance turbulence and mixing, improving heat transfer with manageable pressure losses. Overall, the hexagonal ribbed absorber significantly improves thermal efficiency, offering a promising and cost-effective solution for enhancing solar air heater performance while supporting energy efficiency and environmental sustainability.