Comparative Performance Evaluation of Acute- and Obtuse-Angled Trapezoidal Solar Air Heater Ducts: A Flow and Thermal Perspective

Abstract This research presents a comprehensive analysis of fluid flow and thermal behaviour in solar air heaters featuring three distinct duct configurations: acute-angled trapezoid, rectangular, and obtuse-angled trapezoid. All ducts share a uniform bounding dimensions of 160 mm wide, 80 mm height and an overall heater length of 1000 mm. A three-dimensional numerical model was established in ANSYS Fluent to simulate thermal and flow characteristics and validated against literature data, standard correlations, and selected experimental results. Simulations were initially conducted with uniform-heat flux of 1000 W/m^2 on the absorber plate, followed by variable heat flux scenarios to represent real-world conditions. Performance indicators, including Nusselt number, friction factor, and bulk air temperature rise via the duct (?Tbulk) were systematically evaluated. The combined performance factor (reflecting the thermal enhancement and pressure drop) and bulk air temperature rise were used to determine the optimal duct geometry. Notably, the obtuse-angled trapezoidal duct offers a geometric advantage by allowing multiple surfaces to incident solar radiation, unlike the other two geometries. This design feature enhances heat absorption and distribution. Comparative analysis revealed that the obtuse-angled duct outperforms the acute-angled and rectangular ducts, with improvements in ΔTbulk of approximately 25.08% and 60.86% during afternoon, and 5.51% and 36.34% during morning hours, respectively. These results demonstrate that the obtuse-angled trapezoidal duct significantly enhances heat transfer while maintaining a moderate pressure drop, establishing it as a viable and more efficient alternative to conventional duct geometries.