Numerical investigations on the impact of base angle of an acute-angled trapezoidal solar air heater duct on the flow and thermal characteristics under laminar flow environment

This study presents a novel assessment of thermal and hydraulic performance for various solar air heater duct geometries under laminar flow. A generic isosceles acute-angled trapezoidal duct is proposed to systematically compare triangular, trapezoidal, and rectangular shapes. By varying the base angle from 45° to 90°, the geometry transitions through one triangular, four trapezoidal, and one rectangular form. 3D computational simulations were performed using ANSYS Fluent, with a constant heat flux of 1000 W/m2 and Reynolds numbers ranging from 500 to 2000. The simulation results were validated using literature data and selective experiments. The performance metrics include average Nusselt number, friction factor, and rise in air temperature. The primary objective of SAH duct is to achieve high outlet air temperatures. At Re = 500, the triangular duct showed superior performance, with 46.51% improvement in thermal performance over the rectangular duct. A comparative analysis with published data under turbulent flow revealed that the proposed SAH geometries perform better under laminar conditions. After identifying the optimal duct shape, further enhancement was examined by introducing dimple-shaped protrusions on absorber plate. Adding dimples increased Nuavg and favg by approximately 20.45% and 7.97%, respectively, indicating improved heat transfer with minimal hydraulic penalty. This research supports the development of energy-efficient SAHs optimized for laminar flow, ideal for small-scale drying applications requiring high temperatures at low airflow rates. Potential applications include: drying agricultural produce and food processing. The findings offer practical insights into the design of compact, high-performance SAH.