Enhancing the Thermal Performance of a Triangular-Tube Solar Air Heater through Geometric Modifications

The thermal performance of a solar air heater featuring triangular tubes with an impingement-return flow is highly dependent on its geometric design. This study extends prior numerical analysis by systematically evaluating the impact of specific geometrical parameters on the collector's efficiency. A comprehensive stationary 3D model, employing the finite element method (COMSOL Multiphysics), is utilized to simulate conjugate heat transfer, turbulent forced and free convection airflows using the RNG κ-ε model, and heat exchange by surface radiation. The governing parameters under scrutiny are the ratio of the inlet to total cross-sectional area and the vertex angle (which is a function of the height-to-base ratio) of the triangular ducts, whose total cross-sectional area is kept constant. For the studied test cases, numerical findings revealed that decreasing the area ratio from 2/3 to 1/3 improves thermal efficiency by 13%, while decreasing the vertex angle from 100° to 50° improves it by 14%. Under a solar irradiance of 800 W/m² and an air mass flow rate of 0.025 kg/s, the proposed triangular-tube heater demonstrated superior performance, achieving a thermal efficiency of 86%. This compares to 74% for an equivalent circular tube and 46% for the conventional flat-plate collectors.