Thermo-hydraulic analysis of rhombus-shaped solar air heater ducts with varying interior angles

Abstract

This study examines the thermo-hydraulic performance of rhombus-shaped solar air heater (SAH) duct, focusing on the effect of interior angle, varied from 40° to 140°. Eleven duct configurations are analysed using computational fluid dynamics (CFD) simulations in ANSYS Fluent. The novelty lies in exploiting the rhombus geometry: when oriented on its vertex, both upper surfaces act as absorbers, allowing greater solar exposure. Since solar intensity swings with time of day, non-uniform heat flux combinations are applied to the two absorber surfaces to replicate real conditions. Key performance indicators include Nusselt number, rise in air temperature across the duct, and friction factor, across Reynolds numbers from 5000 to 28,000. A few selected computational results are validated with empirical correlations and experimental data. Among all the duct variants, the rhombus duct with 90° interior angle shows the best performance. At Reynolds number 28,000, this duct achieves maximum enhancements: Nusselt number increases by 17.24% and friction factor by 7.3% compared to 40° interior angle. The rise in air temperature enhanced by 60.2% from morning to afternoon for this duct. Compared with a square duct, this best rhombus duct delivers up to 1.85 times higher rise in air temperature, particularly in afternoon conditions. Literature shows most SAH research stress on triangular and rectangular ducts, with little focus on rhombus geometries. Conventional designs expose only one absorber plate, achieving peak performance near mid-day and underperforming in morning and evening. This work demonstrates that selecting an optimal rhombus angle enhances heat transfer with moderate pressure drop, making rhombus ducts, a promising option for efficient SAH designs.