Flow and thermal behavior of solar air heater with grooved roughness

A simple and eco-friendly solar air heater (SAH) converts renewable solar energy into useful thermal energy. The implementation of roughness on the absorber plate of a solar air heater improves its air heating capability. The hot air thus produced is used in different applications such as agricultural, dairy, food processing, and cement industries. Towards that, a 2D numerical study is carried out with rectangular groove as a roughness with the RNG k−Ɛ turbulent model. The study has been carried out to find a SAH with higher performance. Two thermodynamic models are adopted to identify and qualify the best channel. The performance of the best SAH channel has been identified by the highest thermal enhancement factor (TEF) and to increase the exergetic ratio (Ɛth). A detailed study of introducing rectangular groove on heat transfer, pressure drop, thermal enhancement factor (TEF) and exergetic ratio (Ɛth) is conducted, by varying the geometrical parameters of rectangular grooves in the range of Re from 7000 to 17000. Geometrical parameters such as relative length (α), relative pitch (β) and relative height (λ) are varied in the range of 0.0067–0.040, 0.0333 to 0.10, and 0.1 to 0.5, respectively, to optimize the performance of the SAH. Higher exergetic ratio is found around the same parametric values where we get the highest TEF value. The maximum TEF is found around 1.455 at α = 0.020, β = 0.0667, and λ = 0.30 at Re of 17000 and the corresponding exergetic ratio is found around 0.9949. The length of the groove is found to augment both heat transfer and pressure drop of the solar air heater due to increased disruption of thermal and hydrodynamic boundary layers. Rate of augmentation of heat transfer decreases in case of α≥ 0.0333 due to formation of an additional stagnant zone inside the groove. The flow and thermal analysis shows that although local heat transfer within the rectangular groove is less, the rectangular groove improves the channel's overall heat transfer. Convective heat transfer coefficient is found relatively higher where there is a change in shear stress gradient.