Thermal performance analysis of semicircular and triangular cross-sectioned duct solar air heaters under external recycle

The present work is focused on the analytical and numerical investigations related to thermal and thermohydraulic efficiencies of a solar air heater (SAH) with semicircular (Model-I) and triangular (Model-II) cross-sectioned ducts. V-down rib roughness is considered on the bottom side of the absorber plate under the external recycle operation and effect on the thermal performance is analyzed using the analytical investigation. It is observed that the employment of recycle operation substantially improves the thermal performance of these two SAH models as compared to the single-pass operation (without recycle). The external recycle of the air exiting from the SAH outlet increases the heat extraction rate and strengthen the heat transfer coefficient for the air flowing through the roughened SAH duct and further increases the thermal performance. Moreover, SAH models are operated for different hydraulic diameters. Whereas, the best results of thermal performance are obtained at the least value of the hydraulic diameter. It is predicted from the analytical investigation that the Model-I of the semicircular duct with recycle is 3% and 17% efficient in thermal efficiency as compared to Model-II of the triangular duct with and without recycle, respectively, at a hydraulic diameter of 0.06 m. While, the maximum value of the thermal and thermohydraulic efficiencies obtained for Model-I with recycle operation is found to be 75% and 72%, respectively. Although, the numerical investigation is carried out using Ansys-Fluent software to clearly present the associated air heating and pressure drop reduction abilities of both the solar air heater ducts.