Thermo-Hydraulic Performance Characteristics and Optimization of Protrusion Rib Roughness in Solar Air Heater

To enhance the thermal performance of solar air heaters (SAHs), protrusion ribs on the absorber are considered to be an attractive solution due to their several advantages. These ribs do not cause a significant pressure drop in the SAH duct and help to enhance the heat transfer to flowing air. On the other hand, a degree of roughness of the protrusion rib on the absorber can be produced by pressing the indenting device without adding additional mass. In this paper, the thermo-hydraulic performances of different roughnesses of the conical protrusion rib on the absorber plate have been evaluated by the mutual consideration of thermal as well as hydraulic performance in term of net effective efficiency. Therefore, an analytical technique has been exploited to predict the characteristics of the net effective efficiency under various operating conditions, such as the flow Reynolds number, temperature increase parameter and insolation. The effects of the conical protrusion rib roughness—namely the relative rib pitch (p/e) and relative rib height e/D) in the ranges of 6–12 and 0.200–0.044, respectively—have been evaluated. The highest value of net effective efficiency of 70.92% was achieved at a p/e of 10 and e/D of 0.0289. The optimization of the rib parameters has been carried out in different ranges of temperature increase parameters for the highest values of net effective efficiency. A unique combination of rib parameters—a p/e of 10 and e/D of 0.044—are observed to lead to the best performance when operating a solar air heater with a temperature increase parameter of more than 0.00789 K·m 2 /W.