An investigation is done in relation to heat transfer enhancement in solar air heaters with circular V-shaped down ribs. Ribs were attached to the absorber plate, having an angle of attack ( = 60 ). The ratio of rib height to hydraulic diameter (e/D) was 0.047, while the range of rib pitch-to-height ratio was (P/e = 4, 8, and 16) mm for three different sheets, with the Reynolds number ranging from (Re = 1825-7759). The roughened wall of the duct is heated by direct sunlight, while the remaining three walls are insulated. The concept of a V-shaped rib was developed to capitalise on this...