Heat transfer and friction factor correlations for solar air collectors with hemispherical protrusion artificial roughness on the absorber plate

•The optical path shinning on the dimple and protrusion was simulated by using TRACEPRO software.•The optical path of hemisphere and the spherical cap dimple have also been simulated.•Nu and f and PF have been determined for different values of roughness and operating parameters.•Correlations for Nusselt number and friction factor have been developed. In order to improve the efficiency of solar air collectors, this paper presents a novel solar air collector with hemispherical protrusion/dimple on the absorber plate, and analyses the performance from the two aspects of optics and thermodynamics. For the purpose of enhancing the absorption rate, the optical path shinning on the dimple and protrusion artificial roughness was simulated by using TRACEPRO software. The optical path of hemispherical and the spherical cap dimple was simulated too. The conclusions show that the hemispherical dimple is the best in term of the optics. Then the heat transfer performance of the hemispherical protrusion (back of the dimple) was investigated by experiments. The investigation has covered Reynolds number (Re) ranging from 3000 to 11,000, relative roughness height (e/Dh) from 0.033 to 0.1and relative pitch (p/e) from 3.5 to 5.5. In order to determine the enhancement in heat transfer and increment in friction factor, values of Nusselt number and friction factor have been compared with those of smooth duct under similar flow conditions. Correlations for Nusselt number and friction factor have been developed for solar air collector with such hemispherical protrusion artificial roughness, which can provide reference for the design of this kind of collector.