Correlations for solar air heater duct with dimpled shape roughness elements on absorber plate

► An experimental study has been carried out on roughened solar air heater. ► Nusselt number and friction characteristics for dimple shaped roughness element. ► Correlations for Nusselt number and friction factor have also been developed. An experimental investigation has been carried out for a range of system and operating parameters in order to analyse the effect of artificial roughness on heat transfer and friction characteristics in solar air heater duct which is having dimple shaped elements arranged in angular fashion (arc) as roughness elements on absorber plate. Duct has an aspect ratio (W/H) of 11, relative roughness pitch (p/e) range of 10–20, relative roughness height (e/Dh) range of 0.021–0.036, arc angle (α) range of 45–75° and Reynolds number (Re) ranges from 3600 to 18,000. A considerable increase in heat transfer and friction loss has been observed. The experimental data have been used to develop Nusselt number and friction factor correlations as a function of roughness parameters and operating parameters.