Heat transfer, friction factor and thermal efficiency results of air flowing through three sides concave dimple roughened duct

Abstract

The present work is an outcome of experimental investigations conducted upon three sides concave dimple roughened solar collector. The results reveal that three sides roughened solar air heaters have high values of heat transfer compared to one side roughened ones in the range of 25–86 % for varying relative roughness pitch and 21–81% for varying relative roughness height. The rise in friction factor of three sides over one side roughened duct for varying relative roughness pitch and relative roughness height was 11–34% and 15–41%, respectively. Maximum Nusselt number is obtained at a relative roughness pitch of 12 and a relative roughness height of 0.036. Maximum values of friction factor are obtained at a relative roughness pitch of 8 and a relative roughness height of 0.045. Augmentation in thermal efficiency in three sides over one side duct is 44–56 % for a varying relative roughness pitch and 39–51 % for a varying relative roughness height.