Nusselt number and friction factor correlations for solar air heater duct having protrusions as roughness elements on absorber plate

An experimental investigation has been carried out to study the effect of heat transfer and friction characteristics of turbulent flow of air passing through rectangular duct which is roughened by circular protrusions arranged in angular arc fashion. The roughened wall is uniformly heated while other three walls are kept insulated. The thermal and friction characteristics are governed by duct aspect ratio (W/H), hydraulic diameter (D), relative roughness pitch (P/e), relative roughness height (e/D), arc angle (alpha) and Reynolds number (Re). Experiments encompassed that the Reynolds number ranges from 3600 to 18,100, P/e ranges from 12 to 24, e/D ranges from 0.015 to 0.03 and arc angle of protrusions arrangement ranges from 45 degrees to 75 degrees. The maximum enhancement in heat transfer and friction factor is 2.89 and 2.93 times as compared with smooth duct. These experimental results have been used to develop correlations for Nusselt number and friction factor. (C) 2012 Elsevier Inc. All rights reserved.