Performance analysis of roughened solar air heater becomes hectic due to absence of statistical correlation for heat transfer and friction factor. For understanding flow behavior, researchers have developed statistical correlations for different roughness geometries. This paper presents the outcome of experimental investigations upon 1 & 3-sides concave dimple roughened ducts. The results are presented as variation in Nusselt number & friction factor with Reynolds number. The geometrical & flow parameters has been varied as relative dimple pitch (p/e), relative dimple height (e/D-h), relative dimple depth (e/d) & Reynolds number in the range of 8-15, 0.018-0.045, 1-2 & 2000-13500 respectively. Statistical correlations for Nusselt number and friction factor in terms of roughness and flow parameters were derived based on data collected from experiment. Optimized value of flow & roughness parameter yielding maximum performance is determined. The maximum enhancement in Nusselt number for varying relative dimple pitch, relative dimple height & relative dimple depth was respectively of the order of 2.6 -3.55,1.91 to 3.42, 3.09 to 3.94 times and that of friction factor was of the order of 1.62-2.79,1.52 to 2.34 and 2.21 to 2.56 times over those of 1-side roughened ones. (C) 2018 Elsevier Ltd. All rights reserved.