Metal duct or channels of rectangular cross section with black selective coating has an application in solar air heaters. Increase in heat transfer rate is achieved by suitably modifying the absorber surface with fins, corrugations, honey comb structure etc. In the present article numerical study is performed using computational fluid dynamics (CFD) on a rectangular metal duct having transverse square wire rib roughness of small diameter created artificially on the absorber surface. The design parameters of the analysis are the flow Reynolds number, hydraulic diameter roughness pitch and height. The 3D analysis is performed using ANSYS FLUENT 12.1 code with Renormalization-group (RNG) k-e model having the mesh model under five lakh nodes to investigate the heat transfer and fluid flow characteristics. The results of smooth duct are compared with results from literature and found to be in good agreement. The ratio of increase in heat transfer to friction factor referred as energy gain ratio with the use of artificial roughness is estimated for range of parameters and optimum performance condition is arrived. The maximum value of energy gain ratio is about 1.31 times more compared to the plain duct for the range of operating parameters investigated. © 2006-2014 Asian Research Publishing Network (ARPN).