CFD simulations are performed in an artificially roughened solar air heater duct with an aspect ratio of 5:1. Results for two surface conditions are presented: smooth and one ribbed configurations. For the roughened solar air heater, the underside of the top wall i.e. absorber plate is roughened with semicircular sectioned transverse ribs having relative roughness height (e/D) of 0.042. The relative roughness pitch (P/e) studied are 7.14, 10.71, 14.29 and 17.86. For each P/e, simulations are performed at six Reynolds numbers from 3800 to 18,000 (relevant in solar air heater). Numerical solutions are obtained using commercial software ANSYS FLUENT. The computations based on the finite volume method with the SIMPLE algorithm have been conducted. The model developed in this study for predicting heat transfer enhancement due to semicircular sectioned transverse ribs showed markedly good agreement with experimental data (±10%). Finally, heat transfer and flow friction showed to be quite dependent on relative roughness pitch (P/e). The maximum Nusselt number ratio (Nur/Nus) ratio observed is 2.11 for the investigated range of parameters.