Energy is the basic ingredient to sustain life and development. Work means moving or lifting something, warming or lighting something. There are many sources of energy that help to run the various machines invented by man.A 3-dimensional CFD analysis has been carried out to study heat transfer and fluid flow behavior in a rectangular duct of a solar air heater with one roughened wall having combination of circularand square transverse wire rib roughness. The effect of Reynolds number, roughness height, roughness pitch, relative roughness pitch and relative roughness height on the heat transfer coefficient and friction factor have been studied.In order to validate the present numerical model, results have been compared with available experimental results under similar flow conditions. CFD Investigation has been carried out in medium Reynolds number flow (Re ¼3800e18, 000). Average Nusselt number increases with an increase of Reynolds number.The maximum value of average Nusselt number is found to be 43.577 for relative roughness pitch of 5 and for relative roughness height of 0.06 at a higher Reynolds number, 10,000. Average friction factor decreases with an increase of Reynolds number. The maximum value of average friction factor is found to be 1.698 for relative roughness pitch of 5 and relative roughness height of 0.06 at a lower Reynolds number, 5000.