Computational fluid dynamics (CFD) was used to investigate the flow of water and air in smooth and ribbed duct. Temperature was applied for the top and the bottom of the duct where the ribs are located. The heat transfer coefficient were calculate at different location inside the ducts and the results was validated using several heat transfer coefficient correlations that was developed by other researchers. Three shapes of ribs was studied which are rectangular, trapezoidal, and triangle. Three water velocities and three air velocities was studied (0.4, 0.6, and 0.8 m/s), and (0.12, 0.15, and 0.18 m/s), respectively. The heat transfer coefficient increased by adding ribs, it also increased as the velocity of the flow increased.