Heat transfer enhancement plays an important role in areas, such as electronics cooling, compact heat exchangers, power plants, gas turbine airfoil cooling, etc. Modern high-performance gas turbine engines operate at high entry gas temperature, well above the allowable metal temperature. Therefore, highly efficient cooling technologies (film cooling, impingement cooling, and internal cooling) are required for vanes and blades of advanced gas turbines. In this book, the authors are shows the effect of incorporating ribs in the straight channels with variable aspect ratio. The effect of Reynolds number on fluid flow and heat transfer is also investigated.