This study describes an optimum transient flow control condition on heat transfer enhancement around turbulence promotors such as projections and fins mounted in narrow flow passages using 3D-CFD analysis. This paper conducted a relationship between the difference in the flow rate's transient history and the heat transfer performance around a projection installed in a square-cross-sectional duct. The peak velocity of the transient wave pattern strongly affected the peak heat transfer coefficient and heat transfer enhancement behind the rib. On the other hand, the time-averaged relationship between the net heat transfer performance and the pumping power was not dependent on the transient change pattern of the supply flow rate.