The energy loss in a contraction can be decreased substantially by making the contraction more gradual. Figure below shows such a gradual contraction, formed by a conical section between the two diameters with sharp breaks at the junctions.
Figure below shows the data for the resistance coefficient versus the diameter ratio for several values of the cone angle.
As the cone angle of the contraction decreases below the resistance coefficient actually increases, as shown in figure below. The reason is that the data include the effects of both the local turbulence caused by flow separation and pipe friction. For the smaller cone angles, the transition between the two diameters is very long, which increases the friction losses.
