When a tube narrows, the same volume occupies a greater length, as schematically shown in Fig. A1 is the cross section of the broader pipe and that of narrower pipe is A2
By the equation of continuity, v2, =(A1/A2)v1
Fig. Speed of fluid increases as it enters a narrower pipe
Since A1/A2 > v2 > v1
For the same volume to pass points 1 and 2 in a given time, the speed must be greater at point 2.
The process is exactly reversible. If the fluid flows in the opposite direction, its speed decreases when the tube widens