Why for subsonic flow the cross-sectional area of nozzle decreases?
For an incompressible fluid with a single inlet and exit the conservation of mass equation simply reduces to volumetric flow in equals volumetric flow out. It is then immediately clear that with a change in area from the inlet to the outlet, there is a corresponding change in velocity.
The common sense answer is that if you want to put the same amount of constant density material through an area of different sizes, then you'll have to use different rates.
I consider the more confusing (and interesting) question to be why a supersonic nozzle is divergent.
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