The design minimizes energy loss. In the convergent section streamlines are not much disturbed by the accelerating fluid. Beyond the throat it is important not to have significant vena contracta and allow gradual pressure recovery to avoid turbulence and loss of energy. This is achieved by using a 5 to 8 degree slope in this section. If you compare a venturi with an orifice plate this becomes clear. The oriifice plate has a coefficient 0.5 or less.
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