Why divergent portion is empoy in convergent-divergent nozzle

whether
we know that nozzle is passage of constant varying c/s on which kinetic energy increasee,expense of pressure energy..

so why divergent portion,which reduce the velocity of flow.

The use of the divergent portion is different in different types of nozzle.

In a Venturi, which is used for flow measurement, the pressure will drop in the narrow section and be recovered again in the divergent section providing a measurement with minimum disturbance and loss of pressure in the fluid stream.

In a Laval nozzle (such as is used at turbine inlets and on rockets) the flow in the throat section is at > mach 1 (supersonic). In the divergent section the gases then expand and further accelerate, gaining momentum as they leave the nozzle. In hot gases sonic speed can be very high!

I don't understand your reference to "constant varying c/s" so I can't respond to that part of your question.

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