Question

Why convergent secton of a nozzle should be small in length than a cd nozzle

Answer 1

To conserve mass in supersonic flow where there is a very high velocity, if there is a change in area then there is also a change in density. Hence, it is a compressible flow.

Whereas in subsonic flow, there is a relatively lower velocity and the change in area doesn't largely affect the density. Hence, it is a near incompressible flow. But practically, there is always a small change in the density.