Please explain me what will happen to the fluid when it enters to the diverging section of pipe and then to the converging section of pipe

Two principles are important here. First. Continuity equation. Mass flow rate = constant. Therefore as the area of the pipe increases the velocity will decrease. Second principle is Bernoulli's equation. Which is a statement of Conservation of Energy. Now a question for you? How does the pressure change through the transition?

In a C-D nozzle as the flow of fluid or gas converges the flow is compressed and becomes sonic (mach 1) at the throat. Then as the nozzle/pipe diverges the flow will increase in velocity and become supersonic (>mach 1) where temperature and pressure will reduce. For more information look up: Compressible Flow