Diffuser and Nozzle

heavily crowded subway station narrow exit gate at peak hour = nozzle

After the gate exit = diffuser.

Replace the human population with molecules.

The mob should be unruly enough (usually they are- especially in the office hours) to push the persons in front (that is the pressure)

sir,can u give a more detailed explanation in terms of internal and kinetic energy of molecules

Just think of the 2 principles (in steady state):

1 - conservation of energy (you can not increase BOTH pressure and velocity without doing work)

2 - conservation of mass (what goes in, must come out)

Since the area in both the nozzel and the diffuser changes with respect to length , and the mass flow rate in equals the mass flow rate out, then the velocity of inlet will differ from that to the outlet,

Using the energy equation on both the inlet and the outlet the side where velocity increases results in a decrease of pressure ,, and vice versa