Nozzle is said to be an engineering device that can accelerate fluid and fluid velocity of fluid can surely be increased while providing pressure of fluid which gets reduced.
Diffuser is also an engineering device that can decelerate the fluid and fluid velocity can be decreased but pressure of fluid can also be increased.
In case of nozzle, it is important to write steady flow energy equation. Heat energy can also transfer from any system boundary that will be zero and can change potential energy which will also turn out to be zero. The equation is:
The velocity is V1 which is small and it is compared to discharged velocity and also the velocity is available at exit section that appears to be V2.