Nozzles and diffusers are commonly utilized in jet engines,
rockets, and spacecrafts. Any fluid-mechanical device designed to accelerate
a flow is called a nozzle and any fluid-mechanical device designed to
decelerate a flow is called a diffuser. For subsonic
flow (velocity under the speed of sound), a converging duct is a nozzle
and a diverging duct is a diffuser.
Nozzle and diffuser are single-stream (one-inlet-one-exit) systems.
If the inlet is denoted by subscript 1 and exit is denoted by subscript
2, the mass balance and energy balance for single-stream steady-flow
There are several common assumptions that are made in the energy analysis
of nozzles and diffusers:
The fluid has high velocity and thus not spending enough time in the
device for any significant heat transfer to take place.
Nozzles and diffusers are properly shaped ducts and no shaft or electric
work can be transferred in or out.
As a fluid passes through a nozzle or diffuser, it experiences a large
change in its velocity. Hence, the kinetic energy change must be
The fluid experiences small or no change in its elevation as
it flows through the nozzle or diffuser.
Summarizing the above analysis, the energy balance for nozzles and diffusers
(h2 - h1)
+ ( v22 - v12)/2 = 0