Nozzles and Diffusers

A nozzle is a device that is specifically designed to increase kinetic energy of a high-pressure fluid at the expense of its pressure and temperature. A booster rocket is an example of a nozzle.

A diffuser is a device that increases the pressure of a fluid by reducing its speed.

Thus a nozzle and a diffuser perform in an opposite way.

As shown below, the cross-sectional area of a nozzle decreases in the flow direction to increase fluid velocity, while the cross-sectional area of a diffuser increases in the flow direction. 

For nozzles and diffusers, there is negligible change in the potential energy. Since these devices have high velocities, the time spent by a fluid particle within the devices is very short for any significant heat transfer to occur. Thus, the energy equation simplifies to:

Example

Air enters a nozzle steadily at 300 kPa and 77°C with a velocity of 50m/s, and leaves it at 100 kPa and 320m/s. The heat loss from the nozzle is estimated to be 3.2 kJ/kg of air flowing. The inlet area of the nozzle is 100 cm². Determine:

a) The exit air temperature,

b) The exit nozzle area.

Solution