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Thermodynamics: Nozzles & Diffusers

A nozzle is a device that will increase the velocity of a flow at the expense of pressure. For subsonic flows a nozzle will have a wider cross-sectional area at its entrance, that will slope down to a smaller cross-sectional area at its exit. On the other hand if the flow is supersonic, in order for the nozzle to increase the flow velocity the entrance cross-sectional are is smaller than the exit cross-sectional area. Refer to the images below.

Nozzle Diagram

A diffuser on the other hand is the exact opposite of a nozzle, instead of increasing the velocity of a fluid it decreases the velocity of a fluid. In order to do this for subsonic flows the entrance of a diffuser is larger than its exit. While for supersonic flow the entrance is smaller than its exit. Refer to the image below.

Diffuser Diagram

Do to the speed the fluid normally enters and exits a nozzle or diffuser, the rate of heat entering or exiting the boundary layer can be considered zero. The work and potential energy can also be considered zero. Taking these factors into consideration the energy balance of a nozzle or a diffuser can be found by using equation 1. This equation can be modified to fit other terms as needed.

energy balance of a nozzle /  diffuser equation (1)

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