Diffuser (thermodynamics)

A diffuser is "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system”.

In contrast, a nozzle is used to increase the discharge velocity and lower the pressure of a fluid passing through it.

As the area increases, fluid velocity decreases, and static pressure rises.

[3] Diffusers are used in both all-air and air-water HVAC systems, as part of room air distribution subsystems, and serve several purposes: When possible, dampers, extractors, and other flow control devices should not be placed near diffusers' inlets (necks), either not being used at all or being placed far upstream.

An elbow, or kinked flex duct, just before a diffuser often leads to poor air distribution and increased noise.