Diffuser (thermodynamics)

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A diffuser is the mechanical device that is designed to control the characteristics of a fluid at the entrance to a thermodynamic open system. Diffusers are used to slow the fluid's velocity and to enhance its mixing into the surrounding fluid. In contrast, a nozzle is often intended to increase the discharge velocity and to direct the flow in one particular direction.

Flow through nozzles and diffusers may or may not be assumed to be adiabatic. Frictional effects may sometimes be important, but usually they are neglected. However, the external work transfer is always assumed to be zero. It is also assumed that changes in thermal energy are significantly greater than changes in potential energy and therefore the latter can usually be neglected for the purpose of analysis.

[edit] HVAC

A round diffuser in an HVAC system

Diffusers are very common in heating, ventilating, and air-conditioning systems.^[1] Diffusers are used on both all-air and air-water HVAC systems, as part of room air distribution subsystems, and serve several purposes:

To deliver both conditioning and ventilating air
Evenly distribute the flow of air, in the desired directions
To enhance mixing of room air into the primary air being discharged
Often to cause the air jet(s) to attach to a ceiling or other surface (Coanda effect)
To create low-velocity air movement in the occupied portion of room
Accomplish the above while producing the minimum amount of noise

When possible, dampers, extractors, and other flow control devices should not be placed near diffusers' inlets (necks), but instead not be used at all or be placed far upstream. They have been shown to drammatically increase noise production. For as-cataloged diffuser performance, a straight section of duct needs serve a diffuser. An elbow, or kinked flex duct, just before a diffuser often leads to poor air distribution and increased noise.

Diffusers may be round, rectangular, or linear slot diffusers (LSDs), for example. This last type takes the form of one or several long, narrow slots (hence the name), often semi-concealed in a fixed or suspended ceiling.

Occasionally, diffusers are used in reverse fashion, as air inlets or 'returns'. This is especially true for LSDs and 'perf' diffusers. But more commonly, grilles are used as return or exhaust air inlets.