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Diffuser (thermodynamics)
Encyclopedia
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.
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.
Diffusers are very common in heating, ventilating, and air-conditioning
systems. Diffusers are used on 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. They have been shown to dramatically 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, textile 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, grille
s are used as return or exhaust air inlets.
Open system
Open system may refer to:*Open system , one of a class of computers and associated software that provides some combination of interoperability, portability and open software standards, particularly Unix and Unix-like systems...
. Diffusers are used to slow the fluid's velocity and to enhance its mixing into the surrounding fluid. In contrast, a nozzle
Nozzle
A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits an enclosed chamber or pipe via an orifice....
is often intended to increase the discharge velocity and to direct the flow in one particular direction.
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
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....
are significantly greater than changes in potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
and therefore the latter can usually be neglected for the purpose of analysis.
HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
systems. Diffusers are used on both all-air and air-water HVAC systems, as part of room air distribution
Room air distribution
Characterizing how air is introduced to, flows through, and is removed from spaces is called room air distribution. HVAC airflow in spaces generally can be classified by two different types: mixing and displacement.-Mixing systems:...
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, taking advantage of the Coandă effectCoanda effectThe Coandă effect is the tendency of a fluid jet to be attracted to a nearby surface. The principle was named after Romanian aerodynamics pioneer Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development....
- To create low-velocity air movement in the occupied portion of room
- Accomplish the above while producing the minimum amount of noiseNoiseIn common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
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. They have been shown to dramatically increase noise production. For as-cataloged diffuser performance, a straight section of duct
Duct
A duct may refer to:* Duct , various ducts in anatomy and physiology* Duct , for transfer of air between spaces in a structure* Duct tape, a kind of adhesive tape* Ducted fan, motor for aircraft...
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, textile 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, grille
Grille
A grille or grill is an opening of several slits side by side in a wall or metal sheet or other barrier, usually to let air or water enter and/or leave but keep larger objects including people and animals in or out.-Spelling:In the United States, "grille" is used to differentiate the automotive...
s are used as return or exhaust air inlets.
See also
- Air conditioningAir conditioningAn air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
- ASHRAE
- SMACNA
- ACCAAcca-ACCA as an abbreviation:ACCA may refer to:In Biology* Acetyl-CoA carboxylaseIn Business:* American Corporate Counsel Association, now Association of Corporate Counsel , is a group for attorneys who practice in the legal departments of corporations, associations and other private-sector...