A diffuser is "a device for reducing the

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

and increasing the

static pressure In fluid mechanics the term static pressure refers to a term in Bernoulli's equation written words as ''static pressure + dynamic pressure = total pressure''. Since pressure measurements at any single point in a fluid always give the static pres ...

of a fluid passing through a system”. The fluid's static pressure rise as it passes through a duct is commonly referred to as pressure recovery. In contrast, a

nozzle A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe (material), pipe. A nozzle is often a pipe or tube of varying cross ...

is used to increase the discharge velocity and lower the pressure of a fluid passing through it. Frictional effects during analysis can sometimes be important, but usually they are neglected. Ducts containing fluids flowing at low velocity can usually be analyzed using

Bernoulli's principle Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...

. Analyzing ducts flowing at higher velocities with

Mach number The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a boundary to the local speed of sound. It is named after the Austrian physicist and philosopher Erns ...

s in excess of 0.3 usually require

compressible flow Compressible flow (or gas dynamics) is the branch of fluid mechanics that deals with flows having significant changes in fluid density. While all flows are compressibility, compressible, flows are usually treated as being incompressible flow, incom ...

relations. A typical subsonic diffuser is a duct that increases in area in the direction of flow. As the area increases, fluid velocity decreases, and static pressure rises.

Supersonic diffusers

A supersonic diffuser is a duct that decreases in area in the direction of flow which causes the fluid temperature, pressure, and density to increase, and velocity to decrease. These changes occur because the fluid is compressible.

Shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

s may also play an important role in a supersonic diffuser.

Applications

Diffusers are very common in heating, ventilating, and air-conditioning systems.''Designer's Guide to Ceiling-Based Air Diffusion'', ASHRAE, Inc., Atlanta, GA, USA, 2002 Diffusers are used in 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, taking advantage of the Coandă effect * To create low-velocity air movement in the occupied portion of room * Accomplish the above while producing the minimum amount of

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

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 can be as a shape of round, rectangular, or can be as linear slot diffusers (LSDs). E.g., linear slot diffusers take the form of one or several long, narrow slots, mostly semi-concealed in a fixed or suspended ceiling with

airfoil An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more Lift (force), lift than Drag (physics), drag. Wings, sails and propeller blades are examples of airfoils. Foil (fl ...

s behind the slots directing the airflow in the desired direction. Occasionally, diffusers are mostly used in a reverse fashion, as air inlets or returns. This is especially true for a linear slot diffuser and 'perf' diffusers. But more commonly, grilles are used as return or exhaust air inlets.

References

{{Authority control Ventilation Building engineering

Supersonic diffusers

Applications

See also

References