Diffuser (thermodynamics)

A diffuser is a slow component in machinery, electricity plant, fan, vehicle, aircraft and shipbuilding, the Gas-/Flüssigkeitsströmungen and increases the Gas-/Flüssigkeitsdruck. It is basically the reverse of a nozzle dar. He further serves to "recovery" of kinetic energy in the pipe flow. So diffusers technically be used to convert kinetic energy into pressure energy. For this purpose, the flow must be delayed. This is achieved generally by a continuous or discontinuous widening of the flow cross-section, which can be realized in various geometric ways.

Technical Description

A diffuser is subsonic always an enlargement of the flow cross-section in the flow direction of the flowing medium dar.

In the aerodynamics of the diffuser, for example supersonic aircraft used to decelerate the engine inlet air in a turbine to subsonic speed as the air, the blades of the rotors and stators may only flow around in subsonic range. For it comes to supersonic speeds at the blades of the rotors and stators in a jet propulsion, tears flow from, suffocates the combustion chamber, the drive will fail. In addition, shock waves travel through the fluid medium and the Rotor-/Statorflügel, which can destroy the engine.

The tapering in the flow direction of the nozzle, which follows the engine, the air then accelerates again to supersonic speed.

The fluid medium itself is in supersonic speed and is to remain in supersonic speed (eg in the air intake of a Pulsertriebwerkes ), then the nozzle in the flow direction must be wide, not tapered. The article die this paradoxical phenomenon is explained.

In hydrodynamics to influence the phenomenon of cavitation applied and in the aerodynamics in the region of the sound, the diffuser is a very complex part to be calculated.

Diffusers at an opening angle of ( supercritical diffuser ) dissipation arises from separation of the flow from the diffuser wall, characterized there is strong turbulence in the transition areas to the dead spaces. In a sudden cross-sectional enlargement () is also called a " Carnot shock loss ", the corresponding diffuser is called jump diffuser. In such a diffuser, the flow comes to a distance of about eight to ten times that of the large diameter back to rest.

The quality of a diffuser is described in the " diffuser efficiency " and the " pressure recovery point ".

Calculation for incompressible fluids (Mach < 0.3)

Relatively straightforward but the effect of a diffuser is in the case of non- turbulent and inviscid flows ( ie, it does not come due to sudden changes in cross section or the like. Vortices and the friction losses of the medium on the walls can be neglected ). Then the simplified Bernoulli equation

This is the so-called static pressure acting on the diffuser Auswände, the density of the medium and its flow velocity. (Note:. Bernoulli equation is simplified to that height differences are not taken into account) As can be seen, the static pressure must decrease when increasing the flow rate.

Since in a pipe must pass through the same volume per unit of time with a variable cross-section at any point, it can be seen that the flow rate is inversely proportional to a cross section to the flow speed in the cross-section the ratio of the cross-sections, so it must be true:

Because of the above ( simplified ) shall also apply Bernoulli's equation:

Both together gives:

Or formed:

That is, with increasing cross-section ( diffuser: ) the pressure increases ( and decreases the flow velocity ) and with decreasing cross-section ( nozzle: ), the pressure decreases ( and increases the flow velocity ).

For very narrow cross-sections or very viscous media in addition the friction losses must be taken into account, as well as with sharply varying cross -sections, the turbulence occurring must be considered ( see next section).

Mathematical Description

In a sudden change in cross section loss figure applies to those defined in the Fluid Mechanics, :

With the loss tangent and the extended Bernoulli set of hydraulic tube (consideration of dissipation) follows:

Energy theorem:

Mass conservation law: for

Technical application

Diffusers are used in fast-moving, in particular supersonic aircraft, in order to achieve a defined gas pressure in the direct suction of the engines. In order to set the optimum conditions, the diffuser is usually designed to move. Aircraft with a particularly large speed range or which depend on long range, have complex diffusers with adjustable dampers and multi- variable cross -sections.

Diffusers are used in motorsport, often at super sports car and occasionally in sports cars. This negative pressure is created under the floor, which forces the vehicle to the ground and thus allow higher cornering speeds and improved driving performance at high speeds. The diffuser has the task to increase the negative pressure under the vehicle back on the ruling behind the vehicle ambient pressure. In many cases, they are in such a diffuser to a ground-level wings.

In hydrodynamics one finds the diffuser eg in pumps and Wasserstrahl-/Jetantrieben as well as fans in aerodynamics.

Also in hydraulic flow of rivers could be delayed in principle with diffusers; but in most cases interfering body are used, such as near the shore the groynes. Another technological application can be found in the use of suction tubes in water turbines.

Further Reading

• Willi Bohl: Fluid Mechanics. Vogel Verlag, Würzburg 1998, ISBN 3-8023-1740-8.
• Heinrich Dubbel ( Lim. ): paperback for mechanical engineering. Springer Verlag, Berlin et al 2005, ISBN 3-540-22142-5.