Venturi nozzle

from Wikipedia, the free encyclopedia

A Venturi nozzle (also called a Venturi tube , developed by Giovanni Battista Venturi ) consists of a smooth-walled piece of tube with a narrowing of the cross section , for example by two cones facing each other , which are united at the point of their smallest diameter. A pick-up pipe is placed next to it at this point.

Working principle

The fluid air flows through a venturi nozzle . The liquid in the pick-up tube shows the pressure difference.

When a fluid flows through a Venturi nozzle , the dynamic pressure (back pressure ) is maximum and the hydrostatic pressure is minimum at the narrowest point of the pipe . The speed of the fluid increases in the ratio of the cross-sections when flowing into the narrower part, because the same mass flows through the entire pipe per time (law of continuity ). This reduces the pressure in the pick-up tube, which is located in the narrow part. This creates a differential pressure that can be used in measuring devices or to suck in liquids or gases.

For ideal liquids ( incompressible and frictionless), the pressure difference is given by the Bernoulli equation . In ideal gases which applies advanced Bernoulli equation .

Applications

Venturi nozzles can be found in technology in a large number of applications today because they are low-maintenance and cost-effective. They are used in aquarium technology as skimmers as well as in chemistry as so-called Venturi injectors to dissolve gases in liquids or as sensors for flow velocities of gases or liquids. This principle is also used in oilers in the treatment unit of pneumatic systems. Venturi nozzles are defined as “high-pressure welders” in accordance with DIN 19215 and ISO 5167 as industrial standard parts in mechanical engineering.

Some applications are shown below as examples.

Measuring instruments

The Venturi principle is used to measure the flow velocity of liquids and gases, see Venturi flow measurement . The picture in the section Working principle shows a narrowing glass tube with air flowing in from the right . Their pressure is lowest where the cross-section of the pipe is narrowest or the flow velocity is highest. The manometer measures the static pressures in front of and in the constriction, the difference between which depends on the flow velocity and the air density .
The flow in the Venturi tube with critical flow ("critical flow Venturi", "sonic nozzles") reaches supersonic flow in the area of ​​the point with the smallest diameter, see also Laval nozzle . The mass flow is directly proportional to the inlet pressure. This arrangement can be used as a precise standard for flow rates or as a measuring device. There is an ISO 9300 standard for these nozzles.

Carburetor

LPG venturi nozzle

Another application is in engine construction. In order to operate an internal combustion engine with gaseous ( LPG or natural gas ) or liquid fuel (e.g. gasoline ), it must be mixed with oxygen from the ambient air as an oxidizing agent . The right amount of gas is mixed into the sucked in air flow in a venturi nozzle in front of the throttle valve ( carburettor ).

aviation

Venturi tube on an airplane

The Venturi tube was one of the first devices in motorized aviation that could generate negative pressure. The double funnel was mounted on the fuselage in such a way that it was exactly in the direction of the propeller outflow. With the help of the negative pressure generated in the pipe, the gyroscopic instruments such as the gyro compass (gyro), artificial horizon and turning pointers could be operated. In the early days of aviation, the Venturi tube was also used directly as an airspeed indicator on a trial basis.

The Venturi tube has a decisive disadvantage for aviation. The increase in the flow speed of the air leads to a stronger cooling of the same. Therefore the system tends to ice up very easily (see also carburetor freezing ). This has led to the fact that Venturi tubes are no longer used in aviation today, with the exception of classic cars. They are (German by the suction pump vacuum pump ) and the pitot tube replaced. Nowadays, the turning pointer is mostly driven by a DC motor so that it can continue to display correctly if the suction pump and thus the artificial horizon fail.

Decanting spout for wine bottles

Venturi decanter spout

The Venturi tube is used as a decanting spout for wine bottles to improve the taste of red wine . The system is a special spout that is placed on the neck of the bottle. A constriction in the system increases the flow speed of the wine. As a result of the negative pressure generated in relation to the ambient air, air is sucked in through a channel at the narrowest point of the spout and mixed with the liquid; the pressure is equalized with the formation of bubbles. The resulting enlargement of the surface facilitates the development of flavors and aromas .

Plant protection

Air injector nozzles from crop protection

When sprayers venturi nozzles are used, the drift to reduce the droplet. Air is added to the spray liquid to make the droplets bigger.

Air curtain

Air curtain system in an administration building

With air curtain systems , the curtain can be distributed evenly over the entire width of the system by directing the air flow through a narrow gap.

Exhaust gas cleaning

In the exhaust gas purifying venturis are used for co-deposition of gaseous and particulate pollutants. In or shortly before the cross-sectional constriction, scrubbing liquid is added and atomized by the high gas velocity. Venturi scrubbers are able to separate particles significantly smaller than 1  µm .

Mathematical description

Venturi tube

From Bernoulli's equation:

With

follows

Due to the conservation of mass according to the law of continuity

still applies

This results in the mass flow in the Venturi tube

See also

literature

  • Dieter Schulz, Weisweiler: Laboratory test volume flow measurement in the suction line of a radial fan. Fluid Dynamics Laboratory, Friedberg University of Applied Sciences.
  • Ernst Götsch: Aircraft technology. 4th edition. Motorbuch, Stuttgart 2005, ISBN 3-613-02006-8 .
  • Jeppesen Sanderson (Ed.): Private Pilot Manual. Jeppesen Sanderson, Englewood 1997, ISBN 0-88487-238-6 .
  • Wolfgang Kühr: Technology I. Schiffmann, Bergisch Gladbach 1989, ISBN 3-921270-05-7 ( The private aircraft pilot . Volume 1).

Web links

Commons : Venturi Effect  - collection of images, videos, and audio files

Individual evidence

  1. VDI 3679 sheet 1: 2014-07 wet separator; Basics, waste gas cleaning of particulate matter (wet separators; Fundamentals, waste gas cleaning of particle collections) . Beuth Verlag, Berlin. P. 40.
  2. ^ Hans Güntheroth: Venturi scrubber for separating smoke, mists and aerosols . In: Dust: magazine for dust hygiene, dust technology, keeping the air clean, radioactive suspended matter . 21, No. 9, 1961, pp. 430-434.