Venturi effect

A venturi (also venturi developed by Giovanni Battista Venturi ) comprises a smooth-walled pipe section with a constriction of the cross section, for example by means of two oppositely directed cones which are joined at the location of its smallest diameter. At this point, besides a decrease tube is placed.

Action principle

Flows through the venturi nozzle, a gaseous or liquid medium, is minimum at the narrowest point of the tube, the dynamic pressure (dynamic pressure ) and the maximum static pressure. The speed of the flowing gas (or liquid) increases in proportion to the cross-sections of the passage through the constricted portion, as all passes through the same amount. Simultaneously, the pressure loss in the pipe, which sits right in the narrow part decreases. This creates a differential pressure, which is then used in various measuring instruments and for aspirating liquids or gases.

The differential pressure is given with fluids ( incompressible and without friction) by the Bernoulli equation. For ideal gases, the extended Bernoulli equation. The flow reaches the speed of sound, the venturi is of the Laval nozzle.

Applications

Venturi nozzles can be found today in the art in a variety of applications because they work maintenance and cost. They are used in the aquarium skimmer technology as well as in the chemical industry as so-called Venturi injectors to dissolve gases in liquids or as sensing devices for flow rates of gases or liquids. Moreover, this principle is used in the preparation unit oilers pneumatic systems. As industrial standard parts of mechanical engineering Venturi nozzles are defined as " Einschweißdruckgeber " according to DIN 19215 and ISO 5167.

In the following can be exemplified some applications.

Measuring instruments

The venturi principle is applied for measuring the flow rate of liquids and gases, see venturi flow metering. The picture in the section Working principle shows a narrowing glass tube with inflowing air from the right. The pressure is at its lowest, in which the cross-section of the tube at its narrowest and the flow velocity is highest. The gauge measures the static pressure before and in the constriction, the difference is dependent on the flow velocity and the air density.

Carburetor

Another application is found in the engine. An internal combustion engine with gaseous ( LPG or natural gas) or liquid fuel to be operated (for example, gasoline), it must be mixed with oxygen from ambient air as the oxidant. The gas is in a venturi nozzle before the throttle valve in the right amount mixed into the intake airflow system ( carburetor ).

Aviation

The venturi was one of the first devices in the motor flying, with which one could create a vacuum. The double funnel was on the aircraft fuselage mounted so that it lay exactly in the downstream direction of the propeller. With the help of the negative pressure generated in the pipe then artificial horizon and turn indicator could gyro instruments such as Gyro (gyro ) are operated. In the early days of aviation, the Venturi tube was also tentatively used directly as the airspeed indicator.

The venturi tube has a significant disadvantage for flying. Increasing the flow velocity of the air leads to a greater cooling thereof. Therefore, the system tends easily to the icing (see also carburettor icing ). This has led to the venturi tubes are no longer used, except for vintage cars today in aviation. They have been replaced by the suction pump ( German vacuum pump ) or the pitot tube. The turn indicator is today usually driven by a DC motor in order to further correct display on failure of the suction pump and thus of the artificial horizon.

Dekantierausguss for wine bottles

As Dekantierausguss for wine bottles the venturi is to improve the taste of red wine use. The system is a special spout, which is placed on the bottle neck. A constriction in the system increases the flow rate of the wine. By the thus generated negative pressure relative to the ambient air, air is sucked through a channel at the narrowest point of the nozzle and mixed with the liquid; there is a pressure equalization with bubbling. Elaboration of the resulting increase in the surface facilitates the development of flavor and aroma substances.

Plant protection

At sprayers venturi nozzles are used to reduce the drift of the drops. Here, the spray liquid is mixed with air in order to make larger the drops.

Door Heaters

When air curtains the veil can be evenly distributed over the entire width of the system, by directing the flow of air through a narrow gap.

Mathematical Description

From Bernoulli's equation:

With

Follows

Due to the mass conservation

Shall continue to

Thus, the mass flow in the venturi results to