Venturi effect

The static pressure in the first measuring tube (1) is higher than at the second (2), and the fluid speed at "1" is lower than at "2", because the cross-sectional area at "1" is greater than at "2".

A flow of air through a venturi meter, showing the columns connected in a manometer and partially filled with water. The meter is "read" as a differential pressure head in cm or inches of water.

Idealized flow in a Venturi tube

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after Giovanni Battista Venturi (1746–1822), an Italian physicist.

Background

In fluid dynamics, an incompressible fluid's velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its dynamic pressure must decrease in accord with the principle of conservation of mechanical energy. Thus any gain in kinetic energy a fluid may accrue due to its increased velocity through a constriction is balanced by a drop in pressure.

By measuring the change in pressure, the flow rate can be determined, as in various flow measurement devices such as venturi meters, venturi nozzles and orifice plates.

Referring to the adjacent diagram, using Bernoulli's equation in the special case of steady, incompressible, inviscid flows (such as the flow of water or other liquid, or low speed flow of gas) along a streamline, the theoretical pressure drop at the constriction is given by:

p_{1}-p_{2}={\frac {\rho }{2}}\left(v_{2}^{2}-v_{1}^{2}\right)

where $\scriptstyle \rho \,$ is the density of the fluid, $\scriptstyle v_{1}$ is the (slower) fluid velocity where the pipe is wider, $\scriptstyle v_{2}$ is the (faster) fluid velocity where the pipe is narrower (as seen in the figure).

Choked flow

The limiting case of the Venturi effect is when a fluid reaches the state of choked flow, where the fluid velocity approaches the local speed of sound. When a fluid system is in a state of choked flow, a further decrease in the downstream pressure environment will not lead to an increase in the mass flow rate. However, mass flow rate for a compressible fluid will increase with increased upstream pressure, which will increase the density of the fluid through the constriction (though the velocity will remain constant). This is the principle of operation of a de Laval nozzle. Increasing source temperature will also increase the local sonic velocity, thus allowing for increased mass flow rate but only if the nozzle area is also increased to compensate for the resulting decrease in density.

Expansion of the section

The Bernoulli equation is invertible, and pressure should rise when a fluid slows down. Nevertheless, if there is an expansion of the tube section, turbulence will appear and the theorem will not hold. Notice that in all experimental Venturi tubes, the pressure in the entrance is compared to the pressure in the middle section. The output section is never compared with them.

Experimental apparatus

Venturi tube demonstration apparatus built out of PVC pipe and operated with a vacuum pump

A pair of venturi tubes on a light aircraft, used to provide airflow for air-driven gyroscopic instruments

Venturi tubes

The simplest apparatus, as shown in the photograph and diagram, is a tubular setup known as a Venturi tube or simply a venturi. Fluid flows through a length of pipe of varying diameter. To avoid undue aerodynamic drag, a Venturi tube typically has an entry cone of 30 degrees and an exit cone of 5 degrees.^[1]

Venturi tubes are used in processes where permanent pressure loss is not tolerable and where maximum accuracy is needed in case of highly viscous liquids.

Orifice plate

Venturi tubes are more expensive to construct than a simple orifice plate which uses the same principle as a tubular scheme, but the orifice plate causes significantly more permanent energy loss.^[2]

Instrumentation and measurement

Venturis are used in industrial applications and in scientific laboratories for measuring the flow rate of liquids.

Flow rate

A venturi can be used to measure the volumetric flow rate, $\scriptstyle Q$ .

Since

{\begin{aligned}Q&=v_{1}A_{1}=v_{2}A_{2}\\p_{1}-p_{2}&={\frac {\rho }{2}}(v_{2}^{2}-v_{1}^{2})\end{aligned}}

then

Q=A_{1}{\sqrt {{\frac {2}{\rho }}\cdot {\frac {\left(p_{1}-p_{2}\right)}{\left({\frac {A_{1}}{A_{2}}}\right)^{2}-1}}}}=A_{2}{\sqrt {{\frac {2}{\rho }}\cdot {\frac {\left(p_{1}-p_{2}\right)}{1-\left({\frac {A_{2}}{A_{1}}}\right)^{2}}}}}

A venturi can also be used to mix a liquid with a gas. If a pump forces the liquid through a tube connected to a system consisting of a venturi to increase the liquid speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), the gas will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of liquid and gas will appear. See aspirator and pressure head for discussion of this type of siphon.

Differential pressure

As fluid flows through a venturi, the expansion and compression of the fluids cause the pressure inside the venturi to change. This principle can be used in metrology for gauges calibrated for differential pressures. This type of pressure measurement may be more convenient, for example, to measure fuel or combustion pressures in jet or rocket engines. The first large-scale Venturi meters to measure liquid flows were developed by Clemens Herschel who used them to measure small and large flows of water and wastewater beginning at the end of the 19th century.^[3]

Examples

The Venturi effect may be observed or used in the following:

Cargo eductors on oil product and chemical ship tankers
Inspirators that mix air and flammable gas in grills, gas stoves, Bunsen burners and airbrushes
Water aspirators that produce a partial vacuum using the kinetic energy from the faucet water pressure
Steam siphons using the kinetic energy from the steam pressure to create a partial vacuum
Atomizers that disperse perfume or spray paint (i.e. from a spray gun).
Carburetors that use the effect to suck gasoline into an engine's intake air stream
Wine aerators, used to infuse air into wine as it is poured into a glass
The capillaries of the human circulatory system, where it indicates aortic regurgitation
Aortic insufficiency is a chronic heart condition that occurs when the aortic valve's initial large stroke volume is released and the Venturi effect draws the walls together, which obstructs blood flow, which leads to a pulsus bisferiens.
Protein skimmers (filtration devices for saltwater aquaria)
In automated pool cleaners that use pressure-side water flow to collect sediment and debris
The barrel of the modern-day clarinet, which uses a reverse taper to speed the air down the tube, enabling better tone, response and intonation
Compressed air operated industrial vacuum cleaners
Venturi scrubbers used to clean flue gas emissions
Injectors (also called ejectors) used to add chlorine gas to water treatment chlorination systems
Steam injectors use the Venturi effect and the latent heat of evaporation to deliver feed water to a steam locomotive boiler.
Sandblasting nozzles use Venturi effect to accelerate air and media mixture.
Emptying bilge water from a moving boat through a small waste gate in the hull—the air pressure inside the moving boat is greater than the water sliding by beneath
A scuba diving regulator to assist the flow of air once it starts flowing
In recoilless rifles to decrease the recoil of firing
Ventilators
The diffuser on an automobile
Race cars utilising ground effect to increase downforce and thus become capable of higher cornering speeds, from the late 1970s until such technology was banned from competition in the early to mid-1980s
Large cities where wind is forced between buildings - the gap between the Twin Towers of the original World Trade Center was an extreme example of the pheonomenon, which made the ground level plaza notoriously windswept.^[4] In fact, some gusts were so high that pedestrian travel had to be aided by ropes.^[5]
In windy mountain passes, resulting in erroneous pressure altimeter readings^[6]
The leadpipe of a trombone, affecting the timbre
Foam proportioners used to induct fire fighting foam concentrate into fire protection systems
Trompe air compressors to entrain air into a falling column of water
The Mistral wind in Southern France increases in speed through the Rhone valley.
Low-speed wind tunnels can be considered very large Venturi because they take advantage of the Venturi effect to increase velocity and decrease pressure to simulate expected flight conditions.^[7]

Venturi tubes are also used to measure the speed of a fluid, by measuring pressure changes at different segments of the device. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a Venturi is all that is needed. When the fluid flows through the Venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.^[2]

References

↑ Nasr, G. G.; Connor, N. E. (2014). "5.3 Gas Flow Measurement". Natural Gas Engineering and Safety Challenges: Downstream Process, Analysis, Utilization and Safety. Springer. p. 183. ISBN 9783319089485.
1 2 "The Venturi effect". Wolfram Demonstrations Project. Retrieved 2009-11-03.
↑ Herschel, Clemens. (1898). Measuring Water. Providence, RI:Builders Iron Foundry.
↑ Dunlap, David W (December 7, 2006). "At New Trade Center, Seeking Lively (but Secure) Streets". The New York Times.
↑ Dunlap, David W (March 25, 2004). "Girding Against Return of the Windy City in Manhattan". The New York Times.
↑ Dusk to Dawn (educational film). Federal Aviation Administration. 1971. 17 minutes in. AVA20333VNB1.
↑ Anderson, John (2017). Fundamentals of Aerodynamics (6th ed.). New York, NY: McGraw-Hill Education. p. 218. ISBN 978-1-259-12991-9.

External links

3D animation of the Differential Pressure Flow Measuring Principle (Venturi meter)
UT Austin. "Venturi Tube Simulation". Retrieved 2009-11-03.

This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.

[1] Nasr, G. G.; Connor, N. E. (2014). "5.3 Gas Flow Measurement". Natural Gas Engineering and Safety Challenges: Downstream Process, Analysis, Utilization and Safety. Springer. p. 183. ISBN 9783319089485.

[wolfram-2] 1 2 "The Venturi effect". Wolfram Demonstrations Project. Retrieved 2009-11-03.

[3] Herschel, Clemens. (1898). Measuring Water. Providence, RI:Builders Iron Foundry.

[4] Dunlap, David W (December 7, 2006). "At New Trade Center, Seeking Lively (but Secure) Streets". The New York Times.

[5] Dunlap, David W (March 25, 2004). "Girding Against Return of the Windy City in Manhattan". The New York Times.

[6] Dusk to Dawn (educational film). Federal Aviation Administration. 1971. 17 minutes in. AVA20333VNB1.

[7] Anderson, John (2017). Fundamentals of Aerodynamics (6th ed.). New York, NY: McGraw-Hill Education. p. 218. ISBN 978-1-259-12991-9.