Venturi effect 1

Venturi effect

The pressure in the first measuring tube (1) is higher than at the second (2), and thefluid 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, showingthe columns connected in a U-shape (a

manometer) and partially filled with water. Themeter is "read" as a differential pressure head in

cm or inches of water.

Flow in a Venturi tube

The Venturi effect is the reduction in fluidpressure that results when a fluid flowsthrough a constricted section of pipe. TheVenturi effect is named after GiovanniBattista Venturi (1746–1822), an Italianphysicist.

Background

The Venturi effect is a jet effect; as with afunnel the velocity of the fluid increases asthe cross sectional area decreases, with thestatic pressure correspondingly decreasing.According to the laws governing fluiddynamics, a fluid's velocity must increase asit passes through a constriction to satisfy theprinciple of continuity, while its pressuremust decrease to satisfy the principle ofconservation of mechanical energy. Thusany gain in kinetic energy a fluid mayaccrue due to its increased velocity througha constriction is negated by a drop inpressure. An equation for the drop inpressure due to the Venturi effect may bederived from a combination of Bernoulli'sprinciple and the continuity equation.

The limiting case of the Venturi effect iswhen a fluid reaches the state of chokedflow, where the fluid velocity approachesthe local speed of sound. In choked flow themass flow rate will not increase with afurther decrease in the downstream pressureenvironment.

However, mass flow rate for a compressiblefluid can increase with increased upstreampressure, which will increase the density ofthe fluid through the constriction (thoughthe velocity will remain constant). This isthe principle of operation of a de Laval nozzle. Increasing source temperature will also increase the local sonicvelocity, thus allowing for increased mass flow rate.

Referring to the diagram to the right, using Bernoulli's equation in the special case of incompressible flows (such asthe flow of water or other liquid, or low speed flow of gas), the theoretical pressure drop at the constriction is givenby:

Venturi effect 2

where is the density of the fluid, is the (slower) fluid velocity where the pipe is wider, is the (faster) fluidvelocity where the pipe is narrower (as seen in the figure). This assumes the flowing fluid (or other substance) is notsignificantly compressible - even though pressure varies, the density is assumed to remain approximately constant.

Experimental apparatus

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

Venturi tubes

The simplest apparatus, as shown in the photograph and diagram, is atubular setup known as a Venturi tube or simply a venturi. Fluid flowsthrough a length of pipe of varying diameter. To avoid undue drag, aVenturi tube typically has an entry cone of 30 degrees and an exit coneof 5 degrees.

Orifice plate

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

Instrumentation and measurementVenturis are used in industrial and in scientific laboratories for measuring the flow of liquids.

Flow rateA venturi can be used to measure the volumetric flow rate, .Since

then

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

Venturi effect 3

Differential PressureAs fluid flows through a venturi, the expansion and compression of the fluids cause the pressure inside the venturi tochange. This principle can be used in metrology for gauges calibrated for differential pressures. This type of pressuremeasurement may be more convenient, for example, to measure fuel or combustion pressures in jet or rocketengines. The first large-scale Venturi meters to measure liquid flows were developed by Clemens Herschel who usedthem to measure small and large flows of water and wastewater beginning at the end of the 19th century.[1]

ExamplesThe 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).•• Foam firefighting nozzles and extinguishers• 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 PulsusBisferiens.

• 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.•• Sand blasters used to draw fine sand in and mix it with air• 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• Modern vaporizers to optimize efficiency• In Venturi masks used in medical oxygen therapy• In recoilless rifles to decrease the recoil of firing•• Ventilators• The diffuser on an automobile•• Large cities where wind is forced between buildings• In windy mountain passes, resulting in erroneous pressure altimeter readings[2]

• The leadpipe of a trombone, affecting the timbre• Foam proportioners used to induct fire fighting foam concentrate into fire protection systemsThe Bernoulli Principle and its corollary, the Venturi effect, are essential to aerodynamic as well as hydrodynamicdesign concepts. Airfoil and hydrofoil designs to lift and steer air and water vessels (airplanes, ships and submarines)are derived from applications of the Bernouoli Principle and the Venturi effect, as are the instruments that measure

Venturi effect 4

rate of movement through the air or water (velocity indicators). Stability indication and control mechanisms such asgyroscopic attitude indicators and fuel metering devices, such as carburetors, function as a result of gas or fluidpressure differentials that create suction as demonstrated and measurable by gas/fluid pressure and velocityequations derived from the Bernoulli Principle and the Venturi Effect.A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose in which fluid is flowing:the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.Venturi tubes are also used to measure the speed of a fluid, by measuring pressure changes at different segments ofthe device. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a Venturi is allthat is needed. When the fluid flows though the Venturi the pressure in the two ends of the tube will differ, forcingthe liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.[]

References[1] Herschel, Clemens. (1898). Measuring Water. Providence, RI:Builders Iron Foundry.

External links• 3D animation of the Differential Pressure Flow Measuring Principle (Venturi meter) (http:/ / www. youtube. com/

watch?v=oUd4WxjoHKY)• UT Austin. "Venturi Tube Simulation" (http:/ / www. ce. utexas. edu/ prof/ KINNAS/ 319LAB/ Applets/ Venturi/

venturi. html). Retrieved 2009-11-03.

Article Sources and Contributors 5

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Image Sources, Licenses and ContributorsFile:Venturifixed2.PNG  Source: http://en.wikipedia.org/w/index.php?title=File:Venturifixed2.PNG  License: Public Domain  Contributors: User:HappyAppleFile:VenturiFlow.png  Source: http://en.wikipedia.org/w/index.php?title=File:VenturiFlow.png  License: GNU Free Documentation License  Contributors: user:ComputerGeezer and Geof.Original uploader was ComputerGeezer at en.wikipediaFile:Venturi.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Venturi.gif  License: Creative Commons Zero  Contributors: User:Thierry DugnolleFile:Green Hope High School (Physics Laboratory Venturi Tube) 2006.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:Green_Hope_High_School_(Physics_Laboratory_Venturi_Tube)_2006.jpg  License: GNU Free Documentation License  Contributors: NaderMoussa

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