Venturi effect From Wikipedia, the free encyclopediaJump to: navigation, search

The pressure at "1" is higher than at "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 U-shape (a manometer) and partially filled with water. The meter 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 fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi (1746–1822), an Italian physicist.

Contents

 [hide] 

1 Background 2 Experimental apparatus

o 2.1 Venturi tubes o 2.2 Orifice plate

3 Instrumentation and measurement o 3.1 Flow rate o 3.2 Differential Pressure

4 Examples 5 See also 6 External links 7 References

[edit] BackgroundThe Venturi effect is a jet effect; as with a funnel the velocity of the fluid increases as the cross sectional area decreases, with the static pressure correspondingly decreasing. According to the laws governing fluid dynamics, a fluid's velocity must increase as it passes through a constriction to satisfy the principle of continuity, while its pressure must decrease to satisfy 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 negated by a drop in pressure. An equation for the drop in pressure due to the Venturi effect may be derived from a combination of Bernoulli's principle and the continuity equation.

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. In choked flow the mass flow rate will not increase with a further decrease in the downstream pressure environment.

However, mass flow rate for a compressible fluid can 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.

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

where is the density of the fluid, is the (slower) fluid velocity where the pipe is wider, is the (faster) fluid velocity where the pipe is narrower (as seen in the figure). This assumes the flowing fluid (or other substance) is not significantly compressible - even though pressure varies, the density is assumed to remain approximately constant.

[edit] Experimental apparatus

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

[edit] 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 drag, a Venturi tube typically has an entry cone of 30 degrees and an exit cone of 5 degrees. To account for the assumption of an inviscid fluid a coefficient of discharge is often introduced, which generally has a value of 0.98.

[edit] 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.[1]

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

[edit] Flow rate

A 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 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.

[edit] Differential Pressure

Main article: Pressure head

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.

[edit] 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 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. 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 The leadpipe of a trombone, affecting the timbre Foam proportioners used to induct fire fighting foam concentrate into fire protection

systems

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 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 though 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.[1]

vA pitot (  / ̍ p iː t oʊ / ) tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century[1] and was modified to its modern form in the mid-19th century by French scientist Henry Darcy.[2] It is widely used to determine the airspeed of an aircraft and to measure air and gas velocities in industrial applications. The pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit.[3]

Contents

 [hide] 

1 Theory of operation 2 Operation 3 Industry applications 4 See also 5 References 6 External links

[edit] Theory of operationThe basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure.

The measured stagnation pressure cannot of itself be used to determine the fluid velocity (airspeed in aviation). However, Bernoulli's equation states:

Stagnation pressure = static pressure + dynamic pressure

Which can also be written

Solving that for velocity we get:

Note: The above equation applies only to incompressible fluid.

where:

is fluid velocity;

is stagnation or total pressure;

is static pressure;

and is fluid density.

The value for the pressure drop – or Δp due to Δh, the reading on the manometer:

Δp = g Δh

Where:

is the density of the fluid in the manometer

Δh is the manometer reading

The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure. The static pressure is generally measured using the static ports on the side of the fuselage. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diaphragm is proportional to the dynamic pressure, which can then be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement is typically contained within the airspeed indicator, which converts the dynamic pressure to an airspeed reading by means of mechanical levers.

Instead of separate pitot and static ports, a pitot-static tube (also called a Prandtl tube) may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.

[edit] OperationPitot tubes on aircraft commonly have heating elements called pitot heat to prevent the tube from becoming clogged with ice. The failure of these systems can have catastrophic consequences, as in the case of Austral Líneas Aéreas Flight 2553, Birgenair Flight 301 (investigators suspected that some kind of insect could have created a nest inside the pitot tube: the prime suspect is a species called the black and yellow mud dauber wasp), Northwest Airlines Flight 6231, AeroPeru Flight 603 (blocked static port), and of one X-31.[4] It has also been suggested that pitot tube icing may have been a contributing factor for the crash of Air France Flight 447 [5] although this claim has been contested.[6]

[edit] Industry applications

Pitot tube from a F/A-18

In industry, the velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube. The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential pressure gauge (alnor) for determining the velocity inside the ducted wind tunnel. One use of this technique is to determine the amount of cooling that is being delivered to a room.

The fluid flow rate in a duct can then be estimated from:

Volume flow rate (cubic feet per minute) = duct area (square feet) × velocity (feet per minute)Volume flow rate (cubic meters per second) = duct area (square meters) × veloci