Reduced pressure caused by a flow restriction in a tube or pipe
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The Venturi effect is the reduction in
fluid pressure that results when a moving fluid speeds up as it flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th-century Italian
physicistGiovanni Battista Venturi.
The effect has various engineering applications, as the reduction in pressure inside the constriction can be used both for measuring the fluid flow and for moving other fluids (e.g. in a
vacuum ejector).
By measuring 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
where is the
density of the fluid, is the (slower) fluid velocity where the pipe is wider, and 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 velocity, unless the fluid is compressed.
The 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 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. 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
A pair of Venturi tubes on a light aircraft, used to provide airflow for air-driven gyroscopic instruments
Venturi tubes
The simplest apparatus is a tubular setup known as a Venturi tube or simply a Venturi (plural: "Venturis" or occasionally "Venturies"). 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 often used in processes where permanent pressure loss is not tolerable and where maximum accuracy is needed in case of highly viscous liquids.[citation needed]
Orifice plate
Venturi tubes are more expensive to construct than simple
orifice plates, and both function on the same basic principle. However, for any given differential pressure, orifice plates cause significantly more permanent energy loss.[2]
Instrumentation and measurement
Both Venturi tubes and orifice plates are used in industrial applications and in scientific laboratories for measuring the flow rate of liquids.
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.
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] While working for the
Holyoke Water Power Company, Herschel would develop the means for measuring these flows to determine the water power consumption of different mills on the
Holyoke Canal System, first beginning development of the device in 1886, two years later he would describe his invention of the Venturi meter to
William Unwin in a letter dated June 5, 1888.[4]
where constant terms are absorbed into k. Using the definitions of density (),
molar concentration (), and
molar mass (), one can also derive mass flow or molar flow (i.e. standard volume flow):
However, measurements outside the design point must compensate for the effects of temperature, pressure, and molar mass on density and concentration. The
ideal gas law is used to relate actual values to
design values:
Substituting these two relations into the pressure-flow equations above yields the fully compensated flows:
Q, m, or n are easily isolated by dividing and taking the
square root. Note that pressure-, temperature-, and mass-compensation is required for every flow, regardless of the end units or dimensions. Also we see the relations:
The Venturi effect may be observed or used in the following:
Machines
During
Underway replenishment the helmsman of each ship must constantly steer away from the other ship due to the Venturi effect, otherwise they will collide.
Cargo
eductors on oil product and chemical ship tankers
Water aspirators produce a partial vacuum using the kinetic energy from the faucet water pressure
Steam siphons use the kinetic energy from the steam pressure to create a partial vacuum
Atomizers disperse perfume or spray paint (i.e. from a spray gun)
Carburetors use the effect to suck
gasoline into an engine's intake air stream
Cylinder heads in piston engines have multiple Venturi areas like the valve seat and the port entrance, although these are not part of the design intent, merely a byproduct and any venturi effect is without specific function.
Wine aerators infuse air into wine as it is poured into a glass
Sandblasting nozzles accelerate and air and media mixture
Bilge water can be emptied 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 uses the Venturi effect to assist maintaining the flow of gas once it starts flowing
Race cars utilising
ground effect to increase
downforce and thus become capable of higher cornering speeds
Foam proportioners used to induct
fire fighting foam concentrate into fire protection systems
Trompe air compressors entrain air into a falling column of water
The bolts in some brands of paintball markers
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.[6]
Architecture
Hawa Mahal of Jaipur, also utilizes the Venturi effect, by allowing cool air to pass through, thus making the whole area more pleasant during the high temperatures in summer.
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 phenomenon, which made the ground level plaza notoriously windswept.[7] In fact, some gusts were so high that pedestrian travel had to be aided by ropes.[8]
In the south of Iraq, near the modern town of
Nasiriyah, a 4000-year-old flume structure has been discovered at the ancient site of
Girsu. This construction by the ancient
Sumerians forced the contents of a nineteen kilometre canal through a constriction to enable the side-channeling of water off to agricultural lands from a higher origin than would have been the case without the flume. A recent dig by archaeologists from the British museum confirmed the finding.
^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.
ISBN9783319089485.