Venturi effect
From Wikipedia, the free encyclopedia
The Venturi effect is a special case of Bernoulli's principle, in the case of fluid or air flow through a tube or pipe with a constriction in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli effect. It is named after Giovanni Battista Venturi, (1746–1822), an Italian physicist.
A fluid passing through smoothly varying constrictions is subject to changes in velocity and pressure in order to satisfy the conservation of mass-flux (flow rate). The reduction in pressure in the constriction can be understood by conservation of energy: the fluid (or gas) gains kinetic energy as it enters the constriction, and that energy is supplied by a pressure gradient force from behind. The pressure gradient reduces the pressure in the constriction, in reaction to the acceleration. Likewise, as the fluid leaves the constriction, it is slowed by a pressure gradient force that raises the pressure back to the ambient level.
The limiting case of the Venturi effect is choked flow, in which a constriction in a pipe or channel limits the total flow rate through the channel, because the pressure cannot drop below zero in the constriction. Choked flow is used to control the delivery rate of water and other fluids through spigots and other valves.
In the diagram shown at right, using Bernoulli's equation in the special case of incompressible fluids (such as the approximation of a water jet), the theoritecal maximum pressure drop in the constriction would be given by (ρ/2)(v12 - v22). For example, for seawater ρ = 1.023 g /cc; 1.5 inch diameter pipe with a 0.25 inch restriction flowing 5 gpm; v at the pipe is 28 cm/sec; v at the restriction would have to be 995 cm/sec; the pressure drop would be -51,000 Pascals or about ½ bar!
[edit] Experimental apparatus
- 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.
- A venturi can also be used to mix a fluid with air. If a pump forces the fluid through a tube connected to a system consisting of a venturi to increase the water 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), air will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of fluid and air will appear.
- 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 and is less accurate.
In Chronic Aortic Regurgitation, after the initial large stroke volume is released, the Venturi effect draws walls together, transiently obstructing flow causing a Pulsus Bisferiens.
[edit] Practical uses
The Venturi effect is visible in:
- the capillaries of the human circulatory system, where it indicates aortic regurgitation
- In the effective burning of human bodily waste by use of half a 55 gallon barrel and shovel held at a constrictive angle.
- large cities where wind is forced between buildings.
- inspirators that mix air and flammable gas in barbecues, gas stoves, Bunsen burners and Airbrushes.
- water aspirators that produce a partial vacuum using the kinetic energy from the faucet water pressure
- Steam siphon 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.
- Protein skimmers, a filtration device for saltwater aquaria.
- In automated pool cleaners that use pressure-side water flow to collect sediment and debris.
- The modern day barrel of the 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
A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose that is carrying water. If the flow is strong enough, the constriction will remain even if the hose would normally spring back to its normal shape: 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.
As with Bernoulli's principle, the Venturi effect is also used as an academic demonstration of lift in an airplane wing. Other principles at play in this phenomenon include Circulation (fluid dynamics) and the Coandă effect.
