文氏管
最明显的特征就是当文氏管入口压力保持不变时，降低出口压力到一定程度（文氏管压力损失大于初始汽蚀压力损失）时，文氏管喉部发生汽蚀，当出口压力进一步降低时，汽蚀区逐渐扩大，变成深度汽蚀，汽蚀以后最明显的特征是：通过文氏管的流量不在受文氏管出口压力和下游压力的影响，只取决于入口的压力，提高入口压力就可提高流量，而且下游的压力脉动被文氏管隔离不会传到文氏管上游，防止了整个管路的压力振荡。 Venturi effect
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The pressure at "1" is higher than at "2" because the fluid speed at "1" is lower than at "2".
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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.
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.
Background
The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force. An equation for the drop in pressure due to the Venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity.
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.
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 (p1−p2) at the constriction would be given by:
导平
where is the density of the fluid, v1 is the (slower) fluid velocity
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where the pipe is wider, v2is 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
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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
小肥羊军博店a coefficient of discharge is often introduced, which generally has a value of 0.98.
A venturi can be used to measure the volumetric flow rate Q.
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 a discussion of this type of siphon.
[edit] Orifice plate
[edit] Examples
The Venturi effect may be observed or used in the following:
∙Large cities where wind is forced between buildings
∙Air flows over the top of an aircraft wing. The stable air around the plane and the foil above the cord line of the wing create a venturi around the wind going directly over the聚乙二醇单甲醚
wing. Hence creating a low preasure on top of the wing lifting it upward.
unrelated topic in ordinary differential equations, see Bernoulli differential equation.
A flow of air into a venturi meter. The kinetic energy increases at the expense of the fluid
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