Mechanics

Venturi tube

The Venturi tube is an instrument that indicates the variation of pressure exerted by a liquid that is moving through tubes with variable areas.

Venturi tube indicates pressure variation for flowing fluids
Conceived by Giovanni Battista Venturi, the so-called Venturi tube is an equipment that indicates the variation in pressure of a fluid flowing in regions with different cross-sectional areas . Where the area is smaller, there will be greater velocity, so the pressure will be less . The reciprocal is true.

 A fluid flowing through a tube that has different cross-sectional areas, the central region has a smaller area. The passage of the liquid generates a certain pressure on the walls of the tube. Note that there are three manometers that make the determination of the pressure of the liquid, in the central part, where the cross-sectional area is smaller, the indicated pressure is smaller.

Explanation for the pressure variation in the Venturi tube

The explanation of why there is a pressure difference starts with the Bernoulli equation , which can be written as follows:

1 + ½ ρ.v 2 = constant

The terms of this equation are:

p = Pressure exerted by the fluid (pa);

ρ = fluid density (kg/m 3 );

v = Flow velocity (m/s).

Applying this equation to two distinct regions of a tube, through which a fluid flows, we will have:

1 + ½ ρ.v 2 = p 2 + ½ ρ.v 2

The so -called continuity equation shows us that the smaller the flow area of ​​a fluid, the greater its velocity , this will guarantee a constant flow rate of the fluid, that is, the same volume of fluid per second flowing at all points of a tube . This idea becomes clear when we observe a river, in regions where the distance between the banks is greater, the speed of the current is lower. Already at points of proximity between the edges, the speed is noticeably higher, so we can write:

The terms of this equation are:

1 ev 2 = Fluid flow velocity

1 and A 2 = Drainage area

The equality proposed by Bernoulli’s equation for regions with different cross-sectional area will only be maintained if there is a variation in pressures.
Returning to the initial image of the text, we notice that the central region of the tube, as it is thinner, will present higher flow velocity, therefore, the equality in the Bernoulli equation will only be maintained if the pressure in this region is lower. The Venturi tube is the equipment that indicates this change in pressure values, so the manometers in the image show different values, higher pressures for larger cross-sectional areas and vice versa.

applications

The image below shows a spray gun used for painting. The paint to be used remains stored in the cylindrical container, with the passage of an air stream through the gun, the pressure in the region of the mouth of the container will be reduced, thus, because the internal pressure of the container is greater, the paint will be expelled and will mix with the air.

A similar technique is used by the fire department in the event of fires involving flammable liquids. This class of fire cannot be extinguished with water, so a water-based foam is used to smother the flames, removing the supply of oxidizer (oxygen) and extinguishing the fire . Foam is made with a mixture of water, air and a foam generating liquid (LGE). Air is introduced into the water-LGE mixture through a Venturi tube.

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