Venturi tube . The Venturi effect (also known as Venturi tube) is that a fluid in motion within a closed duct decreases its pressure with increasing speed after passing through an area with a smaller section. If the end of another duct is inserted into this duct, the fluid contained in this second duct is sucked. This effect, demonstrated in 1797 , is named after the Italian physicist Giovanni Battista Venturi ( 1746 – 1822 ). The Venturi effect is explained by the Bernoulli Principle and the continuity principleof dough. If the flow rate of a fluid is constant but the section decreases, the speed necessarily increases after crossing this section. By the energy theorem, if the kinetic energy increases, the energy determined by the pressure value necessarily decreases.
[ hide ]
- 1 Biography
- 2 Venturi tube
- 3 Operation of a Venturi Tube
- 4 Applications of the Ventura effect
- 5 See also
- 6 Sources
Giovanni Battista Venturi Italian Physicist ( Bibiano 1746 – Reggio Emilia 1822). Professor in Modena and Pavia . In 1813 he dedicated himself to physics research . In this field he dealt in particular with colors and various questions of optics, however, he is uniquely recognized for his studies in the field of hydraulics . He showed in 1797 that the contraction of the flow at the entrance of a cylindrical tube, caused: local reduction of pressure and generation of eddies . The replacement of the cylinder by two conical sections, which he called a divergent cone nozzle and which would later be called a venturi tube in his honor, eliminates eddies and therefore increases flow.
A Venturi tube is a device initially designed to measure the speed of a fluid taking advantage of the Venturi effect. However, some are used to accelerate the speed of a fluid by forcing it to pass through a narrow cone-shaped tube. The classic application of speed measurement of a fluid consists of a tube formed by two conical sections joined by a narrow tube in which the fluid moves consequently at a higher speed. The pressure in the Venturi tube can be measured by a vertical U-shaped tube connecting the wide region and the narrow ductwork. The difference in the height of the liquid in the U-tube allows measuring the pressure at both points and consequently the speed. When using a Venturi tube, a phenomenon called cavitation must be taken into account.. This phenomenon occurs if the pressure in any section of the tube is less than the vapor pressure of the fluid. For this particular type of tube, the risk of cavitation is in the throat of the tube, since here, as the area is minimum and the speed is maximum, the pressure is the lowest that can be found in the tube. When cavitation occurs, bubbles are generated locally, which travel along the tube. If these bubbles reach higher pressure zones, they can collapse thus producing local pressure peaks with the potential risk of damaging the tube wall.
Venturi Tube Operation
In the Venturi Tube the flow from the main line in section 1 is accelerated through the narrow section called the throat, where the fluid pressure decreases. The flow is then expanded through the diverging portion to the same diameter as the main pipe. Pressure branches are located on the pipe wall in section 1 and on the throat wall, which we will call section 2. These pressure branches are attached to the two sides of a differential manometer in such a way that the deflection h is an indication of the pressure difference p1 – p2. Of course, other types of differential pressure gauges can be used. In the case of hydraulics where friction losses are taken into account, the most convenient is to develop an equation that contains them. After doing some calculations and some simplifications, you can come up with the following equations that make solving certain types of problems more practical and quick.
Q = K (12.6 h – Hf) 1/2
K = SE [2 g / ((dE / dG) 4 – 1)] 1/2
SE = 0.7854 * dE2
dG = Throat diameter
dE = Diameter in the conduit pipe
h = Difference in level on the manometer (expressed in meter of mercury)
Hf = Losses due to rubbing (expressed in m)
It is prudent to keep in mind that this equation works in the international system (m, s) and that the manometric liquid is mercury. Friction losses are reported in units of length, since they are treated as a decrease in the pressure head. This equation works for incompressible flow. The discharge depends on the manometric difference regardless of the orientation of the Venturi meter; it is not relevant if the meter is horizontal, vertical or inclined.
Ventura effect applications
The Venturi Tube is a device, which can be used in many technological applications and applications of daily life
- In the Automotive Industry : in the car’s carburetor , its use can be observed in what is the Fuel Supply .
Engines require air and fuel to run. A liter of gasoline needs approximately 10,000 liters of air to burn, and there must be some metering mechanism that allows the mixture to enter the engine in the correct proportion. This dispenser is called a carburetor, and is based on the Venturi principle: by varying the inside diameter of a pipe, the speed of the air passage is increased. The Venturi Tube allows mixing of the air with the fuel for combustion, without which the car’s engine could not start, hence the principle of this tube is used as an important part of the automotive industry. The Venturi Effect on the carburettor consists in passing an air stream at high speed, caused by the descent of the piston by a quantity of gasoline that is feeding by a tank forming a gaseous mass. The richness of gasoline depends on the diameter of the pump.
- In the Cleaning area:
This tube also has other applications such as for cleaning. Normal urban air carries around 0.0006 grains of suspended matter per cubic foot (1.37 mg / m3), which is a practical limit for most industrial gas cleaning; The amount of dust in normal air at manufacturing plants is often as high as 0.002 g / ft3 (4.58 mg / m3). The amount of dust in the blast furnace gas, after passing through the first dust collector, is of the order of 10 g / ft3 (22.9 g / m3), just like the hot raw gas of gas generator. All dust content figures are based on air volumes at 60º F and 1 atm (15.6º C and 101000 N / m2). The removal of the suspended matter is carried out by means of dynamic dew washers. The Pease-Anthony Venturi. In this system, the gas is forced through the throat of a Venturi, where it mixes with high pressure water sprays. A tank is needed after Venturi, to cool and remove moisture. Cleaning between 0.1 to 0.3 g / ft3 has been reported. Comparatively, less filtration is applied to clean gases; It is used extensively to clean air and waste gases. Typically, the materials used to filter gases are cotton or wool clothdense fabric, for temperatures up to 250º F; for higher temperatures or metal fiber fabric is recommended glasswoven. The leaking gases must be well above their dew point, as condensation on the filter cloth will clog the pores. If necessary, saturated gas should be reheated. Fabric is often shaped into “bags,” tubes 6 to 12 in. In diameter and up to 40 feet in length, which are suspended from a steel frame (bag chamber). The gas inlet is located at the lower end, through a head to which the bags are connected in parallel; the exit is made through a cover that surrounds all the bags. At frequent intervals, the operation of the whole unit or part of it is interrupted, to beat or shake the bags, or to introduce clean air in the opposite direction through them, to dislodge the accumulated dust, which falls to the gas intake head and from which it is removed by a worm conveyor. Dust content down to 0.01 g / ft3 or less can be reduced at a reasonable cost. The apparatus is also used for the recovery of valuable solids entrained by gases.
- Wind energy collection methods :
The collection of wind energy can be divided into two ways: Direct collection: The energy is extracted through surfaces directly in contact with the wind , for example, windmills and sails. Indirect collection: In this case, an intermediate element intervenes for its collection, for example the sea surface. Indirect Uptake Indirect uptake uses either machines of the preceding type associated with static organs or entirely static organs, or an intermediate fluid. Static organ and dynamic machine: The principle is based on the use of a Venturi Tube; This arrangement allows for a given propeller and a given wind to increase rotational speed and power, as well as aerodynamic performance by suppression of marginal losses. Applied directly to a horizontal axis machine the interest is little, since this tube considerably complicates the installation. It should be noted that this Venturi Tube on propellers with few blades. Systems using several Venturi Tubes in series have been proposed. A more interesting idea could be that of Nazarethat proposes a huge vertical Venturi that would allow real artificial waterspouts to be made, especially if this installation were made in hot countries. These are systems that “manufacture the wind” based mainly on the temperature differences that would exist at the two extremities of the tower. The wind machine would be located on the neck. It will be theoretically possible to develop powers ranging from 500 to 1000 MW, using towers 300 to 400 meters high. It seems that there are many difficulties in building the tower, but already in some nuclear power plants there are 150-meter-high aerial cooling towers.
- Entirely static organs
These mainly use Venturi tubes that modify the distribution of dynamic and static pressure. Systems have been proposed that allow water to be raised by grouping in series a certain number of Venturi Pipes, which appear to be promising.
- Venturi hat:
Another clear application of the Venturi Tube principle is the Venturi Hat. Working principle: The hot air, which comes out of the main duct, is carried by the cold air that enters from the bottom when it “hits” the pipe, producing the vacuum effect at the end of the duct, this action makes this hat has a high index of effectiveness, proportional to the wind speed, working optimally with the slightest breeze. This type of hat is special for very windy areas. Long tests were carried out to achieve effectiveness in adverse weather conditions.