preamble:
the venturi effect
Savery-Newcomen pump as the flame eater engine both use atmospheric forces and both need heat to create vacuum, those machines can sustain themselves, so you can make them run everywhere, burning combustibe and heavy maintenance is needed that's why we have to find an other solution. In all of the site, i mean by vacuum, partial vacuum.
The venturi effect:
The venturi effect give us the ability to creat vacuum with a fluid under pressure, the venturi effect is widely used in the industry, it is also well used for gripping windshields, in bec Bunsen, in carburetors, for sucking fertilizer for irrigation, mixing chemicals, exhaust ventilation, water waste treatment, aquariums oxygenation or simply to determine the flow in a pipe etc...
the venturi effect was discovered by Giovanni Battista Venturi in 1797.
Born 15 March 1746, Died 24 April 1822. physicist (wikipedia)
the vacuum can be created either by air or water under pressure through the venturi tube, but a high air pressure does not exist naturally unlike water if we consider the potential energy to create a high static pressure. When the water flows, the potential energy can be converted into kinetic energy by accelerating the water by passing it through a restriction diameter of the penstock. According to the venturi principle, when the fluid velocity increases, its static pressure decreases, this is how we create a depression and at the same time the suction effect. The Daniel Bernoulli equation and mass conservation can explain the venturi effect.
Fluid dynamics and Bernouilli equation:
Birth February 8, 1700 Groningen (Netherlands)
Died March 17, 1782 Basel (Switzerland)
Swiss physicist and mathematician (wikipedia)
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All people who studied fluid dynamics at the university could not escape Bernouilli equation, and guess what kind of exercises comes first ? the venturi effect of course !
using fluid mechanics can help you to design a performent venturi tube, especially when it comes to the bottle neck. you first need to know the flow rate at your disposal, no matter what method you use, even a bucket with a chronometer. you need also to know about the static pressure, however don't exceed 10 bars in the inlet (because of turbulence issues downstream) if you are planning to use a regular venturi tube. the static pressure and the flow rate will determine the power of air suction.
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mass conservation equation:
For an incompressible fluid in a steady flow, the amount of water entering a pipe is equal to the amount of outgoing water, the flow rate is considered constant.The flow rate Q = V1 *S1 = V2 *S2 = constant, S is the section through which the fluid flows, and V the speed at that section. as flow rate is constant , when the section S1 is big, V1 is low and conversely when the section S2 is small, speed V2 goes high.
Equation of conservation of energy (Bernoulli equation):
Bernoulli's equation is applicable under the following conditions
- Fluid is perfect and incompressible density ρ (viscosity neglected).
- The flow is permanent.
- The flow is in a perfectly smooth conduct ( no friction)
notice because of those conditions, bernouilli equation will not give you very accurate result as in the reality, loss of head for example is not taken in account, it just give you an idea of where you are going.
Water flow through a venturi tube
From Figure above we will establish the Bernoulli equation:
as
therefore
P: represents the work of pressure forces
ρ v ^ 2/2: represents the kinetic energy or the dynamic pressure
ρgz: is the potential energy of position, ρ is the density of water
unit used: m, Pa, s
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as Q, s1 and s2 are known, you can determine v1 and v2 (conservation of mass relationship)
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P1 is read from manometer (static pressure with flowing water), P2 can be extracted from the formula, and it's
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depend on restriction diameter you wish to test.
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Types of venturi tubes:
you can create the venturi effect with just a disk with the hole in the middle (restriction) in the conduct but this methode is not recommanded because of huge loss of head downstream.
There are two types of venturi tubes, single or multi-stage:
single stage
water
multi-stage
water
very important remark: a good venturi tube must have a convergent angle about 20° and divergent angle 6°-7° to ovoid water separation from walls.
Big venturi built into a dam
Navier-Stock equation and CFD:
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The Navier-Stock's equation has found use in computer simulation which is known by the term computational fluid dynamics (CFD). It's a model using the finite element method to determine the behavior of fluids. Digital simulation is very useful during the design, it helps to understand what is happening in the heart of the system
example of using CFD
The model for this study is a simple venturi effect with a length of 20 m, a diameter of 1 m and a restriction of 0.45 meters in diameter, the convergent angle is 34 °, the divergent angle of 7 °
Boundary conditions:
• input Total pressure: 10 bar
• Pressure output of the tube: 1 atmosphere
• fluid: water at 20 ° c
• Quality of the mesh: the highest level 8
Profile of the static pressure along the tube with the apparent mesh
If the absolute static pressure is negative, it means that there will be the appearance of cavitation, but again we are lucky because the air intake will alleviate the cavitation effect and neutralized it before making damage to the tube.
The venturi tube in the rapid waters:
The installation of a venturi tube in the river rapids allows to benefit from the kinetic energy instead of the potential energy, in this situation the system appears in its best condition for the respect of the landscape and nature as the venturi tube will be completely submerged and not visible from the surface, it is securely fixed and parallel to the flow of water at high speed, oxygen supply will be important. Here no penstock or dam and therefore no negative aspect for marine life but on the contrary, it helps nature, Furthermore we can place multiple tubes behind each other or in parallel,this method will multiply energy to extract. The simulation described below confirms this possibility.
high velocity water
Head
sucked air
mixed air-water leaving the tube
securely fixed tube at the bottom
water entrance
air pipe
Illustration of a venturi tube in rapid waters
Simulation of a venturi tube immersed in a high-speed area:
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In this simulation the tube is immersed to a depth of 2 meters below the surface, the water velocity is 20 m / s (72 km / h). The tube is parallel to the flow
The arrows represent the velocity vectors, they give their directions, sense and modules. We find that the flow speed decelerates at the entrance of the tube from 20 m/s to 6 m s approximately, then suddenly accelerated in the converging portion of the tube because it is the beginning of the narrowing of the diameter of the section, velocity reach its maximum speed around 30 m/s in the bottleneck.
Interpretation of static pressure profil:
When the water slows down at the entrance of the tube, the dynamic pressure of the river turns into static pressure according to the conservation of energy law , static pressure and gaining almost 2 bar. Immediately follows the opposite effect to the entrance of the restriction, static pressure drops dramatically from 3.15 bar to -2.49 bar because of the acceleration of the water at the smallest diameter . The area unconditional flow (air inlet) reaches a deep depression -1.6 bar, this result is very satisfactory and promises a comfortable air suction.
Extent of the negative pressure zone:
By changing the color scale from 0 to 1 bar we can appreciate the extent of the depression area (everything that is not red).
Warning :
In the quest for maximum suction, one is tempted to use a bottleneck increasingly tight, it can result in the water becoming very fast and causes strong turbulence in the divergent portion that will halt completely the flow inside the tube, this will result in a bypass of the tube by the water, the tube becomes inoperative. to use with care !
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It is difficult to establish this kind of tube in an environment where the force of the water is important, the tube must also be protected from debris, if you have a piece of wood or a salmon stuck in the middle, you're screwed !
Power of the venturi tube as vacuum pump:
To answer this important question, let us: Refer to the documents posted by MAZZEI company that builds venturi injectors, they have great experience in this field. You can find this documents here: http://mazzei.net/injector-performance/
In this table we can see that the air intake flow may exceed the water flow. example: at a rate of 16120 l / min at 4.57 bar, mazzei venturi tube can suck up to 21 297 l / min of air !
This is the power of a venturi vacuum pump .
The advantages of using venturi tube:
1.32 m Venturi meter TRIAD company
The venturi tube is the heart of the system and must be designed with care if we want to get the most benefit. the venturi effect is a gift from mother nature.
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Venturi tube has no moving part, so it's cost nothing in maintenance
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Embedded flow meter
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Air suction gives you the ability to run an engine AND filter polluted air
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Air injection into water gives massive oxygenation as result