Venturi effect velocity

The increase in velocity seen as part of the Venturi effect simply demonstrates that a given number of fluid particles have to move faster through a narrower section of tube in order to keep the total flow the same. This means an increase in velocity and, as predicted, a reduction in pressure. The resultant drop in pressure can be used to entrain gases or liquids, which allows for applications such as nebulizers and Venturi masks. 

Figure 2.7 shows a typical pneumatic nebulization system for a premixed flame. The sample is sucked up a plastic capillary tube. In the type of concentric nebulizer illustrated here, the sample liquid is surrounded by the oxidant gas as it emerges from the capillary. The high velocity of this gas, as it issues from the tiny annular orifice, creates a pressure drop which sucks up, draws out and shatters the liquid into very tiny droplets. This phenomenon is known as the venturi effect and is illustrated in Fig. 2.8. 

The total-consumption burner operates essentially as an ordinary pneumatic nebulizer. The oxidizing gas enters the burner through the aspirating gas inlet at a fairly high pressure, about 20-35 psi. It is directed around the capillary aspirator tip where the velocity of the gas produces a Venturi effect, drawing the sample into the stream. The fuel gas, in turn, is directed around the oxidizing stream and mixed with the oxidant and sample, and aids in breaking up the sample. The flame burns at the top of the burner. 

Venturi effect = suction created by conversion of pressure energy to kinetic (velocity) energy. 

A high volumeteric air flow at a low linear velocity is desired. A high volumetric flow gives the parison a minimum time to cool before coming in contact with the mold, and provides a more uniform rate of expansion. A low linear velocity is desirable to prevent a venturi effect (see above). Volumetric flow is controlled by the line pressure and the orifice diameter. Linear velocity is controlled by flow control valves close to the orifice. 

Vacuum pump, Venturi A vacuum pump using the Venturi effect (i.e. entrainment of gas in a high velocity gas or liquid stream) to reach a vacuum as low as 60Torr. 

If it becomes necessary to increase the stack-gas exit velocity to avoid downwash, it may be necessary to remodel the stack exit. A venturi-nozzle design has been found to be the most effective. This design also keeps pressure losses to a minimum. 

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

F.G. Hammitt, D.M. Ericson, J.F. Lafferty and M.J. Robinson, Gas content, size, temperature, and velocity effects on cavitation inception in a venturi, Amer. Soc. Mech. Engin. paper 67-WA/FE-22,1968. 

Although such a case is unusual, it has been found at times. A reason for introducing this discussion is that the venturi tube is one case where there may be considerable difference in velocity profiles at the two sections, and it is desirable to observe that what is ordinarily termed friction may include other effects as well. It is in cases where true friction is very small that these other factors become appreciable. The foregoing discussion also explains the use of a converging section preceding the working section of every wind tunnel or water tunnel where it is desired to have a uniform velocity distribution. 

Accuracy is improved if the flowing gas stream is directed at the probe by a venturi nozzle, or by placing the sensor in the throat of the venturi (Figure 3.74). The venturi ensures a smooth velocity profile and eliminates boundary layer effects while concentrating the flow onto the sensor. These units are available for both liquid and gas services. Other designs are of the insertion probe type. Their flow ranges are a function only of the size of the pipe into which they are inserted, and their performance is a function of the correctness of the insertion depth (as are all Pitot tubes). 

Cavitation is not exclusive of ultrasound. Thus, hydrodynamic cavitation can simply arise from passage of the liquid through a constrictor such as a throttling valve, orifice plate, Venturi tube, etc. On passage through the constrictor, the kinetic energy — velocity — of the liquid increases at the expense of the pressure. Various types of hydrodynamic cavitation reactors have been reported and their most salient features reviewed [17]. Also, the effects of ultrasound and hydrodynamic cavitation on oxidation processes have recently been compared [18]. 

The gas-side mass-transfer coefficients kefl and ko increase with liquid feed rate or with gas velocity at each given position in the venturi scrubber and decrease at constant liquid rate and gas velocity with increasing distance from the point of liquid injection (J7, VI1). The values ofkifl generally increase with increasing liquid flow rate or gas velocity (often referred to as the velocity at the throat). However, ki,a will sometimes exhibit a maximum when the gas velocity increases the explanation is that, at higher gas velocities, an increase in turbulence in the throat of the venturi results in the formation of droplets smaller than the thin filaments first formed at lower gas velocities. Internal circulation is reduced in these smaller droplets, and there is also a reduction in the size of the zone of intense turbulence. These two phenomena lead to a maximum for the values of/cL. as found experimentally by Kuznetsov and Oratovskii (K15) and Virkar and Sharma (VI1). The values of the effective interfacial area a increase with both gas and liquid flow rates. 

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