Venturi effect

In the tapered labyrinth (Figure 5-5), a fixed labyrinth seals against a tapered shaft. As the shaft moves axially, the clearance of the seal changes. This allows an excess seal gas flow. The resulting venturi effect actually drags the lubricating oil into the process stream. Also, since the tapered shaft seal makes it difficult to provide an effective oil slinger, the problem is compounded. 

Droplets are dispersed from nebulizer nozzles by one of five basic mechanisms based on Bernoulli (Venturi) effect. 

A new concrete floor incorporating barriers to radon transport from the subsoil appeared to be only partially successful in reducing radon decay-product concentrations. It was shown that the Venturi effect of the wind across two chimney stacks caused pressure-driven flow of radon from the ground. Covering the fireplaces to eliminate this effect resulted in concentrations below the reference level. 

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. 

Q. How does the venturi effect contribute to the nebulization of the sample ... 

Nebulizers are designed primarily for the atomization of aqueous formulations either as solutions or suspensions, and typically contain additional excipients. These systems are nonpressurized formulations and do not contain propellants. Traditionally, nebulizers operated using one of two basic mechanisms jet nebulization or ultrasonic nebulization. Jet nebulizers (Fig. 3) function using the Venturi effect to... 

Flame atomisation. A burner is fed with a combustible gas mixture and constructed in a robust fashion to resist possible explosions of the gas (Fig. 14.8). The flame, which has a rectangular base of approximately 10 cm by 1 mm is aligned with the optical axis of the instrument. The sample is aspirated by the Venturi effect into the mixture of combustible gases feeding the flame. 

Reduced bore PRV A PRV in which the flow path area bdow the seat is smaller than the flow area at the inlet to the valve, creating a venturi effect. 

The most commonly used atomiser is the chemical flame, based upon the combination of a fuel gas (e.g., acetylene) with an oxidant (e.g., air or nitrous oxide). The sample solution is introduced into the flame using a nebuliser in which the passage of the oxidant creates a partial vacuum by the venturi effect and thus the sample solution is drawn up through a capillary. Thus, an aerosol is produced having a wide variety of droplet sizes. This process is shown in Fig. 2. 

Exposed dishes (1-4) show slightly higher speeds than protected dishes (5-12) Dish 4 however shows the highest speed due to venturi effect. 

A liquid-junction interface has also been suggested and applied for CE-ESI-MS [8]. Electrical contact with this interface is established through the liquid reservoir which surrounds the junction of the separation capillary and a transfer capillary, as shown in Fig. lb. The gap between the two capillaries is approximately 10-20 fim, allowing sufficient makeup liquid from the reservoir to be drawn into the transfer capillary while avoiding analyte loss. The flow of makeup liquid into the transfer capillary is induced by a combination of gravity and the Venturi effect of the nebulizing gas at the capillary tip [8]. 

The main problem encountered with PFT with a neutral selector remains the prevention of the chiral selector from entering the ionization source. This problem becomes particularly important at a high pH where EOF is important. Therefore, an acidic buffer pH or a coated capillary to minimize EOF is of the utmost importance. In addition, it has to be noted that the electrospray ionization process is pneumatically assisted when a sheath-liquid interface is used. The coaxial sheath gas induces an aspirating phenomenon in the capillary which may considerably affect the separation quality. This can be due to the decrease in interaction between analytes and the chiral selector and to a hydrodynamic flow induced by the Venturi effect at the capillary end [33, 34]. 

Blood flow to the coronary arteries arises from orifices located immediately distal to the aorta valve. Perfusion pressure is equal to the difference between the aortic pressure at an instantaneous point in time minus the intramyocardial pressure. Coronary vascular resistance is influenced by phasic systolic compression of the vascular bed. The driving force for perfusion therefore is not constant throughout the cardiac cycle. Opening of the aortic valve also may lead to a Venturi effect, which can slightly decrease perfusion pressure. If perfusion pressure is elevated for a period of time, coronary vascular resistance declines, and blood flow increases however, continued perfusion pressure increases lead, within limits, to a return of coronary blood flow back toward baseline levels through autoregulation. 

Continuous systems are mostly pressurized and normally no aeration occurs. A well-designed system will not incorporate air in the suction of the centrifugal pumps and will have no accidental venturi effect. 

Virtually all modern flame AAS (FAAS) instruments make use of a pre-mix nebuliser in combination with a laminar burner design. A typical design is presented in Figure 5. The flows of gaseous fuel and oxidant gases into the nebuliser create a Venturi effect across the exit of a capillary tube. As a result, liquid sample is aspirated through the capillary (at rates of 2 to 6 mL/min) and exits into the nebuhser chamber as an aerosol (with a rather wide range of droplet sizes). An impact bead or a flow spoiler is typically used to further smash up the droplets and to increase turbulence of the flow... 

This is a cocurrent scrubber with pressure drop advantages (venturi effect) and a high degree of turbulence at the point of contact. The equilibrium limitation as indicated in Figure 13.20 prevails. Venturi units have found applications for rapid reactions and for particulate removal. As shown in Figure 13.21, elaborate means for separating the effluent gas and liquid may be necessary. 

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