Radial Discharge Impeller

For gas-dispersion applications, a radial discharge impeller, such as the straight-blade and disk style impellers in Fig. 12.1, should be used. A straight-blade impeller with a power number NP of 3.86 will be used for this design. Assume that the operating speed will be 100 r/min. 

Looking at mixing as a turbulent flow generated by an impeller, a study of the flows generated by conventional and novel radially discharging impeller styles was done at Exxon s... 

Kolar V., Filip P., Curev A.G., Hydrodynamics of a radially discharging impeller stream in agitated vessels,... 

Kuhni contacters (Eig. 15e) have gained considerable commercial application. The principal features are the use of a shrouded impeller to promote radial discharge within the compartments, and a variable hole arrangement to allow flexibility of design for different process applications. Columns up to 5 m in diameter have been constmcted (176). Description and design criteria for Kuhni extraction columns have been reported (177,178). 

A fan in which the air path through the impeller is intermediate between the axial and centrifugal types giving the benefit of increased pressures but capable of being con-stmcted to provide either axial or radial discharge. Static efficiency is typically 70-75 per cent. 

FIGURE 10 Illustration of average velocity from the radial discharge of a radial flow impeller, showing the definition of fluid shear rate (AV/AY). 

Figure 14. Effect on the angle between the blades on the average velocity at a particular position in the discharge stream from a radial flow impeller...

The impellers used in this study are shown in Figure 1. They are both of the radially discharging type which is characterized by a strong radial jet of fluid that moves out to the vessel walls while entraining fluid from above and below. Near the wall it splits into two circulation zones. The flat blade turbine (FBT) is commonly used in industry and consists of a disc with several paddles fixed normal to the disc which serve to generate the radial flow. The novel impeller design... 

Advances in development resulted in the Oldshue-Rushton and Kiihni extractors [43, 45]. The Kiihni extractor has shrouded turbine impellers for agitation to promote radial discharge characteristies (Fig. 2.19). The stator discs are made from perforated plates and the residence time can be varied by changing the distance and the hole diameter of these plates. These extractors can be used in the dispersion or mixer-settler mode. Therefore, they can be adapted to extreme phase ratios and reach high... 

Bittins K., Zehner P, Power and discharge numbers of radial-flow impellers. Fluid-dynamic interactions between impeller and baffles, Chem. 

In a tank with radial impellers, suitable baffles will produce strong top-to-bottom currents from the radial discharge. The installation of baffles generally increase the power consumption [65]. For axial flow impellers, the need for baffling is not as great as for radial flow impellers, thus axial flow impellers also consume less power than radial impellers. Baffles are normally used in turbulent mixing only. 

Impeller clearance affects total discharge flow and its direction. For example, when axial-flow impellers are placed close to the bottom, they produce a radial discharge. Upward-angled, retreat-curve impellers are always located at the bottom of the vessel, where they produce strong radial flow and produce good circulation while handling level changes and solids effectively. 

As the jet travels away from the impeller, it slows down because of the increased area for flow and because more liquid is entrained. Along the centerline of the impeller, the velocity drops more or less linearly with radial distance, and the product 7 r is nearly constant, as was shown by other studies. The total volumetric flow increases with radius to about l.2qg because of further entrainment and then drops near the vessel wall because the flow has started to divide into the upward and downward circulation loops. The maximum flow of 1.2 compared to the radial discharge velocity of 0.6 2 indicates a total flow twice the direct impeller discharge, in agreement with the factor of 2.1 calculated using Eq. (9.9). 

Radial flow impellers discharge fluid in a radial direction. The subsequent low pressure region created at the centre or eye of the impeller causes fluid to enter from an axial direction. The resulting circulation consists of a radial jet or impeller stream at the level of the impeller driving circulation cells above and below the impeller (in this instance, there will be minimal circulation below the impeller due to its close... 

Now that we have a means for estimating the shear, it is of interest to see how often and for what duration, agglomerates will be subject to high levels of shear. The chamber has a throughflow of Q . The radial or impeller stream discharge Qr can be approximated by ... 

RADIAL FLOW IMPELLERS Rushton turbine (DT) is the most widely used in this category. Therefore, the following discussion is limited to its performance. As shown in Figure 7A.13, for the standard [CIT=DIT=1/3] configuration, DT discharges a liquid stream directed radially to the vessel wall. This stream divides into two portions at the vessel wall—one portion directed down toward the base, while the rest travels upward. The lower flow loop has to travel considerable distance comprising three sections (1) outward along the vessel radius from the impeller tip, (2) downward at the wall, and... 

In this expression, is the flow rate produced by the impeller. The subscript is used to ensure that the flow rate for the liquid phase alone is used in the calculation. To compute Qt for an impeller, a surface needs to be created for the discharge region. This surface would be circular for an axial flow impeller and a section of cylinder waU for a radial flow impeller. By integrating the total outflow through this surface, the flow rate, Q, and subsequently the flow number, Nq, can be obtained. 

Radial flow impellers may either have a disk (Rushton turbine) or be open (FBT) and may have either flat or curved blades (backswept mrbine). Impellers without the disk do not normally pump in a true radial direction since there is pressure difference between each side of the impeller. This is also true when the impellers are positioned in the tank at different off-bottom clearances. They can pump upward or downward while discharging radially. Radial discharge flow patterns can cause stratification or compartmentalization in the mixing tank. Disk-type radial impellers provide more uniform radial flow pattern and draw more power than open impellers. The disk is a baffle on the impeller, which prevents gas from rising along the mixer shaft. In addition, it allows the addition of a large number of impeller blades. Such blade addition cannot be done easily on a hub. A disk can also be used with a pitched blade turbine for use in gas-liquid mixing. 

CFD (as described above) has been used to predict mixing times in liquid-phase systems. In the authors experience, once the tracer input condition has been carefully modeled to match an experiment, reasonable agreement with experimental values has been obtained for axial flow impellers. For radial flow impellers, the predicted mixing times were longer than the measured values this appears to be caused by inadequate description of the vertical transfer between the blade vortices in the impeller discharge stream. The effects of gassing have, however, not been explored. 

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