Flow patterns, axial

The Reynolds stress model requires the solution of transport equations for each of the Reynolds stress components as well as for dissipation transport without the necessity to calculate an isotropic turbulent viscosity field. The Reynolds stress turbulence model yield an accurate prediction on swirl flow pattern, axial velocity, tangential velocity and pressure drop on cyclone simulation [7,6,13,10],... 

Fig. 25. Flow patterns in jet mixed tanks where represents 2ones that are poody mixed (a) side entry and (b) axial.

Numerous studies for the discharge coefficient have been pubHshed to account for the effect of Hquid properties (12), operating conditions (13), atomizer geometry (14), vortex flow pattern (15), and conservation of axial momentum (16). From one analysis (17), the foUowiag empirical equation appears to correlate weU with the actual data obtained for swid atomizers over a wide range of parameters, where the discharge coefficient is defined as — QKA (2g/ P/) typical values of range between 0.3 and 0.5. 

Fig. 11. Two-truck tray dryer. A, air inlet duct B, air-exhaust duct with damper C, axial flow fan D, fan motor, 2—15 kW E, air heaters F, air-distribution plenum G, distribution slots and H, wheeled tmcks and trays. The arrows iadicate air and vapor flow pattern.

FIG. 6-39 Typical stirred tank configurations, showing time-averaged flow patterns for axial flow and radial flow impellers. From Oldshue, Fluid Mixing Technology, McGraw-Hill, New Yo7 k, 1983.)... 

In another land of ideal flow reactor, all portions of the feed stream have the same residence time that is, there is no mixing in the axial direction but complete mixing radially. It is called a.plugflow reactor (PFR), or a tubular flow reactor (TFR), because this flow pattern is characteristic of tubes and pipes. As the reaction proceeds, the concentration falls off with distance. 

There are three types of mixing flow patterns that are markedly different. The so-called axial-flow turbines (Fig. 18-3) actually give a flow coming off the impeller of approximately 45°, and therefore have a recirculation pattern coming back into the impeller at the hub region of the blades. This flow pattern exists to an approximate Reynolds number of 200 to 600 and then becomes radial as the Reynolds number decreases. Both the RlOO and A200 impellers normally require four baffles for an effective flow pattern. These baffles typically are V12 of the tank diameter and width. 

FIG. 18 12 Typical flow pattern for either axial- or radial-flow impellers in an unhaffled tank. 

For Reynolds numbers greater than 2000 baffles are commonly used with turbine impelTers and with on-centerhne axial-flow impellers. The flow patterns illustrated in Figs. 18-15 and 18-16 are quite different, but in both cases the use of Baffles results in a large top-to-bottom circulation without vortexing or severely unbalanced fluid forces on the impeller shaft. 

In the region of laminar flow (Vr < 10), the same power is consumed by the impeller whether baffles are present or not, and they are seldom required The flow pattern may be affected by the baffles, but not always advantageously. When they are needed, the baffles are usually placed one or two widths radially off the tank wall, to allow fluid to circulate behind them and at the same time produce some axial deflection of flow. 

FIG. 18-15 Typical flow pattern in a baffled tank with a propeller or an axial-flow turbine positioned on center. 

The fluidfoil impellers in large tanks require only two baffles, but three are usually used to provide better flow pattern asymmetiy. These fluidfoil impellers provide a true axial flow pattern, almost as though there was a draft tube around the impeller. Two or three or more impellers are used if tanks with high D/T ratios are involved. The fluidfoil impellers do not vortex vigorously even at relatively low coverage so that if gases or solids are to Be incorporated at the surface, the axial-flow turbine is often required and can be used in combination with the fluidfoil impellers also on the same shaft. 

Axial-flow turbines are often used in blendiug pseudoplastic materials, and they are often used at relatively large D/T ratios, from 0.5 to 0.7, to adequately provide shear rate in the majority of the batch particularly in pseudoplastic material. These impellers develop a flow pattern which may or may not encompass an entire tank, and these areas of motion are sometimes referred to as caverns. Several papers describe the size of these caverns relative to various types of mixing phenomena. An effec tive procedure for the blending of pseudoplastic fluids is given in Oldshue (op. cit.). 

The spiral assembly can be fitted with covers to provide three flow patterns (1) both fluids in spiral flow, (2) one fluid in spiral flow and the other in axial flow across the spiral, and (3) one fluid in spiral flow and the other in a combination of axial and spiral flow. 

Intermittent (I) - In this flow pattern the liquid inventory in the pipe is non-uniformly distributed axially. Plugs or slugs of liquid that fill the pipe are separated by gas zones that contain a stratified liquid layer flowing along the bottom of the... 

The forces applied by an impeller to the material contained in a vessel produce characteristic flow patterns that depend on the Impeller geometry, properties of the fluid, and the relative sizes and proportions of the tank, baffles and impeller. There are three principal types of flow patterns tangential, radial and axial. Tangential flow is observed when the liquid flows parallel to the path described by the mixer as illustrated in Figure 7. 

Figure 7-7. Axial flow pattern produced by a marine propeller. (Source Holland, F. A. and Bragg, R. Fluid Flow for Chemical Engineers, 2nd ed., Edward Arnold, 1995.)...

Axial flow deviees sueh as high-effieieney (HE) impellers and pitehed blade turbines give better performanee than eonventional pitehed blade turbines. They are best suited to provide the essential flow patterns in a tank that keep the solids suspended. High-effieieney impellers effeetively eonvert meehanieal energy to vertieal flow... 

Figure 7-9. Agitator flow patterns, (a) Axial or radial impellers without baffles produoe vortex, (b) Off-oenter looation reduoes the vortex, (o) Axial impeller with baffles, (d) Radial impeller with baffles. (Source Wales, S. M., Chemioal Prooess Equipment—Seleotion and Design, Butterworths Series in Chemical Engineering, 1988.)...

Table 7-4 shows flow patterns and applications of some commercially available impellers. Generally, the axial flow pattern is most suitable for flow sensitive operation such as blending, heat transfer, and solids suspension, while the radial flow pattern is ideal for dispersion operations that require higher shear levels than are provided by axial flow impellers. Myers et al. [5] have described a selection of impellers with applications. Further details on selection are provided by Uhl and Gray [6], Gates et al. [7], Hicks et al. [8] and Dickey [9]. 

In eertain eases, the primary proeess objeetive is to keep solid partieles in suspension. Areas of applieation involve eatalytie reaetions, erystallization, preeipitation, ion exehange, and adsorption. Axial flow and pitehed-blade turbines are best suited in providing the essential flow patterns in a tank to keep the solids in suspension. The suspended solid is eharaeterized by two parameters ... 

Figure 5-4A. Axial-flow pattern produced by a pitched-blade turbine. By permission, Oldshue, J. Y. [25],...

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