Turbulent impeller

Turbulent impellers are classified as axial or radial flow impellers. Axial flow impellers cause the tank fluid to flow parallel to the impeller s rotation axis. Radial flow impellers cause the tank fluid to... 

There are many ways to obtain mixing in a vessel. This chapter focuses on the turbulent impeller type of mixers, as they are frequently applied in the chemical process industries. 

Fig. 7.2. Typical turbulent impellers [87]. By permission from Tatterson (personal communications, 2006).

It is important to note that some impellers and mixer configurations do not pump weU. For example, the retreat curve blade impeller in an unbaffled vessel creates solid body rotation and poor pumping. Pumping with close-clearance impellers such as anchors and helical ribbon can be very high or, sometimes, very poor, depending on conditions and the materials being pumped. Turbulent impellers in laminar applications only pump locally. Often, the rest of the tank goes unmixed. 

Cavitation is a function of pressure drop occurring across the impeller blades, the fluid temperature and the vapor pressure of the fluid. If the fluid is near its boiling point, cavitation will occur with the turbulent impellers. Cavities are bubbles that are gathering behind the blades of a rotating impeller, these are characterized as... 

Clearance of turbulent impeller from the tank bottom (m)... 

The power number depends on impeller type and mixing Reynolds number. Figure 5 shows this relationship for six commonly used impellers. Similar plots for other impellers can be found in the Hterature. The functionality between and Re can be described as cc Re in laminar regime and depends on p. N in turbulent regime is constant and independent of ]1. 

There are two main reasons why a pump should not operate below its MCSF (/) the radial force (radial thmst) is increased as a pump operates at reduced flow (44,45). Depending on the specific speed of a pump, this radial force can be as much as 10 times greater near the shut off, as compared to that near the BEP and (2) the low flow operation results in increased turbulence and internal flow separation from impeller blades. As a result, highly unstable axial and radical fluctuating forces take place. 

Figure 6-40 shows power number vs. impeller Reynolds number for a typical configuration. The similarity to the friction factor vs. Reynolds number behavior for pipe flow is significant. In laminar flow, the power number is inversely proportional to Reynolds number, reflecting the dominance of viscous forces over inertial forces. In turbulent flow, where inertial forces dominate, the power number is nearly constant. 

Impeller Discharge Rate and Fluid Head for Turbulent Flow. 18-11... 

Impeller Reynolds Number The presence or absence of turbulence in an impeller-stirred vessel can be correlated with an impeller Reynolds number defined... 

Inertial forces are developed when the velocity of a fluid changes direction or magnitude. In turbulent flow, inertia forces are larger than viscous forces. Fluid in motion tends to continue in motion until it meets a sohd surface or other fluid moving in a different direction. Forces are developed during the momentum transfer that takes place. The forces ac ting on the impeller blades fluctuate in a random manner related to the scale and intensity of turbulence at the impeller. 

When fluid viscosity is low and flow is turbulent, an impeller moves fluids by an increase in momentum from the blades mich exert a force on the fluid. The blades of rotating propellers and turbines change the direction and increase the velocity of the fluids. 

As in the case of turbulent flow, then, smaU-diameter impellers (D < Df/3) are useful for (1) rapid mixing of diyparticles into hquids,... 

The drop size varies locally with location in the vessel, being smallest at the impeller and largest in regions farthest removed from the impeller owing to coalescence in regions of relatively low turbulence... 

The fluidfoil impellers (shown in Fig. 18-2) usually give more flow for a given power level than the traditional axial- or radial-flow turbines. This is also thought to be an advantage since the heat-transfer surface itself generates the turbulence to provide the film coefficient and more flow should be helpful. This is true to a limited degree in jacketed tanks (Fig. 18-34), but in helical coils (Fig. 18-35), the... 

Figures 18-36, 18-37, and 18-38 show some approaches. Figure 18-36 shows velocity vectors for an A310 impeller. Figure 18-37 shows contours of kinetic energy of turbulence. Figure 18-38 uses a particle trajectory approach with neutral buoyancy particles.

Damage will be confined to the bubble-collapse region, usually immediately downstream of the low-pressure zone. Components exposed to high velocity or turbulent flow, such as pump impellers and valves, are subject. The suction side of pumps (Case History 12.3) and the discharge side of regulating valves (Fig. 12.6 and Case History 12.4) are frequently affected. Tube ends, tube sheets, and shell outlets in heat exchanger equipment have been affected, as have cylinder liners in diesel engines (Case History 12.1). 

Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

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