Impeller turbulent flow characteristics

Impeller and Flow Characteristics For Turbulent, Baffled Systems Simple Ratio Relationships... 

Results on the FBT agree with earlier studies of Mujumdar et al ( ) who used a hot wire anemometer for their measurements. Recent mixing literature (1, 4) suggests that the turbulent flow characteristics of the novel HIT impeller should be of value in alkylation. 

Flow characteristics in a mixing vessel can influence process performance. The impeller is a device which imparts motion to the medium in which it operates. The characteristics of the flow which are of greatest interest are the mean fluid velocity at all points within the fluid and the turbulent fluctuations superimposed on the mean velocity. Paul and Treybal ( ) have discussed how the detailed flow characteristics can influence process performance. This paper will show how impeller style can influence the flow characteristics. 

Flow in baffled stirred reactors has been modeled by employing several different approaches which can be classified into four types, and are shown schematically in Fig. 10.3. Most flow simulations of stirred vessels published before 1995 were based on steady-state analyses (reviewed by Ranade, 1995) using the black box approach. This approach requires boundary conditions (mean velocity and turbulence characteristics) on the impeller swept surface, which need to be determined experimentally. Although this approach is reasonably successful in predicting the flow characteristics in the bulk of the vessel, its usefulness is inherently limited by the availability of data. Extension of such an approach to multiphase flows and to industrial-scale reactors is not feasible because it is virtually impossible to obtain (from experiments) accurate... 

Lamade [315] has evaluated the suspension characteristics for three different enameled stirrers (PFAUDLER impeller, pitched-blade, paddle stirrers) in the turbulent flow range. He found a process relationship of the form ... 

For highly turbulent flow (i.e., constant Kfg), the required pump head is a quadratic function of the flow rate Q. This relation, which is superimposed on the pump characteristic curves (see line SI in Figure 5.9), is the operating line for the system. The actual pump head and the resulting flow rate are determined by the intersection of the operating line and the pump impeller characteristic curve. For the specified flow rate, the best (least cost) pump/impeller/motor combination that will provide this flow rate should be selected. 

Extensive basic research by many authors together with measurements taken in industrial vessels has shown that the blend time characteristic n is a constant for stirred vessels fitted with baffles. This applies generally for turbulent flow regimes, but also for laminar flow induced by positive displacement impeller systems (Figure 13.6). The numerical value of this constant depends mainly on the type of impeller and the diameter ratio. 

In fully turbulent flow, viscous forces become negligible relative to turbulent stresses and can be neglected (except for their action at the dissipative scales of motion). This has an important implication above a certain Reynolds number, all velocities will scale with the tip speed of the impeller, and the flow characteristics can be reduced to a single set of dimensionless information, regardless of the fluid viscosity. One experiment in the fiiUy turbulent regime can be applied for all tanks that are exactly geometrically similar to the model, at all Reynolds numbers... 

Figure 2-16 Scaling of flow characteristics, (a) Scaling of velocity profiles with tip speed in fully turbulent flow. (From Nouri et al., 1987.) (b) Scaling of dissipation with for the Lightnin A310 impeller, D = 0.475T. (From Zhou and Kresta, 1996b.)...

Referring to Table 5-3 for turbulent, baffled systems, if power is held constant and the system has too large a shear characteristic and apparently too small a volume or flow, the impeller can be increased 20% and the new speed at constant Pow er, P, wall be ... 

With more specific reference to agitated systems, information is lacking on the effects of gas bubbles on basic properties like mean flow pattern, impeller discharge rate, or turbulence characteristics. The observations presented in Section II, based mainly on one-liquid-phase data, must therefore be considered as the best available approximations for the flow regimes in gas-liquid agitated systems. There have been a few papers of somewhat basic nature with direct application to these systems and these will be discussed in the remainder of this section. 

Impeller size (DIT ratio) influences whether flow or turbulence governs the process of mixing. Oldshue [5] shows, for equal process results, that impeller diameter affects power and torque characteristics, as shown in Figure 9.5. The values for power and torque are normalized to the values for an impeller with DIT = 0.333. 

This scaling, however, introduces a factor of (D/T). This may work well where the bulk characteristics of the flow dominate, but it is not an accurate measure of turbulence if local characteristics are needed. For the same power input per unit tank volume, or holding eq. (2-18) constant with variations in impeller type, diameter, and off-bottom clearance, Zhou and Kresta (1996a) provided an extensive set of data and showed that the local dissipation can vary by up to a factor of 100. This is illustrated for the Intermig on the Visual Mixing CD affixed to the back cover of the book. The best order-of-magnitude estimate of the maximum dissipation uses the swept volume of the impeller instead of the total tank volume ... 

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