Impeller flow characteristics

Boyce, M.P., A Practical Three-Dimensional Flow Visualization Approach to the Complex Flow Characteristics in a Centrifugal Impeller, ASME Paper No. 66-GT-83, June 1983. 

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

As mentioned previously, axial flow impellers are typically used for solids suspension. It is also typical to use radial flow impellers for gas-liquid mass transfer. In combination gas-liquid-solid systems, it is more common to use radial flow impellers because the desired power level for mass transfer normally accomplishes solids suspension as well. The less effective flow pattern of the axial flow impeller is not often used in high-uptake-rate systems for industrial mass transfer problems. There is one exception, and that is in the aeration of waste. The uptake rate in biological oxidation systems is on the order of 30 ppm/hr, which is about to the rate that may be required in industrial processes. In waste treatment, surface aerators typically use axial flow impellers, and there are many types of draft tube aerators that use axial flow impellers in a draft tube. The gas rates are such that the axial flow characteristic of the impeller can drive the gas to whatever depth is required and provide a very effective type of mass transfer unit. 

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. 

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. 

Since data are available on the flows generated by the two impellers, as well as on process performance some speculations can be made on the role of the novel HIT impeller in alkylation. A summary of the relative flow characteristics and alkylate quality are given in the table below. 

The HIT impeller has been shown to generate emulsions with 20% more surface area than did the FBT when compared at the same input power level. This is in line with Brown and Pitt s correlations if the observed flow characteristics for the HIT impeller are used. To increase the emulsion surface area by this amount using the FBT, the power level must be approximately doubled. When the FBT power level is doubled, the improvement in product quality is in line with that obtained with the HIT impeller. This suggests the key role of the impeller in alkylation is to generate emulsion surface area. 

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... 

Because of the excessive computational requirements, there are restrietions on the number of computational cells that can be used for the simulations. Such a limitation may make a priori predictions of the desired flow characteristics such as energy dissipation rates, shear rates near impeller blades etc. less accurate. 

The basic idea is to describe a snapshot of the flow in a stirred vessel with a fixed relative position of blades and baffles. It is assumed that the main flow characteristics of a stirred vessel at the particular time instant in question can be captured approximately from the solution of the steady-state equations, provided that artificial cell volnme adjustments and momentum sources are implemented to represent the effect of the impeller rotation. 

The use of computer generated solutions to problems and computational fluid dynamics is also another approach of comparing impellers and process results. There are software packages available. It is very helpful to have data obtained from a laser velocity meter on the fluid mechanics of the impeller flow and other characteristics to put in the boundary conditions for these computer programs. 

There is a different power number for each impeller, reflecting its uniqne shape and drag-producing characteristics. The power number values given in Table 9.1 are only applicable for tnrbulent flow. Mannfactnrers offer a variety of impellers, the characteristics of which are given in Table 9.1. 

Note that Equation (9.85) is basically the same general form as the familiar Dittus-Boelter equation for heat transfer in tubes. The basic heat-transfer mechanism is identical. It is dependent on the flow of fluid next to the heat-transfer surfaces, whether these are the vessel walls or some internals. Differences in the correlations are therefore mainly due to the differences in flow characteristics generated by the different impellers relative to the surface under consideration. This is reflected in the value of K. 

The rest of this section is a comparative appraisal of the flow characteristics of commonly used impeller systems (Figure 13.2) [1]. 

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.)...

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