Turbulent mixing impeller flow characteristics

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. 

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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