Swept diameter, impeller

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 ... 

If it is turning at suitable speed, the impeller will break the gas up into very fine bubbles, which are swept out radially from the tip. In the imme ate vicinity of the impeller tip, the specific interfacial area is consequently very large. Coalesc-nece of the bubbles in the slower-moving liquid in other parts of the vessel increases the bubble diameter and lowers the local area. For moderate impeller speeds, only the gas fed to the impeller produces the specific interfacial area designated as but at higher impeller speeds, especially in small vessels, gas drawn from the space above the liquid contributes importantly to produce the larger mean specific area a. For disk flat-blade turbines, the specific area can be estimated from [23, 24, 84] ... 

The pumping capacity of an impeller is proportional to the impeller tip speed N it d and the swept area of the impeller n d h. Here, N is the impeller speed, d is the impeller diameter, and h is the height of the impeller blades. Assuming that the impeller blades scale with the reactor size, D, we can assume that djD and hiD are constant on all scales, and consequently the pumping rate is given by... 

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