Wake fraction

The wake angle 0W defined in Fig. 9.7 is closely related to the shape of a bubble. A smaller 6W represents a flatter bubble, whereas a large 0W represents a more rounded one. A large wake angle 0W yields a smaller wake fraction /w which is defined as the volume ratio of wake to bubble. As shown in Fig. 9.9,0W is related to particle diameter dp. An increase in... 

Figure 9.9. Relationship of the wake fraction and the wake angle for bubbles in a three-dimensional fluidized bed, based on the data of Rowe and Partridge (1965) (from Clift and Grace, 1985).

So far, the influence of bubble wake on mean bubble velocity i b relative to the column wall has not been mentioned, since Eq. (5-3) has been formulated on the basis of uy,o, which already includes the effect of the wake (although it lacks a correction for wake fraction). In bubbling-bed models (FIO, F12, K24, L5, S18) an upward flow of solid carried by the bubbles and bubble wakes leads to a downflow of solid (that has been assumed uniform) in the remainder of the bed. Then the bubble velocity b relative to bed wall should be smaller than the slip velocity of the bubble Ms relative to emulsion, since the bubble phase is retarded by downflow of... 

The bubble wake fraction is given by the volume ratio of the wake to the bubble ... 

The wake fraction is normally determined from experimental analysis [82]. 

Figure 8.6 Wake fraction of three-dimensional bubbles. [After D. Kunii and O. Levenspiel, Fluidization Engineering, 2 ed., with permission of Butterworth-Heinemann, Boston, MA,...

Baeyens and Geldart (1973) also studied the solid circulation and found that for most Group B powders, the wake fraction, p , had an average of about 0.35... 

Table 4 Wake Fraction, Drift Fraction, and Correction Factor for Two-Phase Theory for Some Common Materials...

The wake volume Vu, is defined as the volume occupied by the wake within the sphere that circumscribes the bubble. The wake fraction is normally determined from experimental analysis [48]. 

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