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5.3.4. Cloud height and homogeneity
The velocities of the jet in the region above the impeller begin to decay after reaching a certain height. Negative buoyant jet behaviour is observed at this point and it results in a sudden con- centration gradient and is termed as ‘cloud height’. Beyond this height the velocity is not able to drag the solids. The cloud height was calculated for the stirred tanl‹s at volume fractions 0.04 and
0.07. Apart from just suspension speed, the cloud height is an important parameter for the representation of homogeneity. The cloud height between 0.45 and 0.55T shows just suspension. The cloud height below this height shows poor homogeneity. And a cloud height that reaches 0.9T or above shows the highest quality of homogeneity. The cloud height in the stirred tanl‹ is shown as the iso—contours of the average volume fraction in the stirred tank (Fig. 10).
Fluctuations in the velocity and the solid concentration below the cloud height was observed to be negligible. Due to the sudden change in the flow field and concentration, a zone of high turbulent fluctuations and macroinstabilities forms at the cloud height. The velocity changes frequently and the cloud height fluctuate around a constant value.
At low solid concentration, the influence of solids on the liquid flow field is less. At the same time the l‹inetic energy dissipation of the continuous phase for the suspension of the dispersed phase is lesser as compared to the high loading systems. As a result, the magnitude of the axial velocity is not altered to a great extent. The homogeneity in the stirred tank is, therefore, achievable at
452 D. Wodnerkor er at. f Advanced 'owder Technolo 23 (2012) 44?I-45J
(b) (c) (d)
Fig. 10. The cloud height in stii red tanlts. (a) 0.01 volume fraction 1000 rpm (b) 0.07 volume fraction 600 rpm (c) 0.07 volume fiaction 800 rpm (d) 0.07 volume fi-action 1000 rpm.
low impeller speeds. The cloud height in this case is observed to be higher for the low solid loading system than the high solid loading system (Fig. 10).
Homogeneity is a measure of quality of suspension. The qual— ity of suspension increases with the impeller speed. The increase in the impeller speed results in a higher kinetic energy of the con- tinuous phase which is available for disposal to the solids. A strong velocity field for continuous phase is present at high impeller speeds in various zones of a stii-red tan1‹. The velocity of the jet near the top surface is also larger. The magnitude of up— ward axial velocity near the walls is found to be higher when compared with those at lowei impeller speeds. This result in the attainment of a higher cloud height at higher impeller speeds as is evident from Fig. 10. It is in accordance with the results ob- tained by Sardeshpande et al. [33] for impeller speeds above ‘speed of just suspension’. The non-monotonic behaviour in the cloud height is observed at very low impeller speeds [33]. This ef- fect is due to the presence of pseudo—bottom formed because of the accumulation of undispersed solids at the bottom causing ‘false—bottom' effect. As a result the off-bottom clearance and the amount of suspended solids increases with increasing the impeller speed and hence, the cloud height deci’eases. Since, the impeller speed studied were not very low that can cause ‘false bottom effect’, this non-monotonic behaviour was not observed. The height of the interface between the clear liquid layer and the solid suspension layer is lower in the region above the impel- ler as the recirculating jet forces solids downwards near the axis. This phenomenon is prominent at low impeller speeds due to the weak upward axial flow and the dominance of the drawing action right above the impeller.