001

Pitched blade turbine I plane passing through center of the blades

FIGURE 10.15 Simulated results at typical r-z plane for a pitched blade turbine (from Ranade et al., 2001a). Reproduced in colour plate section between pages 210 and 211.

conclusions can be drawn from the comparisons of normalized radial velocity shown in Figs 10.19a-c. For the case of turbulent kinetic energy, Fig. 10.20a indicates that predicted turbulent kinetic energy values are much higher than those observed in experimental data obtained within the impeller swept region. Outside the impeller swept region, however, agreement between predicted and experimental data is much better. The snapshot approach was also shown to be capable of capturing the influence of blade angle and blade width of a pitched blade turbine on generated flow (Ranade and Dommeti, 1996b). Thus, the snapshot approach can be used to simulate flow generated by impellers with complex blade shapes. It can be used to evaluate the influence of impeller blade shape and size on the generated flow field.

10.3.3. Simulation of Flow Generated by Multiple Impellers

Results described so far suggest that the snapshot approach can be used to make a priori predictions of the complex flow generated in stirred vessels for impellers of any shape. A number of industrial stirred tank reactors make use of two or more impellers mounted on the same shaft. When more than one impeller is used, the flow complexity is greatly increased, especially when there is interaction between the flow generated by the two impellers. The extent of interaction depends on relative distances between the two impellers (and clearance from the vessel bottom). In order to examine whether the computational snapshot approach can be used to simulate

Iso-surface of Z-vorticity (a>DU^= -7) Impeller rotation: from right to left

FIGURE 10.16 Presence of trailing vortices (pitched blade turbine) (from Ranade et al., 2001a).

Iso-surface of Z-vorticity (a>DU^= -7) Impeller rotation: from right to left

FIGURE 10.16 Presence of trailing vortices (pitched blade turbine) (from Ranade et al., 2001a).

interaction between multiple impellers, the case of a dual Rushton turbine, studied by Rutherford et al. (1996), was considered.

The flow structure in vessels agitated by dual impellers is determined mainly by the flow characteristics of the impellers and interactions between them. When the clearance between the two impellers is sufficiently high, they are likely to act independently of each other. For smaller clearances, the two impeller streams may interact, resulting in complicated and often unstable flow patterns. Rutherford et al. (1996) experimentally studied the flow generated by dual Rushton turbines in cylindrical baffled vessels. They report three stable flow patterns observed with different values of lower impeller clearance (C1), impeller separation (C2) and upper impeller submergence (C3). These three patterns are qualitatively shown in Fig. 10.21. The parallel flow pattern shown in Fig. 10.21a was observed when two impellers were well separated (C1 = C3 = 0.25T, C2 = 0.5T). In this pattern, each impeller generated its own characteristic upper and lower ring vortex leading to formation of four stable ring vortices. When impeller separation was decreased (C1 = C2 = C3 = T/3), the flow pattern shown in Fig. 10.21b was observed. It was termed 'merging flow'

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