Power number

Power consumption was evaluated at pseudo-steady state as described in Appendix A. We considered the flow field in the CE12500 to be pseudo steady when the axial flow rate and the azimuthal momentum averaged over one impeller revolution were both steady. Figure 4.22 shows distribution of forces on the impeller for simulation S6. Shear forces are on the left and total forces, i.e. pressure and shear forces, are on the right. Pressure forces represent the main contribution to the total force, as can be observed by the absolute value of shear and total forces. Shear is maximum at impeller tip and behind the blade, where vortices generated by the upper and lower profiles of the blades merge. Total force is at maximum at the upper and lower profiles of the blades. Figure 4.23 shows the distribution of total forces on the baffle. From this distribution and its variation in time important information can be obtained about the mechanical stresses generated by the blades. Distribution of total forces is used to calculate the power consumption, as described in Appendix A. Table 4.7 summarizes the values of power numbers obtained for the simulations.

Figure 4.22: Shear (left) and Total force (right) distribution on impeller.

Figure 4.23: Total force distribution on baffle.

Table 4.7: Power numbers calculated for the different simulations.

S1	S2	S3	S4	S5	S6
2.07	0.978	0.896	0.853	0.834	0.812

Values of power number are compared against the power characteristic obtained from Nagata (1975) as described in Appendix B and shown in Figure 4.24.

Figure 4.24: Power number versus Reynolds number: CE12500

We compared our results against data supplied by Tycon $-$ Technoglass (Sassetto and Artusi, 2000). These data are referred to as reference curve. The reference curve and the two baffles curve collapse onto each other coincident. Power number values calculated by the simulations are in good agreement with the power characteristic obtained from Nagata (1975). The power characteristic and the values of power number computed for the laboratory scale vessel may not be compared with those obtained for the industrial size vessel. Even if the reactors are geometrically similar, the CE12500 has two beaver tail baffles, while the laboratory vessel is simulated in the configuration with a single beaver tail baffle. As a consequence, power numbers calculated for the CE12500 are larger than those calculated for the laboratory vessel.