# CFD Assessment of the Hydrodynamic Performance of Two Impellers for a Baffled Stirred Reactor
## 3 Results and Discussion
Radial profile of the radial velocity below the impeller (i-), Figure 8c, show that, the radial flow was towards the wall of the reactor and the velocity increased right from the axis and attained a maximum. After attaining the maximum value, radial velocity decreased and became small near the wall. From Figure 8 it resulted that the radial component of the velocity generated by the original impeller had lower values than the new designed impeller for all three investigated sections. Figures 9 and 10 show the tangential velocity comparation. It can be noticed that the tangential flow for the new impeller was in the direction of the impeller rotation at all the axial levels, Figure 10. For the new impeller, the value of the tangential velocity increased and decreased steeply as going from the axis towards the tip of the blades. The maximum velocity was comparable with the maximum axial velocity at all the levels. From Figure 10c it resulted that below the original impeller the tangential velocity component had a value close to zero across the entire section. This also explains the poor operation of the original impeller fitted to the stirring mechanism. Analyzing the flow field generated by the original impeller it can be observed, from Figures 6c, 8c and 10c, that dead zones exist right below the impeller and that is the reason why the original impeller did not prevent the sedimentation of the solid particles. From the analysis of the velocity components distribution, it resulted that the new impeller generated a much stronger mixed axial-radial flow than the original impeller. This flow pattern developed by the new impeller has the form of a jet, which spreads radially as it progresses towards the base of the reactor, entraining liquid adjacent to the impeller. After hitting the bottom wall of the reactor, part of the axial momentum gets converted to the radial component along the base towards the side walls of the reactor. Due to this flow pattern, with no dead zones, there was no sedimentation of the solid phase when the new impeller was operating. A calculation was performed to check if the existing electrical motor, which drives the stirring mechanism, can provide sufficient power for the new impeller. The electrical motor generated a power of 3.6 kW and the power needed by the new impeller to operate had the value of 2.32 kW. It resulted that the existing electrical motor can be used to drive the stirring mechanism fitted with the new impeller, so no supplementary cost was generated. To compare the performances of the two impellers from the point of view of consumption of power is recommended to calculate the power number, \(N_{P}\), with the following equation [15; 17]:  
\[N_{P}=\frac{P}{\rho_{l}\cdot n^{3}\cdot d^{5}} \tag{1}\]The value of the power number for the original impeller was 0.43 and for the new designed impeller it was 1.06. Although the power number for the new designed impeller was higher than the original impeller, the hydrodynamic performances were superior. Although it was performing better than the original impeller, the new impeller was not operating at its best, because there was still a consistent volume of liquid, 20%, with no solid particles in it. This means that the part of the volume of liquid with no solid particles in it will be wasted, because no chemical reaction will take part in that region of the reactor. Figure 4: Final state of the volume fraction distribution of the solid phase: (**a**) old impeller; (**b**) new impeller. Figure 5: Axial velocity distribution of the liquid phase, old impeller vs. new impeller. Figure 6: Axial velocity distribution of liquid phase at various axial levels, old impeller vs. new impeller: (**a**) above impeller; (**b**) impeller level; (**c**) under impeller. Figure 8: Radial velocity distribution of liquid phase at various axial levels, old impeller vs. new impeller: (**a**) above impeller; (**b**) impeller level; (**c**) under impeller. Figure 7: Radial velocity distribution of liquid phase, old impeller vs. new impeller.