5.4. Effect of impeller clearance c/D

The impeller location plays also an important role in the mixing system. For this purpose, three geometries are tested which are c/D = 0.03, 0.26 and 0.5, respectively.

P = η

vesselvolume

The element dv is written as:

Qv dv (4)

The flow fields are independent of the impeller clearance

from bottom of the tank (Figure 17). The axial velocity becomes negligible in half top of the tank when the impeller is placed near the bottom of the tank (c/D = 0.03). When the stirrer is positioned above the middle of the vessel, the

dv  = rdrdθdz (5)

lower part is not disturbed. If the impeller is located closer to the bottom of the tank, increased viscous resistance

Qv  = (2τ2 + 2τ2

+ 2τ2  + τ2  + τ2

+ τ2 )/η2 (6)

occurs, which limits the flow around the agitator, which

rr θθ

zz rz

rθ zθ

Figure 15. Flow fields for double helical ribbon impeller, at n = 0.8, Reg = 50.

Figure 16. Power number for double helical ribbon impeller, n = 0.7,

Reg = 50.

Table 2.  Power number for double helical ribbon impeller, n =   0.8,

Reg = 5, h/D = 0.5.

 c/D  0.03   0.26 0.5

 Np  15.17 13.26 10.01

leads to a high power consumption (Table 2). Thus, the middle of the tank is the most appropriate position for this type of mixer and for this range of Reynolds.

5.5. Comparison between the simple and dou- ble helical ribbons impeller

To determine the influence of impeller design on hydrody- namics induced, two geometrical models have been made. At a vertical position corresponding to Z∗ = 0.43, the axial component of velocity is followed for both configurations (Figure 18). The spiral shape of the double ribbon agitator

yields an intensive fluid flow and enlarges the cavern size (Figure 19).

The biggest concerns of researchers are to design an im- peller which requires a minimum of power consumption in a short time. We study the power required for the operation of agitation in the two cases considered, for different values of structural index and Reynolds number. See Figure 20 and Figure 21, the power consumption is important for the double band regardless of the nature of the fluid or the impeller rotational speed, and this is due to inertial forces.

6. Conclusion

Using the Computational Fluid Dynamics (CFD) method, three-dimensional simulation of non-Newtonian flow gener- ated by the simple and the double helical ribbons impellers was investigated. Numerical results concerning velocity fields and power number are presented in this paper.

Results showed that increasing impeller rotational speed results better axial circulation and diminishes the power consumption, but for Reg > 20 just a slight decrease in

energy consumption is marked.

When the stirrer is positioned near the tank bottom, that results a type of breaking flow near the vessel base, on the other hand, the lower part of the tank is not disturbed if the agitator is placed near the free surface of liquid. Thus, the middle of the tank is the most appropriate position for this kind of impeller.

The impeller size plays also an important role, to enlarge the cavern size it is necessary to increase the blade size, but that yields higher power consumption.