Rushton turbine, flow

In the case of highly elastic liquids mixed by a Rushton turbine, flow reversal may occur in the low Reynolds number region, ReM< 30, leading to values of the power number as much as 60 per cent higher than for inelastic liquids. In the intermediate region, 50 1000, the power... 

As in many other areas of CFD, advances in impeller treatment are occurring rapidly, both in developing new treatments and in making the established treatments more accurate and more efficient computationally. For example, Derksen and Van den Akker (1999) have applied an adaptive force field technique in a model of Rushton turbine flow. VanderHeyden et al. (private communication) at Los Alamos have developed a technique of switching a momentum source term at certain mesh node points on and off in a pattern that simulates the effect of a rotating impeller. These and other approaches promise increased fidelity with greater computational efficiency. 

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-thind the diameter of the bioreactor (43) (Fig. 3). In this system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-fiUed or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power requited to drive it through the broth, to fall and this has important consequences for the performance of the bioreactor with respect to aeration (35). k a has been related to the power per unit volume, P/ U, in W/m and to the superficial air velocity, in m/s (20), where is the air flow rate per cross-sectional area of bioreactor. This relationship in water is... 

A general flow map of different hydrodynamic conditions (Fig. 23) consists of regions of flooding, dispersion, and recirculation on a plot of N vs for a Rushton turbine. For a low viscosity aqueous/air system, the gas flow numbers for the three conditions are given hy FI = 30Fr[D/TY for flooding, = 0.2Fr° (F/r)° for complete dispersion, and =13FF D/TY for recirculation. 

Ranade, V.V., 1997. An efficient computational model for simulating flow in stirred vessels a case of Rushton turbine. Chemical Engineering Science, 52, 4473-4484. 

The three basic types of impeller which are used at high Reynolds numbers (low viscosity) are shown in Figures 10.55a, b, c. They can be classified according to the predominant direction of flow leaving the impeller. The flat-bladed (Rushton) turbines are essentially radial-flow devices, suitable for processes controlled by turbulent mixing (shear controlled processes). The propeller and pitched-bladed turbines are essentially axial-flow devices, suitable for bulk fluid mixing. 

Fig. 2. This plot shows to which degree, according to a LES, the turbulence in a plane midway between two baffles in a stirred vessel provided with a Rushton turbine can be typified. For clarity, not all grid points in such a plane have been used for this plot. According to Lumley (1978), the borders represent different types of turbulent flows 3-D isotropic turbulence, 2-d axis-symmetric turbulence, 2-D turbulence, and 1-D turbulence. Most but not all points are concentrated in the lower part of this Lumley triangle. Reproduced with permission from Hartmann et al. (2004a).

Note The respective impellers used are a classical Rushton turbine (DT), a hydrofoil impeller (A315) manufactured by Lightnin, and a Pitched Blade impeller (PBT). The cases 1 through 4 all relate to a superficial gas rate of 3.6mm/s only, with impeller speeds varying between 5 and 10/s (gas flow numbers between 0.01 and 0.02) cases 2 and 3 differ in sparger size and position. 

Figure 11 Power characteristics of a Rushton turbine stirrer under given geometric conditions, measured in two differently scaled vessels (scale 1 2) and fitting the flow behavior of the viscoelastic fluid [polyacrylamide (PAA) solution] by changing its viscosity. Source From Ref 13.

Mechanically agitated slurry reactors are widely used in three-phase catalytic and noncatalytic reactions. In aerated slurry reactors, the three regimes outlined in Table V prevail. These regimes are schematically illustrated in Fig. 11. The gas flow rate and stirrer speed where the transition from regimes a to b or b to c with a Rushton turbine stirrer occurs can be estimated from the relationships described in Table VI. 

Simulation of Flow Generated by a Disc (Rushton) Turbine... 

FIGURE 10.21 Schematic of stable flow patterns observed with dual Rushton turbines (from Rutherford et o/ 1996). (a) Parallel flow, (b) merging flow, (c) diverging flow. 

FIGURE 10.22 Comparison of experimental ((a) Rutherford et o/., 1996) and predicted ((b) Deshpande and Ranade, 2001) results for dual Rushton turbines (Parallel flow regime). 

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