Interaction between impeller

Bittins K., Zehner P, Power and discharge numbers of radial-flow impellers. Fluid-dynamic interactions between impeller and baffles, Chem. 

This result makes it clear that particle stress is strongly dependent on the interaction between the particles and the interface, so that electrostatic and also hydrophobic and hydrophilic interactions with the phase boundary are particularly important. This means that the stress caused by gas sparging and also by boundary-layer flows, as opposed to reactors with free turbulent flow (reactors with impellers and baffles), may depend on the particle system and therefore applicability to other material systems is limited. 

Such spatial variations in, e.g., mixing rate, bubble size, drop size, or crystal size usually are the direct or indirect result of spatial variations in the turbulence parameters across the flow domain. Stirred vessels are notorious indeed, due to the wide spread in turbulence intensity as a result of the action of the revolving impeller. Scale-up is still an important issue in the field of mixing, for at least two good reasons first, usually it is not just a single nondimensional number that should be kept constant, and, secondly, average values for specific parameters such as the specific power input do not reflect the wide spread in turbulent conditions within the vessel and the nonlinear interactions between flow and process. Colenbrander (2000) reported experimental data on the steady drop size distributions of liquid-liquid dispersions in stirred vessels of different sizes and on the response of the drop size distribution to a sudden change in stirred speed. 

In microcarrier cultures, cell injury is due to the interactions (collisions) between microcarrier beads on which cells are attached, interactions between microcarriers and small turbulent eddies and interactions between microcarriers and bioreactor internals, such as the impeller and various probes (Papoutsakis, 1991a). To protect cells against injury in microcarrier bioreactors, an increased medium viscosity has been the only documented and studied medium alteration (Croughan et ai, 1989 Papoutsakis, 1991a Lakhotia Papoutsakis, 1992). Dextran (Sigma) has been used to increase the medium viscosity as discussed below. This is a purely physical mechanism of protection. 

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... 

Interactions between cells and shear forces in the boundary layers and wakes near solid objects in the bioreactor, especially impellers... 

Effect ofD/T The effect of DIT on cannot be described by a single parameter in the same manner as the effect of CIT. This is because of the interaction between the two terms involving the impeller diameter in the correlations for (Table 7A.3). These are D alone and as (DITf. The general trend reported is (i) at constant T, increase in D causes decrease in and (ii) at constant D, increase in T results in higher In the absence of an explicit relationship between and DIT, information... 

It has been demonstrated that accurate representation of the impeller is central to obtaining accurate CFD simulations. Four alternatives are available for impeller modeling. The sliding mesh technique requires no measurements or assumptions but consumes large amounts of CPU time. Impeller modeling based on generalized mathematical models assumes that there is no interaction between the impeller and... 

On increasing N to 400 rpm in the lab, the desired product distribution is obtained. This is an indication that there is interaction between the reaction kinetics and the mixing. To keep constant, N in the plant must be 118 rpm. Unfortunately, the plant mixer has a fixed rpm. To keep a constant microscale, we decide to change the impeller diameter ... 

Typical spacing between impellers is one impeller diameter. If impellers are placed closer than this, there is considerable interaction between them. In the... 

Mixing can also play a key role in affecting the morphology of a crystalline product. This effect results from the complex interaction between the impeller and nucleation and growth. The reader is referred to the discussion on mixing and crystallization in Chapter 17. 

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