Impeller modeling

In this section the governing equations employed in the different impeller model implementations are presented. 

In this section the different impeller modeling strategies employed simulating the ffow fields in stirred tanks are examined. 

Based on the impeller modeling analyses mentioned above, among other similar investigations, it is customarily concluded that for most industrial applications the MRF method is an appropriate simulation tool. The method represents a trade-off between accuracy and computational demands. However, the averaging process needed to obtain representative data in the im-... 

The computational time required by these impeller modeling methods were also compared. For baffled tanks the computational time required by these two impeller methods are quite similar as several MRRF simulations have to be run per one SM simulation because a manual change in the relative impeller-... 

Vrabel P, van der Lans RGJM, Luyben KCAM, Boon L, Nienow AW. (2000) Mixing in large-scale vessels stirred with multiple radial or radial and axial up-pumping impellers modelling and measurements. Chem. Eng. Sci., 55 5881-5896. 

INTRODUCTION, 297 Computational Fluid Dynamics, 298 Stirred Tanks, 298 Impeller Modeling, 299... 

Accurate CFD (computational fluid dynamic) simulation of the flow in stirred tanks requires correct specification of both the geometry and the physical conditions of the flow. While specification of the geometry, the gridding, and the solution algorithm is relatively straightforward, some other issues remain difficult. The most challenging problem is definition of a physically accurate, computationally tractable impeller or impeller model which incorporates the effect of the tank geometry. This... 

The rest of this chapter is devoted to examining the physical and numerical issues underlying CFD simulations of flow in stirred tanks. Under physical issues, the general issues of turbulence modeling and degree of swirl are addressed first. The comments in these sections can be applied to any flow field. The specific stirred tank flow field depends on the tank geometry, the boundary conditions at the edges of the computational domain, and the impeller model. These issues are discussed in detail. Many of the numerical issues which were the focus of early CFD research... 

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... 

Figure 2. (continued) Figures (c) and (d) show the distortions produced when incorrect impeller models are used. The simulation conditions are ... 

Figure 3. Profiles of the predicted rate of dissipation of turbulence kinetic energy below the PBT for the high clearance geometry using four impeller models ...

The steady-state implicit impeller model, which uses time-averaged experimental data, can be used to model other steady-state and time-dependent processes, as described below. Because of its simplicity, it has no effect on other scalar transport in the domain. The models that do require special consideration are those involving multiple phases, with separate sets of momentum boundary conditions, as described below. Species blending is also discussed, because it is a calculation that is commonly performed in conjunction with the implicit impeller model. 

Table 5-3 Results of the MRF Impeller Model with Several Turbulence Models as Compared to Experiment...

Combining the Geometric Impeller Models with Other Physical Models... 

The geometric impeller models described above can be used to model both steady-state and time-dependent processes, but attention must be paid to the timescales, where appropriate, and other special requirements of each. In this section, some of these considerations are reviewed for the two most popular of the geometric formulations the MRF and sUding mesh models. 

Figure 5-22 Product distribution, Xs as a function of impeller speed (rpm) for two vessels of different size, with the second reactant being added in the outflow of the impeller. Model predictions are compared with data from Middleton et al. (1986).

Figure 5-37 CPU time and grid size requirements for various impeller modeling options.

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