The Snapshot SS Method

Ranade and Van den Akker [74] introduced the snapshot method for simulating flows in baffled stirred tanks. The snapshot method has many similarities with the traditional IBC method but differs in that the effect of the impeller rotation is approximately modeled mechanistically and not deduced from empirical data analysis solely. The snapshot method thus represents an early attempt to improve on the main limitation associated with the IBC method. 

The basic idea is to describe a snapshot of the flow in a stirred vessel with a fixed relative position of blades and baffles. It is assumed that the main flow characteristics of a stirred vessel at the particular time instant in question can be captured approximately from the solution of the steady-state equations, provided that artificial cell volnme adjustments and momentum sources are implemented to represent the effect of the impeller rotation. 

To achieve steady-state values for comparison with experimental data different calculations have to be carried out at different time instants representing an average for the different positions of the impeller blades relative to the baffles. 

Ranade and Krishnan [75] reported an evaluation of the snapshot approach. It was concluded that the simulations captured most of the key features of near-impeller flows with fair accuracy. 

Ranade and Van den Akker [74], for example, used the snapshot method for simulating gas-liquid flows in baffled stirred tanks using a time after volume averaged two-fluid model for incompressible flows (as described in sect 3.3). These multiphase simulations also predicted the near-impeller flows with fair accuracy. Most important, the cavities due to the accumulation of gas in the low-pressure region behind the impeller blades were detected. 

3 The Inner-Outer (lO) Method and the Multiple Reference Frame Approach (MRF) 

18 Computational domain for the outer simulation, lined region is excluded from the domain. The figure is drawn based on a similar figure published by Brucato et al. [10] 

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