Combining the Velocity Data Model with Other Physical Models

4 Combining the Velocity Data Model with Other Physical 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. 

1 Volume of Fluid Model. In stirred tank applications, the volume of fluid (VOF) free surface model is useful for tracking the shape of the liquid surface during operation. This includes the transition to a parabolic shape during startup, which can lead to the (undesired) drawdown of air. The velocity data model can be used in 2D or 3D for simulations of this type. The VOF model can have a steady or time-dependent implementation, and both are fiilly compatible with this steady-state treatment of the impellers. 

If air drawdown does occur, caution is needed. If air passes through cells where large momentum sources exist, resulting from the velocity data boundary conditions, the liquid-air interface will be broken, resulting in many small bubbles that will mix with the liquid. The VOF model is not equipped to handle this condition accurately, so the simulation should be terminated at this point. Thus, whereas the model can be used to predict if drawdown will occur, it should not be used to predict the flow conditions afterward. 

2 Multiphase Model. Both solids suspension and gas sparging can be simulated using an experimental data model for the impeller. The manner in which the multiphase parameters are input depends on the multiphase model being used. For solids suspension, an Eulerian granular multiphase model is recommended, and separate sets of momentum equations are used for the liquid and solids phases. This model, run in a time-dependent fashion, is fully compatible with the time-averaged representation of the impellers. Experimental velocity data are set as a boundary condition independently for each of the phases. Note, 

---