(76b) Mixing Vessel Simulation: When Is a LES Simulation Really Necessary?

The flow structures in a turbulently stirred tank are highly three-dimensional and complex with vortical structures and high turbulence levels in the vicinity of the impeller. In the last years a lot of effort was spent on describing and predicting the flow and turbulence pattern accurately by means of DES (Detached Eddy Simulation), LES (Large Eddy Simulation) and DNS (Direct Numerical Simulation) CFD-simulations. While these methods are computationally expensive these simulations are typically done for lab-scale equipment and far away from being applicable for everyday industrial scale simulation. Here, still RANS-models (Reynolds-averaged Navier-Stokes models) are widely used due to the relatively low computational cost.

In this contribution we want to discuss the benefits and drawbacks of the DES/LES/RANS models with a focus on applicability and runtime. We will show how often needed quantities like power input and mixing time in a stirred tank can be predicted efficiently and accurately and when the accurate description of turbulence is really needed. Simulation results are compared against own as well as published experimental results.