Mixing times in a turbulent stirred tank by means of LES

In turbulent liquid mixing, the performance of a stirred tank is usually expressed as the time it takes to homogenize a passive scalar concentration starting from a segregated state. Numerical prediction of mixing times requires solving the flow field and the associated passive scalar. A novel numerical approach is presented to passive scalar mixing in an agitated tank. In addition to a large-eddy simulation based on lattice-Boltzmann discretization of the Navier-Stokes equations, the convection-diffusion equation governing passive scalar transport has been solved by finite volume discretization. Numerical diffusion has been effectively eliminated by applying a TVD scheme. With this hybrid approach we study mixing in a Rushton turbine stirred vessel at Re = 24,000. The simulations were designed such that their results can be critically assessed with experimental data. The simulations are in (at least) qualitative agreement with the experiments, and allow for assessment of how mixing times defined in various ways relate. Also the role of the impeller size has been investigated. The numerical method needs improvement in the sense that it is not exactly mass conservative, which likely is due to the fixed-grid approach in combination with a non-fixed (revolving) impeller. (c) 2006 American Institute of Chemical Engineers AIChE J, 52: 3696-3706, 2006.