Determination of mixing time and degree of homogeneity in stirred vessels with large eddy simulation

Abstract

Although reactive mixing is of utmost interest for most industrial mixing-sensitive processes, a thorough understanding of passive scalar behaviour and mixing provides an important step in understanding of the relevant processes, especially when turbulent flow conditions are present over much of the vessel volume. The objective of the present work is to characterise the mixing of an inert scalar in a vessel stirred by a Rushton impeller using the large eddy simulation technique. Recent studies have demonstrated the capability of this approach for the accurate modelling of the turbulent flow field in a stirred vessel. However, to the best of the authors’ knowledge, no comprehensive investigation of mixing characteristics and assessment of mixing time against experimental data has been performed so far. The current simulations provides a very detailed picture of the spatial and temporal evolution of the scalar concentration that cannot be obtained with the standard Reynold's averaged Navier–Stokes approach. The largest scalar fluctuations were detected in the injection plane, in the area close to the impeller where turbulence levels are highest. Scalar concentrations recorded at several other points inside the domain revealed different mixing patterns across the vessel. The predicted mixing time compared well, on average within 18%, with values determined from correlations reported in the literature. This agreement demonstrates the capability of this modelling approach to simulate accurately the mixing characteristics in a stirred vessel and therefore to be employed for the optimisation of industrial vessel designs.