Numerical simulation of turbulent batch mixing in a vessel agitated by a Rushton turbine

In this study, computational fluid dynamics (CFD) modelling of turbulent batch mixing of an inert tracer in a baffled vessel agitated by a six-bladed Rushton turbine has been carried out using the proprietary code FLUENT. The study is intended to evaluate the CFD predictions of key properties related to the mixing against measurements and to provide a detailed insight into the process. Three-dimensional, time-dependent flow and mixing calculations have been performed using the fully predictive sliding-mesh technique for the impeller/tank geometry employed by Distelhoff et al. [M.F.W. Distelhoff, A.J. Marquis, J.M. Nouri, J.H. Whitelaw, Scalar mixing measurements in batch operated stirred tanks, Can. J. Chem. Eng. 75 (1997) 641–652] for mixing studies using a laser induced fluorescence technique. Complementary validation of hydrodynamic predictions in a geometrically similar tank was carried out against the experimental data obtained by Hockey [R.M. Hockey, Turbulent Newtonian and non-Newtonian flows in a stirred reactor, Ph.D. Thesis, Imperial College, London, 1990]. The predicted mean velocity components in the bulk regions of the tank above and below the impeller compare well with the experimental data. However, the turbulent kinetic energy is significantly underestimated in these areas. The predicted tracer concentration variations with time at different locations in the tank, in common with measurements, show initial fluctuations, which eventually approach the fully mixed concentration. However, the time required for the appearance of first peak in the concentration–time plot, peak value of the tracer concentration and the time required for the local tracer concentration to attain the final value depend on the position in the tank. The CFD predicted mixing times at different locations in the tank as well as the overall mixing time show reasonably good agreement with the measured data and with those calculated from published experimental correlations.