Investigation of turbulent fluid flows in stirred tanks using a non-intrusive particle tracking technique

• Turbulent fluid flow in a tank agitated by RT and PBT RPT impellers is studied by RPT.
• The RPT data are compared to LDA data and simulation results.
• Both Eulerian and Lagrangian descriptions of turbulent fluid flow are discussed.
• A novel method to measure the mixing time in stirred tanks is presented.

Fully turbulent fluid flows in a laboratory-scale stirred tank (ST) equipped with a radial flow impeller (Rushton turbine; RT) or an axial flow impeller (pitched blade turbine; PBT) were analyzed using the radioactive particle tracking (RPT) technique. The present study covered the Eulerian and Lagrangian descriptions of fluid motions. The RPT measurement of the turbulent flow field in a tank agitated by an RT was benchmarked with CFD simulations of RANS-based turbulence models and laser-based measurements. There was good agreement between all the methods for the measured and predicted 3D mean velocity profiles at all locations in the ST. The RPT technique was used to measure the turbulent flow field in a tank agitated by a PBT for the first time. The behavior of the wall jet was investigated. There was close agreement between our results and those of previous studies for both systems. Lagrangian mixing measurements showed that particle trajectories can be used to generate Poincaré maps, which in turn can be used as a tool to visualize the 3D flow structure inside mixing systems. Two mixing indices, one based on the concept of stochastic independence and the other on the statistical concept of memory loss in mixing processes, were used to measure mixing times using RPT results. The present study showed that the RPT technique holds great promise for investigating turbulent flows and the mixing characteristics of STs, and for assessing the adequacy of numerical models.