Development and application of 3D-PTV measurements to lab-scale stirred vessel flows

3D Particle Tracking Velocimetry (3D-PTV) is a flow visualisation technique which enables the 3D Lagrangian velocity field to be obtained. Whilst its application to fluid mixing in agitated vessels has been limited due to the large field of view and wide velocity ranges involved, guidelines for reliable 3D-PTV measurements at the lab-scale have been developed in this paper. The flow of water at Re=12,000 in a flat bottom cylindrical tank (T=180 mm) stirred with a 6 blades Rushton turbine (c=D=T/3) has been measured at different camera frame rates (125 – 3600 fps) and tracer concentrations (0.001 – 0.010 px−2). The best compromise between the number of measured data, tracking efficiency and CPU time (206 frame-1, 39% efficiency, 0.026 min frame-1) has been obtained at 125 fps and 0.002 px−2. The Savitzky–Golay filter, used to enhance the measurements signal-to-noise ratio, has been optimized by testing different values of the polynomial order (0–3) and filter width (321 data points). A 2nd order, 11-points filter gave the best results, based on considerations regarding the reduced Chi-squared and velocity distributions. With the best conditions and filter, the uncertainty in the measured tracer positions was in the order of 255 μm. Finally, un unbiased distribution of the flow decorrelation time has been determined from the velocity autocorrelation functions along the trajectories longer than 6 impeller revolutions. This method could be used to compare the macro-mixing performance in different flow systems.