• mixing
• PIV
• settling velocity
• slip velocity
• stirred vessel
• trailing vortex
• turbulence

Mixing in stirred tank reactors is one of the most common operations carried out in the manufacture of fine chemicals, pharmaceuticals, polymers, food, and in water treatment. In recent years Particle Image Velocimetry contributed significantly to mixing study by enabling a deep insight into the velocity fields in stirred tanks.
Two new PIV techniques have been developed to obtain qualitative and quantitative information on the hydrodynamics of stirred vessels.
The former involved a liquid-solid system and was focused on the set-up of the parameters for a good data acquisition and processing.
The use of 3 μm diameter fluorescent microspheres as liquid tracer in solid- liquid flows has been tested and validated: it allowed simultaneous measurements of liquid and solid local velocities just by placing a wavelength filter in front of the lens of the camera.
Experiments with 500 μm diameter particles, showed that the particles led the continuous phase in down-flow and lagged in up-flow regions.
Fluid axial and radial velocities decreased even at the lowest concentration analysed, i.e. 0.1 % by weight, and no remarkable decreases were observed with increasing solid amount up to 0.7 %.
Local relative axial velocities along the primary circulation loop were estimated by subtracting the liquid velocity data from those of the solid particles, showing a direct relationship between the settling velocity in a turbulent fluid and turbulence levels in the tank, as been demonstrated by Magelli et al. ( 1990). As the impeller speed was increased a lower relative axial velocity was observed apart for the discharge flow where the increase in particles inertia caused an opposite effect.
In addition data concerning the flow fields of the impeller swept volume in an open, fully baffled, vessel stirred by a Rushton turbine and a 45° 6- bladed PBT have been acquired with a completely new High Frame Rate PIV system.
The technique was successful in determining the location of the axis of the trailing vortices generating from behind each impeller blade and had the advantage of providing a longer trajectory up to 140° from the blade, while data processing was relatively fast compared with other system such as Laser Doppler Anemometry.
Concerning Rushton turbine the two vortices were characterised both in radial and axial coordinates and were found to remain within r/T = 0.25 until 85° from the blade then moved away reaching r/T= 0.35 at 140°. Both vortices moved slightly toward the top of the tank in agreement with the work of Escudiè et al. ( 2004). However, Sharp and Adrian ( 2001) found both vortices moving away from the impeller plane in opposite directions. Concerning Pitched Blade Turbine the same measurements were performed and compared with Schaefer et al.’s ( 1998) work. The vortex remained close to the blades even up to 120° behind the blade. The axial position was found to be influenced by the number of impeller blades.