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6
Conclusions and Future Works
6.1 Multiphase System
PIV measurements have been carried out in a multiphase fully baffled vessel stirred by a 6-bladed PBT operating in the down- and up-pumping mode.
Four different impeller speeds and seven different solid concentrations up to 0.7 % by weight have been considered.
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.
However, particular attention has to be paid in setting up all the parameters such as laser power, vector validation and filtering, in order to obtain good measurements. It was found that fluorescent particles were much sensitive to the camera focus and even a small degree of out of focus caused a complete lost of vectors during image processing.
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 %.
In the impeller jet stream the liquid was largely influenced by the presence of the leading particles that, having a greater inertia, tended to accelerate it at velocities higher than in the single phase case.
Chapter 6: Conclusions and Future Works
86 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.
The use of fluorescent particles is a powerful tool to investigate multiphase flow but needs some improvements. Actually, the fluorescent microspheres employed could not be used with the organic liquids necessary to match the usually high refractive index of glass and this caused the refractive index matching technique not to be performed.
Two choices are thus available: more expensive glass ballotini would be the best way to have a solid phase with low r.i. and no impurities but the economic factor is far from being irrelevant.
On the other hand different fluorescent tracers resistant to organic liquid are as well required, especially for further works on viscous fluids.
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6.2 High Frame Rate PIV
A new PIV system has been tested in an open fully baffled vessel stirred by a Rushton turbine and a 45° 6-bladed PBT.
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 systems 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°. As Van’t Riet et al. ( 1976) reported no displacements were found as the impeller speed was increased.
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. A reason for this difference could be the absence of baffles in their tank but this did not influence the radial location.
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.
Very good agreement was found showing that a different clearance do not affect the trailing vortices.
Chapter 6: Conclusions and Future Works
88 Development of reliable computational models for the design of stirred tank reactors faces an important challenge and experimental data of the kind revealed by this new High Speed PIV system will be invaluable for model testing and validation.
