Abstract
The influence of elongational and shear gradients in the macroscopic flow field in agitated tanks on dispersion processes is investigated. Measurements of droplet size distribution for a liquid-liquid dispersion process using phase-Doppler anemometry (PDA) reveal that axial-flow impellers, such as the 24°-pitched-blade turbine and propeller, produce smaller droplets than the Rushton turbine at the same average specific power and energy input. These results stand in contradiction to the usual assumption that only the maximum turbulent shear stress determines the breakup process and the Rushton turbine is well known to produce higher turbulent shear stresses. Particle image velocimetry (PIV) measurements of the macroscopic flow field indicate that the 24°-pitched-blade turbine and propeller produce larger areas with higher elongational gradients. Therefore, the proposed consideration of particle breakup due to macroscopic elongational flow in addition to turbulent stresses improves the understanding of dispersion processes in agitated tanks.
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