Gert Langer

Zum Verständnis der hydrodynamischen Beanspruchung von Partikeln in turbulenten Rührerströmungen

Hydrodynamic stress of particles in agitated tanks is of essential significance for the choice of appropriate impellers. Taking into account current approaches and results concerning particle stress in turbulent flows it will be shown that modelling this process on the basis of the theory of local isotropic turbulence only is not sufficient. New results indicate that the macroscopic flow also has to be considered for an better unterstanding of the breakup of particles in turbulent flows. For breakage of flocs under turbulent conditions elongational flow provides an significantly greater effect than shear flow. Measurements of the power consumption of various impellers for viscoelastic polymer solutions show that axial impellers produce considerable elongational flow.

The Influence of Macroscopic Elongational Flow on Dispersion Processes in Agitated Tanks

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.

PDA measurement of drop size distribution for liquid-liquid dispersing in agitated tanks

It is the purpose of this paper to investigate the dispersion characteristics of different axial- and radial-flow impellers at the same average specific power and energy input. Phase-Doppler anemometry (PDA) is used to measure the drop size distribution.