Hydrodynamics and Complete Suspension in Axially Stirred Vessels
Doctoral thesis, 1995
Complete suspension of solid particles in stirred vessels has been subject to numerous investigationsin order to develop appropriate design criteria for solid/liquid suspensions. Previous work on this matter has often suffered from the lack of knowledge of fluid mechanics in the system and models based on overall flow characteristics have been proposed in the literature. The present study was undertaken in order to provide a better fundamental undersatanding of the complex process of complete suspension using advanced theoretical and experimental techniques.
It was found that the steady mean and turbulent flow fields obtained using the standard k-e turbulence model agreed well with experimental LDA-data. An optimal value of the turbulent macroscale in the impeller discharge was determined numerically. Model predictions and results from the LDA-measurements clearly demonstrate the behavior of the turbulent flow induced by a pitched blade impeller in a cylindrical vessel. In dimensionless form, the mean and fluctuating velocities were found to be independent of the impeller speed for the three impeller speeds studied.
Measurements of the turbulent velocity distributions adjacent to the solid surfaces of the vessel exhibit some of a turbulent boundary layer typical features. The linear mean velocity profile in the viscous sublayer, along with the peak of the fluctuating velocities close to the buffer layer, have been shown. The development of the axial flow field along the vessel was characterized at the wall. The added complexity of the rotational flow in a stirred tank compared to the idealized flow over a flat plate was recognized.
Complete suspension of solid particles was analyzed by considering individual particles of various sizes located at the vessel boundaries. Experimental complete suspension data reported in the literature was correlated theoretically using particle force balances formulated at the vessel wall and boundary layer arguments. The breakpoint in the experimental complete suspension results was attributed to a change in the suspension mechanism due to boundary layer effects.
axially stirred tank
particle force balances
turbulent boundary layer