Although partially baffled, retreat-blade impeller systems are critical in pharmaceutical production, there is very little information available to date on their hydrodynamics. Such knowledge is critical to understand processes, such as solid suspension, crystallization and chemical reaction, regularly conducted within them.
This research work is focused on the study of the effect of different vessel geometries on the velocity distribution inside retreat-blade impeller-vessel systems. The velocity distribution within these scaled-down systems was determined using Laser-Doppler Velocimetry (LDV) and Computational Fluid Dynamics (CFD). Each experimental apparatus consisted of a vessel equipped with a retreat-blade impeller and a single beaver-tail baffle. Differently styled vessel bottoms (flat-bottomed and hemispherical bottomed) were used during experimentation. All three velocity components and the turbulence intensity were measured at different radial positions on a number of horizontal planes inside the vessels. Numerical simulations of the velocity distribution and turbulence levels inside each vessel were conducted using a commercial mesh generator (Gambit) coupled with a computational fluid dynamic (CFD) package (Fluent). The full 360°-tank geometry was incorporated in the simulations.
The flow patterns in these partially baffled systems are generally complex. It was found that the flow in the lower section of each vessel is dominated by the tangential component of the velocity as in unbaffled systems, although the presence of the baffle produced a stronger axial flow in the upper region. This indicates that phenomena that are dominated by the hydrodynamics near the vessel bottom, such as solid suspension, may be more difficult to carry out in such vessels as compared to fully baffled systems.