In many industrial applications, mixing vessels have liquid height-to-tank diameter ratio H/T equal to, or larger than, 1. However, there are many instances where this ratio is lower than 1, as in all those cases in which the vessel is emptied. Even when H/T<1, sufficient agitation must still be provided in order to attain the desired process objectives. When the impeller submergence is reduced as a result of lowering the liquid level, the fluid dynamics of even a single-phase stirred liquid can become quite complex, with different regimes possibly existing depending on the geometric characteristics of the system (such as impeller clearance, liquid height, or liquid head above the impeller). The objective of this work is to determine the minimum liquid levels and the critical impeller submergence for different impeller off-bottom distances (CT/T=0.12, 0.18, 0.37, 0.54 and 0.68) and different impeller diameters (D/T=0.26, 0.31 and 0.42) where adequate mixing process can still be achieved, both in a single liquid phase and in solid-liquid suspensions.
Computational Fluid Dynamic (CFD) simulations as well as Particle Image Velocimetry (PIV) were used here to study the system’s hydrodynamics in flat-bottom baffled vessels equipped with a single Disk Turbine (DT) for different height-to-tank diameter ratios when the liquid level is smaller than the tank diameter. A commercial mesh generator (Gambit 2.4.6) coupled with a CFD software package (Fluent 6.3.26) was used to simulate the flow field. A standard κ-ε model as well as a realizable κ-ε model coupled with enhanced wall treatment was used to model the turbulence flow. Results were obtained using a multiple references of frames (MRF) approach. For the case where air entrainment occurred, a Volume of Fluid (VOF) model coupled with MRF simulation was used. Experimental work was also carried out with a single liquid phase as well as with solid-liquid suspensions with 0.5 wt% dispersed solids. The minimum agitation speed for complete solid suspension, Njs, at different filling ratios was obtained visually and then they compared with CFD predictions.
In general, good agreement between the experimental data and the predicted results for the velocities distribution, pumping number and Njs were obtained. Both the experimental and the computational results, for different impeller off bottom clearances and impellers sizes, show that there is a minimum liquid level (or minimum impeller submergence) below which: (1) the macroscopic flow pattern changes substantially; (2) the pumping number drop significantly; (3) air entrainment occurs. When this occurred, it was observed that the average velocity field and turbulence intensity close to the tank bottom decreased substantially, which could be the reason why solid suspension becomes problematic.