The power dissipated by an agitated system is an important characteristic of any mixing process, since mechanical energy is required to homogenize the vessel content, disperse immiscible phases, suspend solids, increase mass transfer, and, in general, produce the desired mixing effect. Power dissipation in agitated systems depends not only on the type of impeller used, agitation speed, and the physical properties of fluid, but also on the geometric characteristics of the system, such as impeller clearance, liquid height, or liquid head above the impeller.
The main objective of this work was to investigate the effect of impeller submergence on power dissipation and Power Number, Po, in a flat-bottomed, baffled, cylindrical vessel stirred with a disk turbine (DT) for different impeller off-bottom clearances (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) at constant impeller tip speed, Utip. Additionally, the effect of impeller tip speed was assessed by changing the agitation speed for a fixed impeller size and position.
Experimental work was carried out with a single liquid phase. Torque was experimentally measured with a strain gage-based rotary torque transducer connected to an external multi-channel signal conditioner, display, and controller. Computational fluid dynamic (CFD) work was conducted here to predict the power number, velocity field, and turbulence intensity close to air-liquid interface for different height-to-tank diameter ratios when the liquid level was 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 for this purpose. 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. The velocity predictions were validated using Particle Image Velocimetry (PIV).
In general, good agreement between the experimental data and the predicted results for the power number and velocity distribution near the air-liquid interface 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 power number decrease significantly; (2) transition from “double-eight” to “single-eight” recirculation can be observed; (3) vortex formation and air entrainment occurs. When this happened, it was observed that the average velocity field and turbulence intensity close to the air-liquid interface increased substantially. This shift in flow pattern could be the reason for the air entrainment and the drop in power number.