Flow regimes and surface air entrainment in partially filled stirred vessels for different fill ratios

Abstract

In many industrial applications, stirred vessels have a liquid height-to-tank diameter ratio (fill ratio), H/T, equal to, or larger than, 1. However, there are many instances when H/T<1, as when a vessel is emptied or filled. 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. The objective of this work was to study the hydrodynamic changes that occur when H/T is decreased, and to determine the conditions for which adequate mixing can still be achieved. A baffled vessel equipped with a disk turbine was used. Particle Image Velocimetry (PIV) was used for the experimental determination of the velocity profiles, impeller pumping capacities and Pumping Numbers. A strain gage system was used to measure power dissipation. Computational Fluid Dynamics (CFD) was used to predict velocity profiles, Power Numbers, and Pumping Numbers using a multiple reference frame (MRF) model coupled, when needed, with a Volume of Fluid (VOF) model. Results show that a critical impeller submergence ratio, Sbcr/D, exists below which: (1) the macroscopic flow pattern generated by the impeller changes substantially, transitioning from a “double-loop” recirculation flow to a "single-loop up-pumping" recirculation flow; (2) the Power Number and radial Pumping Number drop significantly; (3) vortex formation occurs, air entrainment is greatly facilitated, and impeller flooding typically results. The critical Sbcr/D ratio was not affected by agitation speed. Impeller flooding was observed only when the new regime was established and the vortex depth reached the impeller disk. This phenomenon was correlated to a critical value of the Froude Number, Frcr. These results show that stirred vessels can be effectively operated only within certain ranges of the operating variables without compromising their mixing effectiveness. This knowledge can help practitioners avoid operating their equipment in regions where the desired mixing effects are not achievable.