Earlier research works have elucidated that decrease in the clearance of the Rushton Turbine (RT) impeller of the baffled reactor vessel causes a transition from double loop pattern to single loop pattern with significant reduction in the power number. We observe similar flow behaviour when the diameter of the reactor vessel is increased from the standard conditions of the reactor vessel. The mean and turbulent flow fields associated with the baffled reactor vessels of various diameter agitated using the RT impeller were analysed using the Computational Fluid Dynamics technique so as to understand the hydrodynamic reasons causing changes in the flow behaviour with increase in the diameter of the reactor vessel. The increase in the diameter of the reactor vessel generates a low pressure zone below the impeller which deflects the discharge streams and trailing vortices towards the bottom surface of the reactor vessel, causing the formation of single loop pattern. The downward propagation of trailing vortices weakens the flow separation regions behind the impeller blades; which in turn decreases the form drag and power number of the impeller. The higher magnitudes of axial velocity, vortex and turbulence activity from the impeller up to bottom surface of the reactor vessel as well as inferior entrainment of air makes the larger diameter vessel as a suitable option for the solid-liquid suspension process.

Double- to single-loop pattern transition and a significant reduction in the power number with a decrease in the clearance of the Rushton turbine impeller in a baffled reactor was elucidated in earlier research works. The present work investigates the physical reasons behind these phenomena using the computational fluid dynamics approach. The Reynolds Averaged Navier–Stokes equations with standard turbulence model closure were used to model the turbulent flow conditions in the reactor vessels. The Multiple Reference Frame (MRF) approach was adopted to model the impeller baffle interactions in the reactor vessels. The implicit Volume of Fluid (VOF) method was employed to simulate the aeration process in the reactor configurations considered. The development of a low pressure region below the impeller of a low clearance vessel deflects the discharge streams downward, leading to the formation of a single-loop pattern. The downward movement of the discharge streams reduces the vortex activity behind the impeller blades, leading to weaker form drag and a decrease in the power number of the impeller. Similarly, a high clearance vessel provides a low pressure region above the impeller which deflects the discharge streams above the impeller, resulting in a single-loop pattern and a considerable increase in the air entrainment due to superior vortex and turbulence activity present near the free liquid surface. The standard reactor vessel was found to provide superior bulk mixing of fluid as the overall turbulent dissipation rate is 35% more than that associated with low and high clearance vessels.

The present study predicts the periodic behavior of mean velocity fields from the properly verified and validated CFD model to determine the extent of vortex and turbulent activity in a baffled tank stirred by standard six bladed Rushton Turbine. This region includes most of the vortex and turbulent action that controls the mixing and mass transfer processes in the stirred tank. The complexity of periodicity of mean radial and tangential velocities in the radial direction and mean axial velocity in the axial direction is used to fix the proper radial as well as axial extents of the vortex generated by RT. The extent of flow periodicity in the present stirred tank configuration is confined within a cylindrical region around the impeller of radius 0.753 times the impeller diameter and a height of 0.323 times the impeller diameter above and below the impeller center plane.