Two-phase flow and bubble size distribution in air-sparged and surface-aerated vessels stirred by a dual impeller

The aim of this work is to investigate the turbulent hydrodynamic features of a fully baffled gas-liquid stirred vessel equipped with a dual turbine consisting of a radial concave-blade turbine and a downflow pitched blade turbine (PBT). Two different operating modes are analyzed, namely air sparged and surface aerated. A two-phase particle image velocimetry (PIV) technique, based on a single laser sheet source and two cameras, each provided with a filter for separately catching the light scattered by the liquid seeding particles or the bubbles, was adopted for detecting independently and simultaneously the liquid and bubble phase velocities. The mean flow fields of the gas and liquid phases, the rms velocity fluctuation maps of the two phases, and a comparison with the corresponding velocity measurements effected in single-phase conditions are presented. The typical bubble size distribution (BSD) in the two gas-liquid systems obtained by a digital image processing method is also presented, and its dependency on the gas feeding configuration and on the dimensionless Weber number is shown. The effects on the hydrodynamic behavior of the gas-liquid dispersion of liquid viscosity as well as of gas flow rate or impeller speed for the air-sparged and surface-aerated systems, respectively, are discussed. The results suggest that simultaneous detection of the flow fields of the two phases is required for appropriate, detailed evaluation of the system features. Even a limited viscosity change has a significant impact on the system hydrodynamics, since it affects both the flow field and the BSD.