Turbulent gas-liquid flow and bubble size distribution in multiple impeller stirred vessels

The aim of this work is to investigate the turbulent hydrodynamic features of a gas-liquid stirred tank by detecting independently and simultaneously the liquid and the bubble phase velocity by means of a two-phase PIV technique. The investigation was carried out in an open, fully-baffled, flat-bottomed cylindrical vessel (diameter, T=23.2 cm) filled of liquid up to a height H=2T. Agitation was provided by a dual turbine – a 6SRGT manufactured by Scaba of diameter D=T/2 placed at the distance C1=T/3 from the vessel base and a down-pumping PBT of diameter D=0.40T placed at the distance C2=T from the bottom impeller. The gas was introduced by a point sparger placed below the bottom impeller. Air was always used as the gas phase, while two liquids of different viscosity were considered: demineralised water and a moderately viscous water-glycerine solution (=5 mPa∙s). The mean flow fields of the gas and the liquid phases, the r.m.s. maps of the two phases and a comparison with the corresponding velocity measurements in single-phase conditions are presented. The typical bubble diameters in the two gas-liquid systems obtained by a digital image processing method are also presented. The effects of liquid viscosity and of the gas flow rate on the hydrodynamic features are discussed.