Two-phase flow and bubble size distribution in aerated dual 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 measurement system consisted of a single laser sheet source and two cameras each provided with a filter for catching the light scattered by the liquid seeding particles or the bubbles. The investigation was carried out in an open, fully-baffled, flat-bottomed cylindrical vessel (diameter, T=23.2 cm) filled with a liquid up to a height H=2T. Agitation was provided by a dual turbine – a 6SRGT manufactured by SCABA placed at the distance C1=T/3 from the vessel base and a down-pumping PBT placed at the distance C2=T from the bottom impeller. Air was always used as the gas phase, while two liquids of different viscosity were considered. 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 size distribution in the two gas-liquid systems obtained by a digital image processing method are also presented and their dependency on the dimensionless Weber number is shown. The effect of liquid viscosity and gas flow rate on the hydrodynamic behavior of the gas-liquid dispersion are discussed. The results suggest that simultaneous detection of the flow fields of the two phases is required for appropriate 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. The experimental results obtained by the PIV system are also compared with those collected by a pressure transducer. The two techniques show the presence of low frequency events that can appreciably influence the mean flow field in the stirred vessel.