Influence of the Top Impeller Diameter on the Gas Dispersion in a Sparged Multi-impeller Stirred Tank

The impeller configuration with a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT+2WHU and recommended in previous work, was used in this study. The effect of the top impeller diameter, ranging from 0.33T to 0.50T, on gas–liquid flow in a stirred tank of 0.48 m diameter was investigated by experimental and CFD methods. Power consumption, total gas holdup, and local void fraction were measured for the impeller configurations with different top impeller diameters. Results show that while the ratio of top impeller diameter to tank diameter (Dtop/T) increases from 0.33 to 0.50, the relative power demand (RPD) in a gas–liquid system decreases slightly. The increase of total gas holdup with rising gas flow rate becomes less evident as Dtop/T increases from 0.33 to 0.50. Local void fractions at the measurement points above the height of 0.8T increase significantly with the increase of top impeller diameter. When Dtop/T = 0.50, there is an extremely l...