Hollow self-inducing impellers are often used in gas-liquid stirred tanks in many industrial processes. To understand the hydrodynamics characteristics of this type of stirred tank under aerated and unaerated conditions, a computational fluid dynamics (CFD) simulation study was conducted. The predicted gas self-inducing flow rate, power consumption, and impeller power number were compared with the experimental data in the literature and discussed. Under unaerated conditions, liquid level in the hollow shaft relies on the impeller speed, and is slightly influenced by the impeller clearance. Self-inducing impeller power number remains almost unchanged with impeller speed, while increasing with the rise of the impeller clearance. Under aerated conditions, gas is easy to accumulate in the centre of the upper and lower circulation loops. Gas self-inducing flow rate, global gas holdup, and power consumption increase with higher impeller speed. The critical impeller speed for gas self-induction decreases with higher impeller clearance. At the same impeller speed, gas self-inducing flow rate and global gas holdup increase, while power consumption reduces with increasing of impeller clearance.

A computational fluid dynamics(CFD) simulation was carried out to study on flow field characteristics in dual-Rusthton turbine stirred vessels in laminar and turbulent regimes. Model validation was conducted using experimental data in the literature. The simulation results show that flow pattern and dimensionless velocity distribution vary with Reynolds number in laminar regime, while these parameters remain almost unchanged for different Reynolds numbers in turbulent regime. For vessels with a certain geometrical configuration, flow pattern, dimensionless velocity distribution and impeller power number depend mainly on Reynolds number, and are little affected by working medium and enlargement scale. By changing impeller spacing and offbottom clearance of lower impeller, it is obtained the parallel, merging and diverging flow patterns in turbulent regime, and the changing processes of flow patterns in laminar regime for the three configurations. Total power number has the order of parallel>diverging>merging for the three configurations at the same Reynolds number. With increasing of Reynolds number, the power number of merging configuration shows the largest drop, followed by diverging configuration, and the lowest drop for parallel configuration in laminar regime, while power number rises slightly for the three configurations in turbulent regime.

A computational fluid dynamics (CFD) simulation was performed to study the hydrodynamics characteristics in a Rushton turbine stirred tank in laminar regime. The effects of operating condition, working medium and geometrical parameter on the flow field and power number characteristics were investigated. It is found that the two-loop flow pattern is formed in laminar regime when the impeller is not very close to tank bottom, while its shape and size vary with Reynolds number and impeller diameter. For a given geometrical configuration, the flow pattern, power number and dimensionless velocity profile are mainly depended on Reynolds number, and do not change with working medium and scale-up for a constant Reynolds number. When impeller off-bottom clearance is too low and Reynolds number is relatively high, the fluid flow would transit from two-loop flow pattern to sing-loop flow pattern as that occurs in turbulent regime. Power number falls for larger impeller in laminar regime. Surprisingly, in laminar regime, power number in the baffled tank with small impeller is almost identical to that in the unbaffled tank.

The computational fluid dynamics (CFD) combined with a population balance model (PBM) was applied to simulate gas-liquid dispersion in a stirred tank with low and high gas loadings. The model predictions were validated by using the data in the literature. The simulation results show that the flow patterns and gas dispersion characteristics are very different in the stirred tank for low and high gas loadings. A typical two-loop flow pattern forms as that in single-phase stirred tank for low gas loadings, while a triple-loop flow pattern, with two recirculation loops above and one below the impeller is found in the tank for high gas loadings. Shaft power input of impeller agitation plays a major role for gas dispersion with low gas loadings. For high gas loadings, the potential energy due to gas sparging has significant effect on gas dispersion and can not be neglected. Compared to low gas loading, high gas loading causes average gas holdup increased in the stirred tank, while relative local gas holdup in the lower circulation-loop region and near-wall region reduced. The ability of impeller agitation for gas dispersion reduces with high gas loadings, and mean bubble size becomes larger and the volume-averaged bubble size distribution is wider.

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