GAS-LIQUID FLOW IN STIRRED REACTORS: Trailing Vortices and Gas Accumulation behind Impeller Blades

In a gas-liquid stirred reactor, gas tends to accumulate in low-pressure regions behind the impeller blades. Such gas accumulation forming so-called gas cavities, significantly alters impeller performance characteristics. We have computationally investigated gas-liquid flow generated by a Rushton turbine. Rotating Rushton turbine generates trailing vortices behind the blades, which enhance the gas accumulation. Characteristics of these trailing vortices were first investigated by considering a model problem of flow over a single impeller blade. Predicted results were compared with the published experimental data. Circulation velocity and turbulent kinetic energy of the trailing vortices were found to scale with blade tip velocity. Several numerical experiments were carried out to understand interaction of gas bubbles and trailing vortices. Gas-liquid flow in stirred vessel was then simulated by extending the computational snapshot approach of Ranade and Dometti (Chem. Eng. Res. Des. 74, 476-484, 1996). The approach was able to capture the main features of gas-liquid flow in stirred vessels. The detailed analysis of predicted results with reference to experimental data and the results obtained for flow over a single impeller blade will be useful for extending the scope of computational fluid dynamics (CFD) based tools for engineering gas-liquid stirred reactors.