Experimental Study And Simulation Of Gas-liquid Two-phase Flow In Stirred Tanks Using Dual-bubble-size Model

Gas-liquid stirred tanks are commonly used in mixing and reaction in chemical,metallurgical,pharmaceutical and other fields.The complex gas-liquid two-phase flow is a typical multi-scale problem in nature.Understanding the hydrodynamic characteristics of the multiphase flow is very important for the design,optimization and scale-up of gas-liquid stirred tanks.In this paper,the experimental and Computation Fluid Dynamics methods were used to study the gas dispersion characteristics in gas-liquid stirred tank.The objective of this work is to compare the influence of different drag models and bubble treatment methods on the CFD simulation and verify the applicability of the Dual-Bubble-Size drag model in the CFD simulation of gas-liquid stirred tanks.Firstly,the influence of different drag models on the simulation of gas liquid flow field,gas holdup and liquid velocity axial distribution in the discharge region are investigated.The simulation results show that the predicted gas holdup distribution of DBS-Local drag model fits well with the experimental data.Then,the predictive capability of DBS-Local drag model on flow regimes transition and Power Number under different operation conditions is analyzed.The results show that the DBS-Local model can simulate the flow pattern and the Power Number in the stirred tank with the change of impeller rotational speeds.Secondly,in order to verify the applicability of the DBS-Local drag model in the simulation of stirred tank,the gas holdup radial distribution in the circulating flow region in a stirred tank was measured by a double conductivity probe,then the experimental system was simulated by CFD method.The simulated gas holdup distribution of Tomiyama or DBS-Local drag model is in good agreement with experimental data under lower rotational speeds.However,only the DBS-Local drag model can accurately simulate the gas holdup distribution in circulating flow regions.The simulation demonstrates the advantage and potential of the DBS-Local drag model for gas-liquid stirred tanks.In the last section,the CFD-PBM method was used to simulate the gas-liquid two-phase flow in a stirred tank equipped with dual-impeller.The effect of different bubble treatment methods on global gas holdup,local gas holdup distribution and liquid flow field were compared.In addition,the effects of different coalescence kernel models on the bubble size distribution are also compared.The results show that PBM models can not show any advantage compared to the constant bubble size assumption under lower gas flow rate,but there is some certain influence under higher gas flow rate.Due to the complexity of fluid flow in the gas-liquid stirred tank,the traditional coalescence models cannot predict the bubble size distribution accurately.