Two-phase 3D CFD modelling of an airlift pump

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Gray, Geoffrey
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An airlift pump is a vertical tube that utilizes the buoyant effects of a gas to lift a liquid. Unlike a standard mechanical pump, the liquid flow rate through the airlift pump is not directly controlled; rather, it depends on the supplied gas flow rate, the tube length and diameter, and the relative height of the liquid supply free surface (submergence ratio). The present study uses the commercial CFD code ANSYS CFX to model the isothermal, 3-D, transient flow in an airlift pump using water and air. The model applies pressure boundary conditions at both ends of the tube and specifies the mass flow rate of air through multiple openings in the side of the tube. The bottom of the tube is an inlet of water only and the outlet is a two-phase flow opening. A time-dependent, homogeneous, VOF two-phase RANS CFD modelling approach is used with the air treated as an ideal gas. Both the k-epsilon and SST turbulence models were each used in portions of this study. This work found that a complete 3-D domain was necessary for consistent prediction of the airlift performance and physically realistic two-phase flow structures. Three airlift pump configurations (combinations from two tube lengths and submergence ratios) matching cases from experiments in the open literature were simulated. The CFD simulations predicted the overall trends in airlift performance. The agreement between the simulation and experimental results was better at low gas flow rates than at high gas flow rates. The two-phase flow structures of the simulations were studied using stochastic methods. Churn flow was the predominant flow regime observed in the simulation. Airlift pump instabilities characterized by low frequency large amplitude oscillations in the water flow were observed for one case of low gas flow rate. The more the liquid flow rate entering the airlift pump varied with time, the more the two-phase flow in the airlift pump deviated from conventional two-phase flow regime maps. The present study demonstrated that a 3-D CFD model is a valuable approach to the study of the two-phase flow in an airlift pump.
Two-phase flow, CFD, Airlift pump