Computational fluid dynamics analysis of air -water bubble columns
Computational fluid dynamics (CFD) simulations of air-water bubble columns are performed with CFDLib, a cell-centered, finite-volume Fortran code developed by Los Alamos National Laboratory. The code uses an Eulerian two-fluid model, including closures for effective stress and interphase momentum exchange. Numerical studies provide a detailed evaluation of the ability of the multiphase models in CFDLib to predict flow regimes in bubble columns.
A fundamental simulation study illustrates the strong dependence of the CFD flow regime predictions on the two-fluid model formulation, which includes bubble-induced turbulence and interphase force models (i.e., drag, added-mass, lift, rotation, and strain). This initial study is considered a set of numerical "experiments" in which model parameters such as bubble Reynolds number or force model coefficients are adjusted in order to determine which closures are necessary for the two-fluid model to successfully predict known flow regimes.
In the two-fluid model, the bubble Reynolds number Re is controlled by bubble diameter and the gas volume fraction alpha is controlled by the inlet gas flow rate. Flow maps identify the regions in Re-alpha space where flow profiles exhibit a particular behavior, and illustrate where flow transitions occur. These flow maps also reflect a strong dependence on the model formulation applied.
Next, the linear stability of the two-fluid model is investigated. Dispersion relations describe the growth (or decay) rates for disturbances in gas-liquid flow. The linear stability analysis provides further insight regarding how the two-fluid model formulation affects transitions from homogeneous to heterogeneous flow. The roles of the effective viscosity model, the bubble-pressure model, and the interphase force models are examined in detail.
Finally, a validation study compares numerical results against experimental results from researchers at Delft University of Technology. CFD simulations of uniform and non-uniform aeration cases are carried out. Overall, the numerical studies presented in this work demonstrate that the effective viscosity model, the bubble-pressure model, and the full set of interphase force models, with carefully chosen parameters, should be included in order to obtain flow profiles expected for particular sets of operating conditions.