CFD simulation of isolated bubbles rising in Newtonian or non-Newtonian fluids inside a thin-gap bubble column

Abstract

Hydrodynamics of the bubbling process can be complex especially in thin bubble columns, when the gap has the same order of magnitude as the bubble diameter, and with complex fluids. It is then important to understand this phenomenon either by experimental investigation through optical methods such as shadowgraphy and/or Particle Image Velocimetry (PIV) or numerically by Computational Fluid Dynamics (CFD), which, when validated, can allow numerical experimentation in situations which are expensive to implement experimentally or time consuming. In this study, three-dimensional numerical simulations of isolated bubbles rising in Newtonian (water) or non-Newtonian (CarboxyMethyl Cellulose (CMC) and Xanthan Gum (XG) solutions) liquid phases mimicking Chlorella vulgaris cultures at 42 g.L⁻¹ concentration inside a 4 mm gap bubble column are performed using the volume of fluid (VOF) model with the ANSYS FLUENT 17.2 code. Results are validated by comparison with shadowgraphy experiments. Bubble terminal velocity, shape, and trajectory are numerically analysed. Wall shear stress (WSS) induced by the bubble, strain rate, viscosity and flow field around the bubble are also discussed. Numerical results show similar trends as experimental ones despite slightly lower terminal velocity and aspect ratio values are observed in comparison to the experimental results. The trajectory of the bubble is non-rectilinear for water and rectilinear for non-Newtonian fluids as observed experimentally. This numerical study highlights the bubble-liquid and bubble-wall interactions that will help to understand the complex phenomena of bubble rise in non-Newtonian media/microalgae suspensions at high concentrations at the local level in thin-gap bubble columns.