CFD simulation of multiphase flow in an airlift column photobioreactor

A Computational Fluid Dynamics (CFD) simulation using CFX, ANSYS 11.0, has been carried out using a multiphase flow model with an Eulerian-Eulerian approach for an airlift column photobioreactors (PBR). Transient simulations were performed for three inlet air flow, 2, 3 and 5 l/min. The contours for gas holdup, air and water velocity showed that the presence of gas phase (air bubbles) is lower in the downcomer but larger in the riser, which leads to require a vigorous mixing in the riser that will be sufficient for a continuous flow. For air, velocity vectors show that they are smaller in the downcomer than in the riser. Nevertheless, water velocity vectors are organized, pointing down in the downcomer and up in the riser. Water shear stress rate contours analysis showed that, shear stress rate regions are considerably larger in the riser, but lower in the downcomer. Due to fewer restrictions to the liquid phase in the riser, a large amount of energy is dissipated by gas liquid interactions. In the downcomer region, gas phase is almost inexistent, and so are the bubble collisions. Finally, the kinetic energy is larger at the top region of the riser, meanwhile is lower at the downcomer. Similar results are observed for energy dissipation rate.