CFD SIMULATION OF MULTIPHASE FLOW IN AN AIRLIFT COLUMN PHOTOBIOREACTOR FOR THE CULTIVATION OF MICROALGAE

摘要

Due to their better operational advantages, such as good scalability, operational flexibility, high mass and heat transfer characteristics, homogenous shear stress, good mixing and better control over fluid circulation, airlift column photobioreactors (PBR) have become a promising design alternative for microalgae culture, among bubble column and other types of photobioreactors. The good hydrodynamic environment for fragile microorganisms in airlift columns have led to numerous investigations in order to understand the hydrodynamic phenomena of multiphase flow, particularly gas-liquid, inside this type of PBR.In the present paper, a CFD (Computer Fluid Dynamics) simulation in CFX, ANSYS Inc. 11.0, is developed following a multiphase flow model with an Eulerian-Eulerian approach. For evaluating turbulence, the modified k — e model is chosen. Gas holdup, gas superficial velocity and liquid velocity profiles for different heights column are obtained by varying air volumetric flow rates at 2, 5 and 8 L/min. Also gas holdup, gas superficial velocity and liquid velocity contours are obtained for a sequence of five step times. An experimental video validation to compare the multiphase flow behavior has been made with a high-resolution video camera. The experiments are carried out by using an airlift column photobioreactor with air supply by a porous sparger.The profiles of the hydrodynamic variables made for 5 different column heights show that the trends of gas holdup and gas superficial velocity are very similar and do not depend on the variation of inlet air flow. For the liquid velocity profiles, the trends don't show the same behavior, the profiles at lower heights are off-centered and less symmetric and the maximum velocities are reached at h=0.2 m.The time sequences with the variable contours are made to enhance the visualization and understanding of the hydrodynamic behavior, especially when air supply begins and the bubble plume starts to form.