CFD simulation of multiphase (liquid–solid–gas) flow in an airlift column photobioreactor

A 2D computational fluid dynamics simulation was carried out using a multiphase flow model with an Eulerian–Eulerian approach for a microalgae culture in an airlift column photobioreactor. Simulation was performed for a 0.0625l/lculture·min\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.0625\,l/l_{\mathrm{culture}}\cdot \hbox {min}$$\end{document} inlet airflow. Air, water and microalgae velocity contours showed less gas phase present in the downcomer than in the riser, suggesting the necessity of vigorous mixing in the ascendant portion if homogeneous water and solid flow is to be achieved. Air velocity is smaller in the downcomer (shorter velocity vectors) than in the riser. Water velocity vectors point always in the expected direction, down in the downcomer and up in the riser. Microalgae paths, perhaps due to the small size of the microorganisms, follow the water velocity vectors. As there are fewer hydraulic restrictions to the liquid phase in the riser, a large amount of energy is dissipated by gas–liquid interactions. In the downcomer region, the gas phase is almost nonexistent, and bubble collisions are almost nonexistent as well. A quasi-stagnation zone was found at the lower section of the downcomer, showing that the design requires improvement. Finally, the turbulent kinetic energy is larger at the top and middle region of the riser; meanwhile, it is lower at the downcomer. Similar results were observed for the energy dissipation rate.