Abstract

Mixing performance of photobioreactors where the microalgae grow up greatly affect the accessibility of nutrient substance as well as the availability of light for the cells, so it is of vital importance to properly design the geometry of photobioreactor. In this work, a kind of spiral photobioreactor was introduced. The flow characteristics of the fluid were obtained through computational fluids dynamics modeling. Cell trajectories of microalgae were determined by Lagrangian formulation and the light intensity histories of tracked cells were investigated by integrating Lagrangian approach with the irradiative model. The swirl numbers representing the mixing performances of the both photobioreactors were numerically calculated. Results show that strong swirl motions are formed in the cross-sections along axial coordinate of the spiral photobioreactor under laminar state, but no such vortice is observed for tubular photobioreactor. The light intensity histories of the tracked cells imply that the microalgae cells experience the so called light/dark cycle which is necessary to their growth. With high swirl number ranging from 0.1 to 0.45, the mixing performance of the spiral photobioreactor is much better than that of tubular PBR, indicating such innovative geometry is of great potential for mass culture of microalgae in the future.

Key words: Microalgae, mixing performance, spiral tube photobioreactor, flow characteristics, cell trajectories, light intensity history.

Abbreviation

Notation: - pipe diameter (m), - direct solar irradiance, - diffuse solar irradiance, - length of the torus reactor (m), - radial coordinate (m), - pipe radius (m), - Reynolds number=, - the considered cross-sectional surface area, - swirl number, - mean axial velocity component (ms⁻¹), -mean bulk velocity (ms⁻¹), - mean circumferential velocity component (ms⁻¹),- axial coordinate (m), - dynamic viscosity (Pa), - fluid density (kgm⁻³).