Dynamics of fluid and light intensity in mechanically stirred photobioreactor

• Flow structure in a mechanically stirred photobioreactor was modeled by the computational fluid dynamics.
• Four characteristic zones of the movement of the microorganisms in the flow field were observed.
• Light intensity the microorganisms exposed to is significantly different in these zones.
• Control of the light intensity and the light/dark cycles the microorganisms subjected to is realizable.

Turbulent flows in a single-stage and a two-stage impeller-stirred photobioreactor with a simple geometric configuration were analyzed using computational fluid dynamics. The trajectories of the microorganisms entrained in the flow field were traced by the particle tracking method. By projecting these trajectories onto a radial–axial (r − z) plane with a given azimuth angle, we were able to observe four different dynamics zones: circulation, pure rotation, trap, and slow-motion. Within the pure rotation zone, turbulence can be observed near the edges of the impeller. The light intensity and the light/dark cycles subjected by the microorganisms differ significantly in these zones. These differences can be further changed by providing different incident light illuminations on the reactor surface. The dynamics zones can be altered by modifying the geometric configuration of the reactor and the impeller stirring mechanism. In combination with the utilization of different incident light illuminations, the light intensity dynamics and the light/dark cycles subjected by the microorganisms can be controlled such that an optimal photobioreactor design with a high efficiency of light utilization and a high formation rate of the biochemical products can be realized.