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Computational analysis of hydrodynamics and light distribution in algal photobioreactors

Varun Loomba

Photobioreactors (PBRs) of different shapes and sizes are used to efficiently grow microalgae. Study and optimization of the PBR are required to maximize algal productivity. In the present study, a lab scale flat panel PBR of 1 L capacity is computationally analyzed to characterize the fluid flow. Air is bubbled through the medium (water and microalgae), creating loops of liquid in the PBR. Flow profiles of liquid medium and air are simulated using COMSOL Multiphysics software that solves the Navier-Stokes equations.

The results of liquid flow profiles are used to trace the path of microalgae over a certain time span. There is a distribution of light inside because of absorption and scattering of light by microalgae. This is taken into account by solving the radiative transfer equation to obtain the light profile in the PBR. The algal traces are combined with the light intensity values to get the dynamic light exposure of the algal cells as they move inside the PBR. This information is used to analyze the light-dark patterns algae are exposed to, and to calculate the dynamic growth rates of algal cells based on available light intensity. Growth models of varying complexity are applied. The same tasks are performed for various flow conditions that help to determine the optimal conditions to operate the PBR. Design optimization is also done to improve the flow profiles of the liquid in the PBR.

 Velocity profile of liquid movement in an algal photobioreactor sparged with air from four inlet holes, shown along two perpendicular planes inside the photobioreactor