Impact of bubbles on the light field within a photobioreactor: A practical design tool

Abstract

Bubbles can be strong modulators of the light field they interact with. An example of such a configuration is a photobioreactor hosting a microalgae culture where bubbles and cells contributions entangle. To investigate this question, this work computed bubble distribution in a photobioreactor setup using OpenFOAM. Then, Ray Tracing coupled with the Monte Carlo Method was used to calculate the light over 3240 configurations (cell concentrations, strains, pigment profiles, void fraction, etc.). From a physical perspective, this investigation showed that the presence of bubbles lengthens the ray path within the culture medium and increases the apparent absorption. In addition, a sizable amount of the incident light can be reflected and scattered away (about 10% for a 2.15% void fraction). From a numerical perspective, using a dimensionless approach and physics-based formulation, the whole knowledge of the database was condensed into an exponential decay (Beer–Lambert like) model. The methodology was first used on a bubble-free setup (360 profiles), where it delivered accurate local and global predictions of the light and the reflected light fraction. Then, it was expanded to the whole database. All in all, it allowed to compute the local light field, total distributed energy, and reflected light fraction with accuracies (MARE) of 9.50, 2.25, and 6.35%, respectively. It also allowed to distinguish between bubbles and microalgae contributions. Owing to its simplicity and computational efficiency, the model can be used to precisely account for detailed light distribution in large-scale models, opening the way to improving the computation of country-scale microalgae cultivation performances.