Abstract
A numerical study of the motion of algal cells in a representative thin-layer-cascade (TLC) photobioreactor is presented. The goal is to determine the time scale associated with the light/dark (L/D) cycle seen by the cells during their turbulent motion in the liquid culture. Owing to the limited reliability of the available numerical results which deal with time-averaged quantities and thus lack time-resolved information, the present study is based upon the Direct Numerical Simulation of the Navier-Stokes equations, a reliable but consequently expensive numerical approach which does not incur in turbulence modelling errors. Indeed, the simulation is successfully validated in terms of averaged velocity with experimental data. The availability of full temporal information allows algae cells to be followed in time along their trajectories. A large number (up to a million) of tracers is placed in the flow to mimic the algae cell. Their trajectories are statistically studied and linked to the turbulent mixing. Results indicate that, in a typical TLC reactor designed to mimic an experimental setup, cells undergo an L/D cycle with a time scale in the range 0.1–2 s. Such time scale, albeit much longer than the typical time scale of the photosynthesis, significantly benefits the productivity of the algae compared to a steady illumination.
Highlights
A strong interest in cultivation of phototrophic microalgae is driven by their several envisaged application areas: a non-exhaustive list includes biofuels, chemicals, medicine and nutrition (Posten and Walter 2012)
We study through numerical simulations one specific thin-layer cascade (TLC) photobioreactor, and aim at understanding its working principle, by characterising the L/D cycle the algal cells are typically subjected to
The algae cells are modelled as massless tracers which are passively driven around by the velocity field
Summary
A strong interest in cultivation of phototrophic microalgae is driven by their several envisaged application areas: a non-exhaustive list includes biofuels, chemicals, medicine and nutrition (Posten and Walter 2012). Microalgae hold potential in food production and in purification of contaminated water and wastewater (Acien et al 2016; Bernaerts et al 2017). They are important in the food chain as they are the main feeding source for rotifers, fishes, etc.; microalgae possess high nutritional value and contain lipids, proteins, carotene and other essential minerals (Hakalin et al 2014). An open-pond PBR is simple and enjoys low construction costs and low power requirements It can be scaled up in size; cleaning and maintenance are simple and affordable. The sunlight intensity is usually excessive near the irradiated liquid surface and causes photoinibition (see Han et al (2000) and references therein), whereas a large fluid volume beneath the surface receives insufficient light for photosynthesis, leading of the so-called respiration phenomenon, where net photosynthesis cannot take place (Iluz and Dubinsky 2012)
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