Experimental investigation of a local recirculation photobioreactor for mass cultures of photosynthetic microorganisms

• Photosynthetic activity under the “flash effect” was estimated.
• PBR wavy-bottomed surface features a spatial alternation of deep and shallow zones.
• The fluid dynamics of wavy PBR was characterized by Particle Tracking Velocimetry.
• Vortex size, recirculation and straight flow times were identified.
• Microalgal photosynthetic activity is higher under high frequency light pulses.

The present work deals with the experimental fluid mechanics analysis of a wavy-bottomed cascade photobioreactor, to characterize the extent and period of recirculatory and straight-flowing streams establishing therein as a function of reactor inclination and liquid flow rate. The substream characterization via Feature Tracking (FT) showed that a local recirculation zone establishes in each vane only at inclinations ≤6° and that its location changes from the lower (≤3°) to the upper part of each vane (6°). A straight-flowing stream flows opposite (above or below) the local recirculation stream. The recirculation time ranges from 0.86 s to 0.23 s, corresponding, respectively, to the minimum flow rate at the minimum inclination and to the maximum flow rate at the maximum inclination where recirculation was observed. The increase of photosynthetic activity, resulting from the entailed “flash effect”, was estimated to range between 102 and 113% with respect to equivalent tubular and bubble column photobioreactors.

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