Abstract Summary (Max 250 words)
Tubular photobioreactors for microalgae cultivation suffer from strong radial gradients in light intensity, mass transfer limitations, and progressive biofilm formation on transparent walls, all of which reduce productivity and operability. This contribution investigates the use of static and dynamic mixers inserted into tubular photobioreactors to tailor hydrodynamic conditions with three primary objectives: (i) homogenization of the light field within the illuminated liquid layer, (ii) intensification of mixing and mass transfer for efficient utilization of nutrients and CO2, and (iii) enhancement of wall shear stresses to mitigate biofilm growth on transparent tube surfaces. A set of mixers with variable geometry was designed, enabling systematic variation of key geometric parameters relevant for flow pattern, residence time distribution, and wall shear. The performance of mixers was evaluated through a combination of numerical and experimental methods. In order to characterize flow structures and hydrodynamic indicators relevant to light and mass transfer, numerical simulations were employed. Furthermore, laboratory-scale experiments were conducted in order to quantify mixing/homogenization efficiency and pressure drop across the mixers. The resulting data were then compared with those obtained from a commercially available mixer. The present study explores the trade-offs between mixing intensity and hydraulic losses, and identifies geometry ranges where enhanced homogenization and increased wall shear can be achieved without prohibitive pressure effects. The findings provide a rational basis for the design and scale-up of tubular photobioreactors equipped with static or dynamic mixers, with the aim of achieving more robust and energy-efficient microalgae cultivation.
