Flow visualisation in flat panel photobioreactors with optimised gas spargers using fluorescent dyes and microspheres to evaluate mixing mechanisms and microalgae growth

Flat-panel photobioreactors (PBRs) are widely used for microalgae cultivation due to their high surface-area-to-volume ratio, efficient light distribution, and scalability. However, their efficiency is limited by suboptimal gas–liquid mixing and mass transfer, often caused by ineffective spargers. This study investigates flow patterns in a 10 L flat-panel PBR using optimised spargers that generate micro-size bubbles, aiming to improve mixing. Continuous and intermittent airflow conditions were tested at 0.5, 0.8, and 1 L min −1 . Two qualitative flow visualisations, fluorescent dye imaging and motion tracking with fluorescent microspheres, were employed. Flow pattern images were captured using a high-speed camera, while a UV lamp was used to illuminate the fluorescent dye and particles, enhancing visualisation for detailed image analysis. Results confirmed that the sintered metal sparger (5 μm pores), arranged in a novel double orientation with 1 L min −1 , provided optimal mixing, which was validated biologically as the most suitable configuration for Chlorella sorokiniana cultivation. Intermittent airflow (20 s ON, 10 s OFF) provided adequate culture mixing with 33 % lower energy. It resulted in improved Chlorella sorokiniana performance, yielding a higher specific growth rate (0.56 d −1 ), shorter doubling time (1.24 days), greater biomass productivity (0.12 g L −1 d −1 ), and CO₂ fixation (0.21 g L −1 d −1 ) compared to continuous flow. This research advances the understanding of micro-mixing in PBRs and evaluates sparger designs for improved microalgae cultivation and carbon capture. Future studies should integrate computational fluid dynamics to validate the findings. The underlying mechanisms will contribute to developing sustainable and efficient PBRs for CO 2 sequestration and bioresource production in circular economy applications. • Flow visualisation confirmed improved mixing at 1 L min −1 in flat-panel PBRs. • CUS-5 sparger outperformed CUS-10 and CUS-20 in mixing efficiency. • Intermittent air supply of 20s ON/10s OFF reduced mixing time and energy use. • Flow visualisation identifies vortices and dead zones compared to quantitative tests. • Optimised sparger enhanced Chlorella sorokiniana growth and CO 2 fixation.