Numerical simulation on vortex flow and CO₂ bubble aggregation behavior in a raceway pond with diaphragm aerators

To optimize flow field in a raceway pond with diaphragm aerators to promote microalgal CO2 fixation, numerical simulation on three-phases (CO2 bubbles-culture medium-microalgae cells) with CFD (Computational Fluid Dynamics) - PBM (population balance model) was conducted to investigate effects of dynamic bubble aggregation/fragmentation on flow field distribution and mixing performance. The inhomogeneity of turbulent energy distribution increased with decreased bubble movement distance when inclination angle theta > 11 degrees. When bubble generation diameter increased from 0.4 mm to 2.5 mm, average solution velocity and mixture turbulent kinetic energy first increased to peaks and then decreased, bubble aggregation height decreased and aggregation range became larger. When initial bubble velocity increased from 0.0314 m/s to 0.1257 m/s, microalgal flashlight frequency first increased to a peak of 0.47 Hz and then decreased, gas volume fraction and turbulent dissipation rate gradually increased. Finally, the growth rate of Chlorella increased by 22% to 0.71 g/L/d.