Experimental study of the mixing and flow properties of fractal oscillating grids in water.

If a biochemical process deals with micro-organism cultures, the mixing device used in the bioreactor should promote all the transfer phenomena (nutrients, gas-liquid, light, …) while limiting the mechanical stress induced on the micro-organisms. With this comment in mind, we propose to design a photobioreactor (PBR) stirred by fractal oscillating grid systems for the culture of shear-sensitive microalgae. Oscillating cross-barred grids generate almost isotropic and homogeneous turbulence with a weak (almost null) mean flow. Grids with fractal geometries were used as fixed grids to generate downstream, isotropic and homogeneous turbulence in wind and water channels. Therefore, our work is the first implementation of a fractal oscillating grid flow. Flow characterisation experiments namely Planar Laser Induced Fluorescence (PLIF) and PIV (Particle Imaging Velocimetry) are performed, with the aim to determine the best conditions (grid pattern, oscillation frequency and amplitude) a priori for microalgae cultures.