Investigation of single bubbles rising in Newtonian and non-Newtonian fluids inside a thin-gap bubble column intended for microalgae cultivation

Abstract Single bubbles rising in a confined geometry – a parallelepidic bubble column with a 4 mm thickness – in the presence of a non-Newtonian liquid phase are studied using a shadowgraphy technique. The context is the cultivation of microalgae at high cell concentrations. In fact, it necessitates the reduction of the bioreactor thickness to allow light penetration through the whole culture. In the same time, the medium becomes shear-thinning for biomass concentrations higher than 30 g L−1. A 2 g L−1 carboxymethyl cellulose (CMC) and a 1 g L−1 Xanthan Gum (XG) aqueous solutions are studied to mimic the behavior of 42 g L−1 Chlorella vulgaris cultures. Besides, a comparison with water, representing the Newtonian liquid phase at low culture concentrations, is proposed. The bubble equivalent diameter is varied between 1.34 and 3.36 mm using different capillary sizes to generate the bubbles. Results show the complex and coupled effects of confinement and shear-thinning behavior of the liquid phase on bubbles terminal velocity, shape and trajectory. In fact, compared to the case of an infinite liquid medium, confinement increases wall friction on the bubbles, while reduces the liquid phase apparent viscosity in its neighborhood due to increased shear rate. This is why, depending on the bubble size, the bubble terminal velocity in shear-thinning fluids can be higher or lower than in unconfined conditions. For bubbles rising in shear-thinning fluids, contrary to water, it is observed that confinement induces a transition from spherical shape to ellipsoidal shape that occurs sooner in terms of bubbles diameter or Eo number. A correlation for the bubble drag coefficient in confined conditions in presence of shear-thinning fluids is also proposed for confinement ratios db/e between 0.34 and 0.84.