Rising behavior and mass transfer of CO2 bubbles in shear-thinning non-Newtonian fluids

An experimental study of single carbon dioxide (CO2) bubbles ascending in aqueous solutions of carboxymethyl cellulose (CMC) is performed as a means to obtain further understanding of the rising behavior and mass transfer of soluble bubbles in shear-thinning non-Newtonian fluids. High-speed photography and digital image processing technique are employed to quantify the characteristic parameters of the bubble rising process. The rising behavior of single CO2 bubbles in a Newtonian fluid (tap water) is measured as a reference. It has been found that the rheology of the liquid phase significantly influenced the rising behavior of the bubbles. In CMC aqueous solutions, CO2 bubbles mostly followed a straight rising path characterized by increasing aspect ratio and decreasing vertical velocity. Increase in initial bubble size led to bubble deformation in less concentrated CMC aqueous solutions which resulted in an irregular rising path. The evolution of bubble equivalent diameter can be mainly divided into two stages: a fast reduction stage followed by a slow one. Meanwhile, the variation of liquid-side mass transfer coefficient kL of CO2 bubbles in tap water and CMC aqueous solution with different weight percentages is discussed. The mobility of gas-liquid interface is considered to have significant impact on gas-liquid mass transfer. A dimensionless correlation considering the rheology of the liquid phase is proposed to describe the evolution of the motion and deformation of single soluble CO2 bubbles in infinite stagnant shear-thinning non-Newtonian liquids.