Flow and Mass Transfer of Fully Resolved Bubbles in Non-Newtonian Fluids Stefan Radl Institute RNS, Graz University of Technology, Graz, Austria Gretar Tryggvason Dept. of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609 Johannes G. Khinast Dept. of Chemical and Biochemical Engineering, Rutgers University, Piscataway, NJ 08845 DOI 10.1002/aic.11211 Published online June 5, 2007 in Wiley InterScience (www.interscience.wiley.com). In this work, high-resolution 2-D numerical simulations were performed on the motion of deformable bubbles in non-Newtonian fluids and the associated mass transfer. For that purpose, we have implemented a semi-Lagrangian advection scheme and improved the fluid dynamic calculation by the usage of implicit algorithms. Non-Newto- nian fluids are described by generalized Newtonian as well as viscoelastic model fluids. As shear-thinning model we use a Power-Law and a Carreau-Yasuda model, the viscoelastic fluid simulations are based on an Upper-Convected Maxwell model combined with a recently introduced model for the evolution of the effective shear rate. The mathemati- cal challenges arising from the hyperbolic nature of the resulting set of equations are addressed by inclusion of artificial diffusion in the stress equation. In our work, it was found that shear thinning effects have impact on collision rates, and therefore, may influence coalescence of bubbles in non-Newtonian liquids. Furthermore, for the first time, concentration fields of dissolved gas in viscoelastic fluids are presented. The study shows that the fluid elasticity plays a major role for bubble rise velocity, and therefore, mass transfer. As the wake dynamics differ significantly from that in Newtonian liquids, abnormal mixing characteristics can be expected in the bubbly flow of viscoelastic fluids. Ó 2007 American Institute of Chemical Engineers AIChE J, 53: 1861– 1878, 2007 Keywords: numerical simulation, bubbles, non-Newtonian liquids, mass transfer Introduction Because of persistent uncertainties in the underlying mod- els and system properties, the scale-up of bioreactors remains a challenging problem. For example, the rheology of media in bioreactors is non-Newtonian in most cases. 1–8 However, details of the non-Newtonian fluid characteristics, such as viscoelasticity or the tendency for shear thinning, are fre- quently unknown or neglected. Furthermore, in contrast to Newtonian media, there is a lack of general correlations for mass and heat transfer rates, which are needed for the predic- tion of oxygen supply and heat removal. Similar observations can be made for the properties needed to precisely predict transport of metabolites, nutrients, or CO 2 . So far, most stud- ies are based on experimental investigations, which are usu- ally interpreted with simple theoretical models. 3,4,8–11 First- Correspondence concerning this article should be addressed to J. G. Khinast, currently Murie Curie Chair at the Institute RNS, Graz, at khinast@tugraz.at. Ó 2007 American Institute of Chemical Engineers AIChE Journal July 2007 Vol. 53, No. 7 1861