Buoyancy driven bubble rise and deformation in milli/micro channels filled with shear-thinning nanofluids

The motion and deformation characteristics of buoyancy driven air bubbles in milli- and micro- channels filled with nanofluids have been numerically investigated over wide range of Eotvos number using COMSOL Multiphysics 4.3a. The two dimensional unsteady simulations are carried out by adopting a level set method available in the solver. Experimental rheological properties of water based alumina nanofluids are adopted from the literature and these nanofluids obey shear-thinning power-law type non-Newtonian behavior. The range of volume fraction of nanoparticles considered in this work is Φ = 0.015–0.035 which correspond to the power-law behavior index of shear-thinning nanofluid in the range n = 0.78–0.58 respectively. The motion and deformation characteristics of bubbles in nanofluids are elucidated by presenting volume fraction images, streamline patterns and viscosity distributions around the rising bubbles. The volume fractions of nanoparticles present in the nanofluids play different roles on the deformation of mono-size bubbles in milli- and micro- channels. The deformation and breakup of bubbles are realized early in the case of milli-channels as compared to the case of micro-channels for a fixed Eotvos number. Further the Eotvos number at which bubble breakup occurs in both milli- and micro- channels is very different.