The effects of nanoparticles on the lift force and drag force on bubbles in nanofluids: A two-fluid model study

Bubbly flows of air-water and air-nanofluid were investigated numerically using the two-fluid model. Through comparing the predicted bubble velocity and void fraction profiles against the experimental data, the classic two-fluid model, which has been widely validated for two-phase flows of pure liquids, was found to be inapplicable to those of nanofluids because of the empirical nature of the interfacial force formulation. The roles of interfacial forces were believed to be significantly altered in nanofluids rather than in pure liquids due to the spontaneous phenomenon of nanoparticle adsorption at bubble interfaces. Because of the nanoparticle layer, bubbles submerged in nanofluids would partially behave like a rigid sphere and develop a rotation movement. A slanted wake could be induced behind the bubble, generating a lateral Magnus force pointing towards the pipe centre and consequently making the positive-to-negative reversion of lift force occur at a smaller bubble diameter. Meanwhile, the slanted wake would also make bubbles in the viscous regime experience a drag force similar to that in the distorted regime, which makes the viscous-to-distorted transition point occur at a smaller bubble Reynolds number. It was recommended that the most important task when modelling bubbly flows of nanofluids using the two-fluid model is to reformulate the interfacial forces accounting for the effects of nanoparticle adsorption.