Magnetic field induced acceleration or deceleration of bubble rising inside ferrofluids: A fractional step lattice Boltzmann investigation

Abstract

Bubble rising process inside a ferrofluid commonly occurs in industrial applications, but the interface deformation, the bubble motion, and their underlying mechanisms are not fully understood. In the present study, a magnetic field coupling fractional step lattice Boltzmann model is employed to simulate the complex interface behaviors and the magnetic flux density distribution during the bubble rising process. The magnetic field is solved by a self-correcting Poisson equation solver, and the flow properties and the interface are first predicted by the equilibrium distribution functions and then corrected by the nonequilibrium distribution functions and source terms. The acceleration and deceleration of the rising bubble under the combination effects of the hydrodynamic force, the surface tension force, and the magnetic force are investigated. This study aims to reveal the mechanisms of the magnetic effects, especially the magnetic field induced bubble interface deformation, and the acceleration and deceleration effects on the bubble rising process. Applying a vertical uniform magnetic field, the bubble rising velocity accelerates with the magnetic field strength, and various shape types are observed. However, when a horizontal uniform magnetic field is applied, the bubble rising velocity decelerates with an increase in the magnetic field strength. Meanwhile, the initial shape remains elongated only along the magnetic field direction. The numerical results from both the conditions show that the external magnetic field indirectly impacts on the bubble rising process through the squeezing effect generated by the surrounding magnetized ferrofluid. A shape regime map is presented for different bubble shapes during the rising process over wide range of the parameter space. Besides, by employing an appropriate combination of the surface tension and the magnetic field strength and direction, it is possible to either accelerate or decelerate the rising bubble. These findings not only provide valuable insights into the underlying mechanisms of the bubble deformation and the rising velocity, but also offer potential technical support for industrial applications.