Optical visualization of the formation behavior of magnetic particle clusters in a forced convection field

Abstract

Temperature-sensitive magnetic fluids are expected to achieve more efficient heat transfer by controlling the internal flow of suspended magnetic particles, in addition to improving the heat transfer coefficient of the fluid itself. Magnetic clusters are considered to be generated inside the channel under the application of a magnetic field, but their measurement is difficult experimentally, and various assumptions have been used for calculation because of the complicated flow. Therefore, in this study, fluorescent magnetic microcapsules were created to visualize cluster formation and collapse behavior at the magnetic field application position in the forced convection field. Overall, shorter clusters tended to form at larger flow velocities owing to the fluid shear force. When the fluid shear force applied exceeded a certain level, the magnetic clusters collapsed on the wall surface and stopped growing. The average length of the cluster became unstable in the region immediately before the critical shear force. The shear stress at the boundary where the cluster collapsed was correlated with the applied magnetic flux. The cluster formation behavior can be controlled by the flow velocity, magnetic flux density, and particle magnetization. The observations that would aid the achievement of a more efficient forced convection heat transfer by magnetic fluids are presented in this study.