Heat transfer augmentation using a magnetic fluid under the influence of a line dipole

Ferrofluids have promising potential for heat transfer applications, since advective transport in a ferrofluid can be readily controlled by using an external magnetic field. However, unlike conventional free or forced convection, ferrohydrodynamic convection is not yet well characterized. A full understanding of the relationship between an imposed magnetic field, the resulting ferrofluid flow, and the temperature distribution is a prerequisite for the proper design and implementation of applications involving thermomagnetic convection. The literature variously assumes constant magnetic fields, does not completely represent the variation in the imposed field, or its descriptions are inaccurate, since the fields do not comply with the Maxwell's equations of electromagnetism. We address this by simulating two-dimensional forced convection heat transfer in a channel with a ferrofluid that is under the influence of a two-dimensional magnetic field created by a line-source dipole. Our objective is to characterize the heat transfer augmentation due to the thermomagnetic convection and correlate it with the properties of the imposed magnetic field. We determine that magnetic effects on the corresponding flow are localized. The local asymmetry in the thermal boundary layer about the line dipole and the resulting spatial nonuniformity of the fluid susceptibility causes colder fluid to move closer to the line dipole. Thus, the magnetic field induces the production of a local vortex near the cold wall. This alters the advection energy transport, changes the temperature distribution in the flow and enhances the heat transfer. The addition of dipoles is beneficial for heat transfer, since they create additional recirculation zones. Heat transfer is also affected while the spacing between the two dipoles is varied. Thus, an enhancement in the overall heat transfer depends on the net magnetizing current as well as the relative placement of the dipoles. For hydrodynamically similar cases, the heat transfer enhancement produced by a magnetic field can be predicted if information regarding the magnetic moment of the field-inducing magnet and the distribution of the representative line dipoles are known. (C) 2003 Elsevier B.V. All rights reserved.