Effects of external magnetic field arrangements on thermomagnetic convection in ferrofluid around heated Micro-wire

Thermomagnetic convective cooling of a heated micro-wire in quiescent ferrofluid is experimentally investigated under various non-uniform external magnetic fields produced by permanent magnets. In the past studies, in most of the cases for this geometry the external magnetic field is applied under forced convection conditions. This work provides the first published data set of heat transfer from a microwire in quiescent ferrofluid (free convection conditions) under various arrangements of external magnetic fields produced by permanent magnets. For the different arrangements, magnetic flux density and Kelvin body force density are calculated to aid discussions of the observed effects. The experiment tracked the transient temperature rise of a heated micro-wire, highlighting effects of applied magnetic field orientation, symmetry, and strength. The results showed that greater non-uniformity in radial magnetic field could enhance the cooling effects with up to 7% reduction in the heated wire temperature rise. A magnetic field aligned with the heated wire axis produced only a minor enhancement of 3% in heat transfer in contrast to application of radial magnetic fields. Surprisingly, a stronger external magnetic field did not always result in better heat transfer. Magnetophoresis of ferro-particles induced by the external magnetic field was found to be an important consideration when using permanent magnets for ferrofluid applications.