Heat transfer characteristics of Fe_3O_4 ferrofluid flowing in a mini channel under constant and alternating magnetic fields

摘要

Laminar forced convection heat transfer of water based Fe_3O_4 ferrofluid in a mini channel in the presence of constant and alternating magnetic fields is studied numerically. The hot ferrofluid flows into the 20 mm (1) × 2 mm (h) mini channel with isothermal top and bottom cold surfaces and is subjected to a transverse non-uniform magnetic field produced by current carrying wires. Two-phase mixture model is implemented and the governing equations are solved using the finite volume approach. Primarily, the effects of the constant magnetic field location and intensity on the convective heat transfer are investigated. Simulation results show that the heat transfer is enhanced due to the disruption of the thermal boundary layer. However, this effect is more pronounced when the magnetic field source is placed in the fully developed region. In the next section, an alternating magnetic field with frequencies ranging from 0 to 10 Hz is imposed to the ferrofluid at different Reynolds numbers of Re= 10, 25 and 50. A 16.48% heat transfer enhancement is obtained with a constant magnetic field at Re=25 and magnetic field intensity, Mn=1.07 × 10~8. This value is increased up to 27.72% by applying an alternating magnetic field with the same intensity at f=4 Hz. Results also indicate that the heat transfer enhancement due to the magnetic field is more significant at lower Reynolds numbers. The optimum frequency for heat transfer enhancement has been obtained for all the cases which shows that it has an increasing trend with the Reynolds number.