Convective heat transfer characteristics of magnetite nanofluid under the influence of constant and alternating magnetic field

The effects of constant and alternating magnetic field on the laminar forced convective heat transfer of water based magnetite (Fe3O4) ferrofluid in a heated tube are studied experimentally. The ferrofluid flows in a uniformly heated tube with 9.8 mm diameter and 2680 mm length and is influenced by a magnetic field generated by four electromagnets. The local convective coefficients are measured at both thermally developing and fully developed regions for three different volume fractions of phi = 1, 1.5 and 2% and in the Reynolds number range of 400-1200. The magnetic field and the resulting magnetic force distributions are also simulated to get further insight into the heat transfer augmentation. In the absence of a magnetic field, results show that using magnetite ferrofluid with phi = 2 % improves the average convective heat transfer up to 13.5% compared to the DI-water at Re = 1200. This value grows up to 18.9% and 31.4% by application of constant and alternating magnetic field with intensity of B = 500 G, respectively. The heat transfer is shown to be increased with the Reynolds number, ferrofluid concentration, and the intensity of the magnetic field. Under the constant magnetic field, migration of nanoparticles to the tube surface increases the local thermal conductivity and consequently the heat transfer near the electromagnets. Moreover, disruption of the thermal boundary layer and increased flow mixing seem to be the possible reasons for the heat transfer enhancement by the alternating magnetic field. (C) 2015 Elsevier B.V. All rights reserved.