Experimental investigation on the convective heat transfer of Fe3O4/water nanofluids under constant magnetic field

The convective heat transfer characteristics of Fe3O4/water (3 vol%) nanofluids were investigated under a parallel constant and uniform magnetic field. Specifically, the effects of temperature and magnetic field strength on the convective heat transfer of the Fe3O4/water nanofluids were examined in the experiment. The Fe3O4/water nanofluids flew through a uniformly heated circular tube with 3 mm diameter and 600 mm length. In the absence of the magnetic field, the averaged convective heat transfer coefficients using the Fe3O4/water nanofluids were enhanced by up to the range between 1.2% and 2.3% under the laminar-flow regime conditions and between 4.7% and 5.6% under the turbulent-flow regime conditions than the cases with distilled water. Unfortunately, the convective heat transfer coefficients of Fe3O4/water nanofluids were observed reduced when a parallel constant and uniform magnetic field was applied and decreasing with the intensity of the magnetic field. The reason for the heat transfer enhancement under the applied magnetic field was assumed to be the reduction in the boundary layer thickness. In addition, the chain-like structure distribution of magnetite nanoparticles and the Brownian motion of magnetite nanoparticles were also fundamental to the convective heat transfer of magnetite nanofluids. Moreover, a rise in the heat transfer coefficient was observed when the temperature of the nanofluids being high, so compared with effect of the chain-like structure the nanoparticle's Brownian motion had a major effect on the nanofluid heat transfer performance under a weak magnetic field (⩽200G).