| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 107 === Viscosity of magnetic nanofluid with Fe3O4 particles was studied experimentally using a modified Brookfield viscometer (based on circular Couette flow), with its cylindrical test section enclosed by an electric coil for generating the magnetic field through the application of an applied current. The test section together with the coil were immersed in a circulating water tank for removing the heat due to Joule heating, such that the test fluid was kept at a constant temperature during the experiment. Both the modified viscometer and the cooling system were developed by the previous investigators, and here we performed detailed tests of stable magnetic nanofluid fluids, for demonstrating that the magnetic nanofluids can potentially be served as a smart fluid.
Fe3O4 particles were synthesized using the classical co-precipitation method, and their sizes were measured as around 9.7 nm using X-ray diffraction. The particles were then coated with oleic acid for providing repulsive steric force against van der Waals force, avoiding the coagulation of particles in liquid. The coated particles are then dispersed into base fluid for the synthesis of magnetic nanofluids. Two base fluids, vacuum pump oil (VPO) and transformer oil (TO), were used. The viscosities of Fe3O4-VPO nanofluid and Fe3O4-TO nanofluid were measured for different temperature (16oC – 30oC), different volume fraction (0% – 4%), different magnetic field strength (0 – 100 Gauss, measured at the top of the test section), and different times after the fluids were synthesized (aging effect). We found: (1) The viscosity of the nanofluid decreases, but the ratio of the viscosity of the nanofluid to that of the base fluid increases slightly, as the temperature increases. (2) The viscosity of the nanofluid increases with the magnetic field strength and volume fraction. (3) Through a repeated periodic excitation of applied magnetic field (field is on for 5 minutes and then off for 5 minutes within a cycle), it was found that the viscosity of magnetic nanofluid could be tuned “instantaneously” as desired by the magnetic field. In particular, viscosity was enhanced when the magnetic field was on, and recovered when the field was off; and the enhancement was proportional to the field strength. (4) The nanofluids in this study are shear thinning nanofluids under the action of magnetic field. (5) The aging effect is minor for Fe3O4-VPO nanofluid, but is substantial for Fe3O4-TO nanofluid. Thus the Fe3O4-VPO nanofluid is more appropriate for application.
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