Experimental and theoretical investigations on thermal conductivity of a ferrofluid under the influence of magnetic fiel
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THE EUROPEAN PHYSICAL JOURNAL E
Regular Article
Experimental and theoretical investigations on thermal conductivity of a ferrofluid under the influence of magnetic field Catalin N. Marin and Iosif Malaescua West University of Timisoara, Faculty of Physics, Bd. V. Parvan nr. 4, 300223 Timisoara, Romania Received 23 April 2020 / Received in final form 20 August 2020 / Accepted 3 September 2020 Published online: 29 September 2020 c EDP Sciences / Societ` a Italiana di Fisica / Springer-Verlag GmbH Germany, part of Springer Nature, 2020 Abstract. The effect of the strength and orientation of magnetic field with respect to the temperature gradient on the effective thermal conductivity λef f (H), in a kerosene-based ferrofluid with magnetite particles is reported. A new theoretical model to explain the experimental dependence λef f (H), obtained for both the parallel and perpendicular orientation of the magnetic field, relative to the temperature gradient is proposed, based on the Sillars equation (which is applied for the first time to a ferrofluid in this purpose). For computing λtheor , we have considered that the particle agglomerations, arranged in field-induced microstructures, have ellipsoid forms and the ratio a/b between the major axis and the minor axis of the ellipsoid increases with increasing the magnetic field strength. Using the proposed theoretical model, we established for the first time a semi-empirical relationship between the ratio, a/b and the magnetic field, H, both for parallel and perpendicular H relative to the temperature gradient, determining then the dependence on H of λtheor . The theoretical results are in agreement with the experimental measurements. The reported results are of great practical importance and show that ferrofluids may be useful for incorporation in magnetic tuneable heat transfer devices or for other potential thermal applications.
1 Introduction The solid-liquid biphasic systems have been used to enhance the transfer of heat through fluids. In order to prevent sedimentation of the solid particles, they were coated with a surfactant and their size was reduced to the nanometer order. Such biphasic fluids are called nanofluids [1], having higher heat transfer performance compared to the usual fluids [2] and which can be used in industrial applications based on the heat transfer [3]. A special category of nanofluids is that of magnetic fluids, or ferrofluids, which are ultra-stable colloidal systems consisting of magnetic nanoparticles dispersed in a carrier liquid and stabilized with a suitable surfactant, the diameter of particles ranging from approximately 2 to 15 nm [4]. Unlike non-magnetic nanofluids, some of the ferrofluids have the property that in the presence of a static magnetic field, the magnetic nanoparticles form agglomerates in the shape of elongated droplets, along the direction of the magnetic field [5]. The length of the elongated droplets can reach the value of hundreds of micrometers and these so-called droplets vanish after the magnetic field removal [5]. Recent studies on the fo
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