Magnetic nanofluids, also called ferrofluids, are stable colloidal solutions consisting of magnetic nanoparticles dispersed in a basedfluid that can be either a polar or a nonpolar liquid [1]. Enhancement of thermal conductivity of nanofluids can improve the efficiency of heat transfer fluids [2]. In this work, magnetic Fe3O4 nanoparticles were synthesized by co-precipitation technique. X-ray diffraction (XRD), transmission electron microscope (TEM), and Fourier transform infrared (FTIR) techniques were used to characterize the structure, purity, and size of the resultant product. Afterwards, the Fe3O4 nanoparticles were dispersed into deionized water. Tetramethylammonium hydroxide is used to improve the dispersion of nanoparticles in the preparation of nanofluids. The thermal conductivity of Fe3O4 nanofluids was measured using the KD2 Pro Thermal Properties Analyzer (Decagon Devises, USA), which is based on the transient hot wire method. The effect of volume fraction and temperature on the thermal conductivity of nanofluids was also investigated. The results show that the thermal conductivity ratio of nanofluid increases with increasing temperature and volume fraction in the range of measurements. The highest enhancement of thermal conductivity was 11.4% in the nanofluid of 3 vol% of nanoparticles at 30 oC. Experimental results were compared with some known theoretical models.

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