Nanofluids Containing γ-Fe2O3 Nanoparticles and Their Heat Transfer Enhancements
© The Author(s) 2010
Received: 5 March 2010
Accepted: 4 May 2010
Published: 20 May 2010
Homogeneous and stable magnetic nanofluids containing γ-Fe2O3 nanoparticles were prepared using a two-step method, and their thermal transport properties were investigated. Thermal conductivities of the nanofluids were measured to be higher than that of base fluid, and the enhanced values increase with the volume fraction of the nanoparticles. Viscosity measurements showed that the nanofluids demonstrated Newtonian behavior and the viscosity of the nanofluids depended strongly on the tested temperatures and the nanoparticles loadings. Convective heat transfer coefficients tested in a laminar flow showed that the coefficients increased with the augment of Reynolds number and the volume fraction.
Nanofluids, which contain nanoparticles dispersed in base fluids, have been proposed as a new kind of heat transfer media because they can improve the heat transport and energy efficiency and may have potential applications in the field of heat transfer enhancement. The thermal conductivity of the nanofluids can be enhanced obviously when nanoparticles, such as CNTs , Fe , Cu , and Al2O3, are dispersed into the base fluids. Viscosity of the fluids also increases with the augment of the nanoparticles concentrations [5, 6] when nanoparticles are dispersed into the base fluids as well. At the same time, temperature and nanoparticles size  may have effects on the viscosity of the nanofluids. According to the previous studies [7–9], nanofluids can improve the convective heat transfer coefficient considerably comparing to the conventional heat transfer fluids and can be used in thermal devices or systems such as heat exchangers or cooling system to enhance heat transfer.
Magnetic fluids, suspension containing magnetic nanoparticles, show both magnetic and fluid properties and have important applications in industrial [10, 11] and biomaterial fields [12–14]. However, seldom experiments and applications on the heat transfer of magnetic fluids have been reported. The conductivity of magnetic nanofluids could be improved through controlling the alignment of nanoparticles by the external magnetic field . What’s more, with the development of the industry and the technology, the performance elevation of the traditional heat transfer medium using mixture of water and ethylene glycol (EG) is necessary. Kulkarni et al.  investigated the thermal properties of aluminum oxide nanofluids based on the mixture of EG and water. And they found that the heat transfer was enhanced efficiently.
In the present paper, γ-Fe2O3 nanoparticles were chosen to form nanofluids with mixture base fluid composed of 55 vol% deionized water (DW) and 45 vol% EG. Thermal transport properties including thermal conductivity, viscosity, and convective heat transfer coefficient of the nanofluids were further investigated.
Preparation of Nanofluids
Two-step method was used to prepare nanofluids. Commercial spherical-shaped γ-Fe2O3 nanoparticles with diameter of 20 nm were selected as additives, and the mixture of ethylene glycol and deionized water with volume ratio of 45:55 was selected as a base fluid. In a typical procedure, adequate surfactant (sodium oleate) was dissolved into the mixture at first, and then the nanoparticles were gradually added into the base mixture fluid with violent stirring. Afterward, the suspensions were stirred using disperse mill (7,200 r/min) for 40 min. Nanofluids with different volume fractions (φ, φ = Vnanoparticles/Vbase fluids) of 0.005, 0.01, 0.015, and 0.02 were obtained by intensive ultrasonication for 45 min.
Measurement of Thermal Properties
The size of nanoparticles was observed by means of transmission electron microscope (TEM) (JEOL, JEM-2100F). The sample for TEM observation was prepared in a typical procedure. First, the nanoparticles were dispersed into the ethanol solution. Then, the mixture was ultrasonicated for 10 min to obtain stabilized suspension. Finally, the upper layer of the suspension was carefully selected to drop on a copper mesh.
The thermal conductivity of the nanofluids (k nf) as a function of volume fraction of the nanofluids was measured using a transient short hot-wire method. Ethylene glycol was used to calibrate measurement apparatus. The thermal conductivity of ethylene glycol was measured threes times under a temperature at an interval of 5 min. The uncertainty of measurements is estimated to be within ±1.0%.
Viscosity of the base fluids or the nanofluids, η (mPa·s), was measured using a rotary viscometer (Brookfield, DV-II + Pro), which was calibrated using the standard fluid at first. The uncertainty of measurements is estimated to be within ±1.5%. The viscometer contains a sample chamber and a spindle. The fluid or nanofluid was put into the chamber, and the temperature of the sample, ranging from 10 to 60°C in 5°C increments in chamber, was controlled by water bath.
Results and Discussion
We presented a technical route for preparing stable nanofluids composed of γ-Fe2O3 nanoparticles and the mixture of deionized water (DW) and ethylene glycol (EG) (DW-EG) as the base fluid. Sodium oleate was used as surfactant, and it was proved to be beneficial to the dispersion of the nanoparticles in the nanofluids. The viscosity of the γ-Fe2O3 nanofluids fits Newtonian behavior and strongly depends on the temperature and the volume fraction. Thermal conductivities of the nanofluids are higher than that of base fluid, and the enhanced values increase with the volume fraction of the nanoparticles. Though the enhanced ratios of thermal conductivity of the nanofluids are not so encouraging compared with other oxides nanofluids, the convective heat transfer coefficient of the nanofluids has substantial enhancement when compared to that of the base fluid. These results indicate that the enhanced thermal conductivity is not the only mechanism responsible for heat transfer enhancement and other factors such as stability of nanofluids, thermal properties, and viscosity of the nanofluids also should be considered.
This work was supported by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the National High Technology Research and Development Program of China (No. 2006AA05Z232), and Shanghai Nanotechnology Promotion Center (0852nm03200).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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