Viscosity, thermal and electrical conductivity of silicon dioxide–ethylene glycol transparent nanofluids: An experimental studies
Introduction
In 1995, S. Choi presented a paper in which he demonstrated an increase of the thermal conductivity in the suspensions of nanoparticles, which he described as “nanofluids” [1]. From that moment, the thermal properties of such materials are intensively explored all around the world, leading to the development of numerous industrial applications [2], [3], [4]. Some of the main areas of potential use of the nanofluids are the heat exchange process in the buildings [5], the machinery industry [6] and the nuclear power engineering [7]. Another advantage of the nanofluids is a possibility to use them in the alternative energy sources [8] and solar systems [9], [10], [11], [12], the significance of which can not be overestimated in our era of an increasing need of the, “clean” energy. A separate group of nanofluids are the ferrofluids, which also have a lot of sophisticated applications [13] due to their unique properties.
There are two types of the methods of nanofluid preparations: the one-step and the two-step [14] ones. The one-step methods consist in the preparation of nanoparticles directly in the base fluid, and are used most often in the case of metallic nanoparticles. The two-step methods rely on the preparation of nanoparticles (first step), and then their dispersion in a liquid base (second step). In the case of two-step methods it is necessary to use mechanical methods to break up agglomerates. The mechanical mixing and sonication are the most common used techniques.
The mainstream of research of the characteristics of nanofluid is its thermal conductivity. Numerous studies are conducted in both experimental and theoretical fields on the thermal conductivity enhancement in nanofluids [15]. This leads to formation of new complex theoretical models of this issue [16]. Unfortunately, we still do not have a coherent theoretical model describing the mechanism of thermal conductivity of nanosuspensions. Certain models may describe some of materials correctly but can not be entirely applicable to others.
However, one should keep in mind that the practical application of nanofluids is not be possible without deep knowledge of their mechanical properties and, in particular, the viscosity. This is another area of intensive studies of the nanofluids. As in the case of thermal conductivity, theoretical [17] and experimental [18] studies of the rheological properties of nanosuspensions are conducted now. Great efforts have been made to develop a coherent theoretical model describing the viscosity of nanofluids but at the moment there are only models [19] which can be applied only to some individual cases.
One of the areas of study of the physical properties of nanofluids is their electrical properties, which are equally important but probably underestimated by researchers as compare to their thermal or rheological properties. Among the electric properties, the electrical conductivity is most frequently studied. The electric conductivity of nanofluids tends to increase, like thermal conductivity, with an increasing volume concentration of nanoparticles in the base fluid. However, such studies are rather scarce, and the further work in this area is still needed.
Some studies of the thermophysical properties of nanofluids containing SiO2 nanoparticles have been already performed. For example, Kulkarni et al. [20] described the heat transfer properties of ethylene glycol/water mix based nanofluids. The dependence of viscosity on the particle diameter and an increase of the heat transfer coefficient were reported. Dutta and De [21] presented the results of experimental studies of electrical properties of polypyrrole (PPY–SiO2) nanocomposites at various temperatures and concentrations of the nanoparticles. Konakanchi et al. [22] measured pH of the suspension of SiO2 in the mixture of propylene glycol with water. They observed that the pH of nanofluid increases with an increase of the volume fraction of particles. Talib et al. [23] presented the results of thermophysical measurements of dispersions of SiO2 nanoparticles in ethylene glycol/water mixture, and described the potential use of this materials in the proton exchange membrane fuel cell cooling. Shahrul et al. [24] and Anoop et al. [25] evaluated the possibility of using SiO2–water nanofluids in heat exchange processes and compared the properties of this material with the other water based nanofluids. Sharifpur et al. [26] carried out studies of an effect of the ultrasound energy densities on the electrical conductivity SiO2–EG nanofluids with various volume concentrations in temperature range from 20 to 70 °C. They showed that an increase of the energy density causes the decrease in electrical conductivity of SiO2–EG nanofluids. Haghtalab et al. [27] present the results of experiments on absorption of the carbon dioxide in the water-based nanofluids of spherical SiO2 nanoparticles.
This paper presents the results of experimental studies of viscosity and thermal and electrical conductivity of SiO2–EG.
Section snippets
SiO2 nanoparticles
The nanoparticles used in the studies are commercially available They were produced by PlasmaChem GmbH (Berlin, Germany) with >99.8% purity, catalog number PL-SiOF-25g. The particle size distribution declared by the manufacturer is 7—14 nm, and the specific surface over 200 m2 g−1. The thermal conductivity of SiO2 is 1.38 W m−1 K−1, the electrical conductivity 10−21μS cm−1, and the value of density 2220 kg m−3, as presented in [23]. According to available literature, the specific heat of silicon dioxide
Viscosity
The results of measurement of viscosity as a function of shear rates, are summarized in Table 2.
Fig. 3A presents the flow curves of SiO2–EG nanofluids with various volume fractions of particles at constant temperature 298.15 K. It can be seen that the shear stress increases linearly with the shear rate. The liquids which have such character of the flow are usually called Newtonian fluids. The Newtonian model assumes that the shear stress is proportional to the shear rate, and the proportionality
Conclusions
This paper presents the results of our experimental studies of viscosity, thermal and electrical conductivity of various volume fraction suspensions of silicon dioxide in the ethylene glycol.
It is shown experimentally that SiO2–EG nanofluids exhibit the Newtonian fluid behavior, in which the viscosity increases with the fraction of particles in suspension. It is shown that in the measured volume fraction range the viscosity enhancement is linear with increasing concentration of particles, and
Acknowledgments
The authors wish to thank prof. Vitalii Dugaev (Rzeszow University of Technology) for proofreading the manuscript and dr. Piotr Sagan (University of Rzeszow) for the SEM picture of nanoparticles.
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