Viscosity of copper oxide nanoparticles dispersed in ethylene glycol and water mixture

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Abstract

Nanofluids are new kinds of fluids engineered by dispersing nanoparticles in base fluids. This paper presents an experimental investigation of rheological properties of copper oxide nanoparticles suspended in 60:40 (by weight) ethylene glycol and water mixture. Nanofluids of particle volume percentage ranging from 0% to 6.12% were tested. The experiments were carried over temperatures ranging from −35 °C to 50 °C to demonstrate their applicability in cold regions. For the particle volume concentrations tested, nanofluids exhibited Newtonian behavior. An experimental correlation was developed based on the data, which relates viscosity with particle volume percent and the nanofluid temperature.

Introduction

Nanofluids are composites consisting of solid nanoparticles with sizes varying generally from 1 to 100 nm dispersed in heat transfer liquids such as water, ethylene glycol, propylene glycol and so on. In the last decade, nanofluids have gained significant attention due to their enhanced thermal properties. According to Eastman et al [1], when 0.3 vol% of copper nanoparticles are suspended in ethylene glycol, thermal conductivity of the fluid increases by 40%. Pak and Cho [2] report that the convective heat transfer coefficient increases by 75% for an Al2O3 particle concentration of 2.78% at a fixed Reynolds number. Results like these have motivated both the industrial and science communities to explore the heat transfer and rheological properties of nanofluids.

A great deal of energy is expended heating industrial and residential buildings in the cold regions of the world. Due to the severe winter conditions, ethylene glycol or propylene glycol mixed with water in different volume percentages are typically used to lower the aqueous freezing point of the heat transfer medium [3]. Such heat transfer fluids are used in baseboard heaters in homes, heat exchangers, automobiles and in industrial plants in cold regions. These fluids can withstand very low temperatures. At low temperatures, ethylene glycol mixtures have better heat transfer characteristics than propylene glycol mixtures [4]. A 60% ethylene glycol and 40% water by weight fluid mixture is most commonly used in the sub-arctic and arctic regions of Alaska. We have conducted experiments with this fluid mixture by adding copper oxide nanoparticles in order to explore the thermophysical properties of such nanofluids. A thorough understanding of these properties is key to testing for successful application in cold regions. Xuan and Li [5] showed that a nanofluid of low concentration increases the heat transfer coefficient substantially without much penalty in pressure loss. Therefore, copper oxide nanoparticles dispersed in a glycol/water mixture in various volume percentages (0–5% and 6.12%) were tested to investigate the rheological characteristics of these nanofluids over temperatures ranging from −35 °C to 50 °C for their effective usage.

Determining the viscosity of the nanofluid is essential to establishing adequate pumping power as well as the convective heat transfer coefficient, as the Prandtl and Reynolds numbers (functions of viscosity) will be influenced. Until now, only a few studies have addressed the viscous properties of nanoparticles suspensions at cold temperatures. Earlier research at higher temperatures includes investigation of viscosity of carbon nanotubes [6] and graphite nanofluids [7], BaTiO3 suspensions [8], nickel–terpineol suspensions [9], and TiO2 nanoparticles in water [10]. Other investigations have focused on the rheology of aluminum nanoparticle suspensions in paraffin oil [11] and Al2O3 nanoparticles in water [12]. Results for copper oxide in ethylene glycol at room temperature [13] have been presented, but no data is available for subzero temperatures. Several available correlations for nanofluid viscosity are presented below.

Einstein [14] proposed a viscosity correlation for particle suspensions in base fluid when the volume concentration is lower than 5%.μs=μf1+52ϕHere, μs = suspension viscosity, μf = viscosity of base fluid, and ϕ is volume percentage of particles in base fluid.

Bicerano et al. [15] proposed a similar correlation which relates viscosity and volumetric suspensions byμs=μf(1+ηϕ+kHϕ2+)Here η is the virial coefficient and kH is Huggins coefficient.

Brinkman [16] presented a viscosity correlation that extended Einstein’s equation to concentrated suspensions.μs=μf1(1-ϕ)2.5Notice that all three correlations, Eqs. (1), (2), (3) were developed to relate viscosity as a function of volume percentage only; there is no consideration of temperature dependence.

Generally fluids have higher viscosity near their freezing point and fairly low viscosity near their boiling temperature, showing that viscosity is a strong function of the temperature. White [17] presented a correlation for pure fluids between viscosity (μf) and temperature which is given bylnμfμ0a+bT0T+cT0T2Here (μ0, T0) are reference values and (a, b, c) given in the table by White, are dimensionless curve-fit constants. They vary from fluid to fluid; for example for water a = −2.10, b = −4.45 and c = 6.55. Kulkarni et al. [18] proposed a correlation that relates viscosity of copper oxide nanoparticles suspended in water and in the temperature range of 5–50 °C.lnμs=A1T-BHere A and B are the functions of volume percentage ϕ. As this is an aqueous solution, this correlation is not applicable for nanofluids in the subzero temperature range. Our investigation of the relation between temperature and nanofluid viscosity at subzero temperatures will help us develop the next generation of heat transfer fluids applicable in cold regions. A comprehensive review of heat transfer characteristics including viscosity measurements of nanofluids have been recently presented by Wang and Mujumdar [19].

Section snippets

Experimental procedure

In our experiments, we used copper oxide nanoparticles with an average diameter of 29 nm and a particle density of 6.3 gm/cc (Nanopahse, Inc. [20]). Nanofluids with different volume concentrations (1%, 2%, 3%, 4%, 5% and 6.12%) were dispersed in a 60:40 (in weight) ethylene glycol and water mixture. Sample preparation was carried out using a very sensitive mass balance with an accuracy of 0.1 mg. The nanofluid mixture was then stirred and agitated thoroughly for 30 min. with an ultrasonic agitator

Results and discussion

To verify the accuracy of our equipment and experimental procedure, the viscosity of the ethylene glycol and water (60:40% by weight) was measured before the addition of any copper oxide nanoparticles. The obtained readings were compared with data from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) handbook [4], see Fig. 2. The ASHRAE data and the experimental values match nicely (maximum difference of ±2%) with temperatures ranging from −35 °C to 50 °C.

The

Conclusions

  • 1.

    Copper oxide nanofluids exhibit Newtonian behavior in an ethylene glycol and water mixture for concentrations varying from 0% to 6.12% with temperatures ranging from −35 °C to 50 °C.

  • 2.

    The viscosity of nanofluids increases when the volume concentration of nanoparticles increases. For example, the viscosity of 6.12% copper oxide volume concentration is about four times the value of the base fluid at −35 °C.

  • 3.

    As the temperature increases the viscosity of copper oxide nanofluids decreases exponentially.

  • 4.

Acknowledgements

Financial assistance from the Arctic Region Supercomputing Center at the University of Alaska Fairbanks is gratefully acknowledged. Authors are thankful to the Institute of Northern Engineering Petroleum Development Laboratory for providing the experimental facilities to measure viscosity.

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