Enhancement of thermal conductivity of ethylene glycol based silver nanofluids
Graphical Abstract
Silver nanofluid have been synthesized using silver nitrate (precursor), ethylene glycol (reducing agent), and poly(acrylamide-co-acrylicacid) (dispersion stabilizer). The silver particles in colloidal phase have been characterized by UV–visible spectroscopy, Zeta potential and transmission electron microscopy (TEM). The stability and thermal conductivity of these nanofluids were determined with a transient hot-wire apparatus, as a lapse of time after preparation.
Research Highlights
► Stability of Nanofluids. ► Enhancement of Thermal conductivity of Nanofluids. ► Mechanism of Nanofluids. ► Thermal Conductivity.
Introduction
Researches in heat transfer have been carried out over the previous several decades, leading to the development of currently used heat transfer enhancement techniques. Conventional heat transfer fluids have inherently poor thermal conductivities. One of the most commonly used techniques to improve the thermal conductivity of fluids is to suspend small solid particles with high thermal conductivity in fluids. The fluid suspensions of solid particles provide useful advantages in industrial fluid system, including heat transfer fluids, magnetic fluids and lubricant fluids [1], [2], [3], [4], [5], [6], [7], [8], [9]. In the past, mili or micro-sized particles in fluids had not only interfered in the flow of fluids, but also increased the risk of clogging, sedimentation and erosion of pipes and channels.
However, recent developments on nano-sized particles in fluids suggest solutions for these problems [10], [11]. The principal idea is to exploit the very high thermal conductivities of solid particles which can be even hundred or thousand folds greater than those of conventional heat transfer fluids. Nanofluids engineered on the atomic or molecular scale to produce either new or enhanced physical properties were not exhibited by conventional micro-sized bulk solids. Therefore, nanofluids not only solve the problems such as sedimentation of large particles, clogging flow channels, cohesion, corrosion, erosion of pipelines and causing pressure drop which happens conventionally in heterogeneous solid/liquid mixture with millimeter or micrometer particles, but also increase the thermal performance of base fluids remarkably [7], [8], [9], [11], [12], [13]. Several other unique and remarkable properties of nanoparticles such as electronic, electric, optical, chemical, and mechanical properties can attributed to the advancement of modern techniques [14], [15]. Typically, metal nanoparticles are used industrially and are applied to many fields. Contrary to the milli- and micro-sized particle slurries explored in the past, nanoparticles are relatively close in size to the molecules of the base fluid, and thus can realize very stable suspension with little gravitational setting over long periods of time.
Nanofluid is a kind of new engineering material consisting of solid nanoparticles with sizes typically of 1–100 nm suspended in base fluids. The term “nanofluid” was first proposed by Choi in 1995 of the Argonne National Laboratory, U.S.A. [16] as a route for surpassing the performance of common heat transfer liquids. Nanofluids prepared by dispersing nanoparticles into conventional heat transfer fluids are proposed as the next generation heat transfer fluids as they offer exciting new possibilities to enhance heat transfer performance compare to pure liquids. Therefore, nanofluids have attracted great interest due to their potential benefits for numerous applications such as microelectronics, energy supply, transportation and HVAC. As a result, nanofluid technology becomes a new challenge for the heat transfer fluid because of their higher thermal conductivity [17] and stability than those of the conventional heat transfer fluid or the suspensions of the micro-sized particles. Keblinski et al. [11] listed four possible explanations for the cause of an anomalous increase of thermal conductivity such as Brownian motion of the nanoparticles, molecular-level layering of the liquid at the liquid/particle interface, the nature of heat transport in the nanoparticles, and the effects of nanoparticles clustering.
Preparation of nanofluids is the key step in the use of nanoparticles to improve the thermal conductivity of fluids. Researchers have used different types of solid nanoparticles like: (1) metallic particles (Cu, Al, Fe, Au, and Ag); (2) nonmetallic particles (Al2O3, CuO, Fe3O4, TiO2, and SiC); (3) carbon nanotube; and (4) nanodroplet with high thermal conductivity as additives of nanofluid. The base fluids commonly used are water, acetone, decene, engine oil and ethylene glycol. There are mainly two techniques used to produce nanofluids; (i) the single-step direct evaporation method represents the direct formation of the nanoparticles inside the base fluids, and (ii) the two-step method represents the formation of nanoparticles and the subsequent dispersion of the nanoparticles in the base fluids. In either case, the preparation of a uniformly dispersed nanofluid is essential for obtaining stable reproduction of physical properties or superior characteristics of the nanofluids. The single-step process was developed by Akoh et al. [18] and is called the VEROS (Vacuum Evaporation onto a Running Oil Substrate) technique. A modified VEROS process was proposed by Wagener et al. [19] a little later. The processes of drying, storage, transportation, and the dispersion of nanoparticles are avoided, so the agglomeration of nanoparticles is minimized and the stability of the fluids is increased. However, only low vapour pressure fluids are compatible with this process. This disadvantage limits the application of this method. Whereas, the two-step process is extensively used in the synthesis of nanofluids considering the available commercial nanopowders supplied by several companies. Eastman et al. [20], Lee et al. [9], and Wang et al. [15] used this method to produce Al2O3 nanofluids. Also, Murshed et al. [21] prepared TiO2 suspension in water using the two-step method. In this process, nanoparticles were first produced and then dispersed with the base fluids. Generally, ultrasonic treatment, control of pH or addition of surface active agents are some techniques which are used to attain stability of the suspension of the nanofluids against sedimentation by suppressing formation of particle cluster. But, the addition of surface active agents can affect the heat transfer performance of the nanofluids, especially at higher temperature. However, these efforts to improve the thermal conductivity have been interfered with the stability of particle size. When the heat transfer particles were dispersed in solvents, nanoparticles tend to aggregate into large clusters [22], [23], [24].
Literature survey reveals that nanofluids dispersing carbon nanotubes [25], [26], copper[27], aluminium [28], tin [29], iron [30]and their oxide, titania [21], and gold [31] nanoparticles have been widely investigated with water, ethylene glycol, toluene, engine oil, poly oil, decene and FC-72 as base fluid and their findings are published in the peer reviewed journals. However, very few reports are available on silver [32], [33] based nanofluids, which has excellent chemical and physical stability, even though it is not widely investigated. Therefore, in the present study, the chemical reduction method, which is simple single-step process involving simultaneous preparation and direct dispersion of nanoparticles in the base fluid, was applied to prepare silver nanofluid [34]. Among different polymers, as a dispersion stabilizer, poly(acrylamide-co-acrylicacid) (PAA-co-AA) was a final selection due to its good affinity with silver particles. Ethylene glycol that has already been reported was used as the reduction agent for silver nitrate as well as solvent [35]. The size distribution of particles and the stability of silver nanofluids were investigated. The silver particles were characterized by energy dispersive X-ray (EDX) and X-ray diffraction (XRD) techniques. UV–vis spectroscopy and Zeta potential were adopted to observe the trend of particle size distribution with the passage of time. Transmission electron microscopy (TEM) was used to observe the morphology of nanoparticles. Thermal conductivities of silver nanofluids were measured by a transient hot-wire method each after 1, 3, 7, 15, and 30 days of preparation.
Section snippets
Chemicals
Silver nitrate (AgNO3, DC Chem, 99.8%) was used as the metal precursor to prepare the silver nanoparticles. Ethylene glycol (Duck San Pure Chem., 99.5%) was used as a solvent as well as a reducing agent. Poly(acrylamide-co-acrylicacid) (PAA-co-AA, Aldrich, 99%) was used as a dispersion stabilizer to prevent the aggregation of silver particles.
Preparation of silver nanofluid
100 ml ethylene glycol was injected into a flask, then the different amounts of PAA-co-AA were added. The melting of PAA-co-AA in ethylene glycol was
Results and discussion
The silver nanofluid prepared by using a dispersion stabilizer (PAA-co-AA) exhibited a stable brown color at room temperature. As the amount of dispersion stabilizer increased, the mixture became light brown. The formation of silver particles can be described as follows:HO − CH2 − CH2 – OHCH3 − CHO + H2O2AgNO3 + 2CH3CHO 2Ag + H3C − CO − CO − CH3 + 2HNO3
The dispersion stabilizer, PAA-co-AA, of the reduced silver in the colloid phase can play a key role in preventing the aggregation of silver particles during the
Conclusion
The stability of the silver nanofluids has been estimated by UV–vis spectroscopy. Stability of silver nanofluid is strongly affected by the characteristics of the suspended particle and base fluid such as the particle morphology, the chemical structure of the particles and base fluid. The result of UV–vis spectroscopy reveals that when the PAA-co-AA/AgNO3 ratio increase, the particle size decreases. The particle size distribution shows better dispersion behavior in the suspension with the
Acknowledgement
The authors like to acknowledge the financial supports provided by a grant (code CE3-101) from Carbon Dioxide Reduction & Sequestration Research Center, one of the 21st Century Frontier Programs funded by the Ministry of Science and Technology of the Korean government.
References (80)
- et al.
Heat transfer enhancement of nanofluids
Int. J. Heat Fluid Flow
(2000) - et al.
Conceptions for heat transfer correlation of nanofluids
Int. J. Heat Fluid Flow
(2000) - et al.
Thermal conductivity and lubrication characteristics of nanofluids
Curr. Appl. Phys.
(2006) - et al.
Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids)
Int. J. Heat Mass Transf.
(2002) - et al.
Preparation of silver nanoparticles in hexagonal phase formed by nonionic Triton X-100 surfactant
Colloids Surf.
(2002) - et al.
Magnetic properties of ferromagnetic ultrafine particles prepared by vacuum evaporation on running oil substrate
J. Cryst. Growth
(1978) - et al.
Enhanced thermal conductivity of TiO2–water based nanofluids
Int. J. Therm. Sci.
(2005) Non-metallic silver clusters in aqueous solution: stabilization and chemical reaction
Chem. Phys. Lett.
(1989)- et al.
Stability and thermal conductivity characteristics of nanofluids
Thermochim. Acta
(2007) - et al.
Enhancement of thermal conductivity with Cu for nanofluids using chemical reduction method
Int. J. Heat Mass Transfer
(2006)et al.Investigation on the thermal transport properties of ethylene glycol-based nanofluids containing copper nanoparticles
Powder Technol.
(2010)
Dispersion behavior and thermal conductivity characteristics of Al2O3–H2O nanofluids
Curr. Appl. Phys.
Development and characterization of Al2Cu and Ag2Al nanoparticle dispersed water and ethylene glycol based nanofluid
Mater. Sci. Eng. B
Stability and thermal conductivity of nanofluids of tin dioxide synthesized via microwave-induced combustion route
Chem. Eng. J.
Enhancement of thermal conductivity of kerosene-based Fe3O4 nanofluids prepared via phase-transfer method
Colloids Surf A Physicochem. Eng. Aspects
Thermal conductivity of nanoparticle suspensions
J. Appl. Phys.
Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: manifestation of anomalous enhancement and chemical effects
Appl. Phys. Lett.
Study of interaction of ethylene glycol/PVP phase on noble metal powders prepared by polyol process
Bull. Mater. Sci.
Thermal transport in nanofluids
Annu. Rev. Mater. Res.
The role of interfacial layers in the enhanced thermal of nanofluids: a renovated Maxwell model
J. Nanopart. Res.
Effect of particle migration on heat transfer in suspensions of nanoparticles flowing through minichannels
Microfluid. Nanofluid.
Particle migration in a flow of nanoparticle suspensions
Powder Technol.
Role of Brownian motion hydrodynamics on nanofluid thermal conductivity
Appl. Phys. Lett.
A new parameter to control heat transport in nanofluids: surface charge state of the particle in suspension
J. Phys. Chem. B
A model of thermal conductivity of nanofluids with interfacial shells
Mater. Chem. Phys.
Effect of interfacial nanolayer on the effective thermal conductivity of nanoparticle–fluid mixture
Int. J. Heat Mass Transfer
Temperature-dependent thermal conductivity of nanorodbased nanofluids
Appl. Phys. Lett.
A fractal model for predicting the effective thermal conductivity of liquid with suspension of nanoparticles
Int. J. Heat Mass Transfer
Model for heat conduction in nanofluids
Phys. Rev. Lett.
Model for thermal conductivity of carbon nanotube-based composites
Phys. B Condens. Matter
Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities
J. Appl. Phys.
Biosynthesis of silver nanocrystals by Bacillus licheniformis
Colloids Surf B Biointerfaces
Anomalously increased effective thermal conductivities of ethylene glycol based nanofluids containing copper nanoparticles
Appl. Phys. Lett.
Principles of Enhanced Heat Transfer
Heat transfer characteristics of liquid–solid suspension flow in a horizontal pipe
KSME Int. J.
Investigation on convective heat transfer and flow features of nanofluids
J. Heat Transf. Trans. ASME
Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: manifestation of anomalous enhancement and chemical effects
Appl. Phys. Lett.
Antiwear effect of Fullerene C60 additives to lubricating oils
Russ. J. Appl. Chem.
Thermal conductivity of suspensions containing nanosized SiC particles
Int. J. Thermophys.
Measuring thermal conductivity of fluids containing oxide nanoparticles
Trans. ASME
Temperature dependence of thermal conductivity enhancement for nanofluids
ASME J. Heat Transf.
Thermal conductivity of nanoparticle–fluid mixture
J. Therm. Phys. Heat Transf.
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