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Effect of particle size on the viscosity of nanofluids: A review
Halil Dogacan Koca
a
, Serkan Doganay
b
, Alpaslan Turgut
c
, Ismail Hakki Tavman
c
, R. Saidur
d,e
, Islam Mohammed Mahbubul
f
a
Dokuz Eylul University, The Graduate School of Natural and Applied Sciences, Mechanical
Engineering Department, Tinaztepe Campus, Buca 35397, Izmir-Turkey
b
Dokuz Eylul University, The Graduate School of Natural and Applied Sciences, Mechatronics
Engineering Department, Tinaztepe Campus, Buca 35397, Izmir-Turkey
c
Dokuz Eylul University, Engineering Faculty, Mechanical Engineering Department, Tinaztepe
Campus, Buca 35397, Izmir-Turkey
d
Research Centre for Nanomaterials and Energy Technology (NMET), School of Science and
Technology, Sunway University, No. 5, Jalan Universiti, Bandar Sunway, Petaling Jaya, 47500
Selangor Darul Ehsan, Malaysia
e
Department of Engineering, Lancaster University, Lancaster, LA1 4YW, UK
f
Center of Research Excellence in Renewable Energy (CoRERE), Research Institute, King Fahd
University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia
Abstract
Nanofluids are potential new generation heat transfer fluids, which have been
investigated meticulously, in recent years. Thermophysical properties of these fluids
have significant influence on their heat transfer characteristics. Viscosity is one of the
most important thermophysical properties that depends on various parameters. Size
of the particles used in nanofluids is one of these effecting parameters. In this work,
experimental studies considering the particle size effect on the viscosity of the
nanofluid have been reviewed. Firstly, comparison of nanofluid and surfactant type,
production and measurement methods were considered. Viscosity results of selected
studies were evaluated in view of the parameters such as particle size, temperature
and concentration. Furthermore, effective viscosity models of nanofluids, which include
particle size as a parameter were discussed. The results indicate that there is a
discrepancy about the effect of particle size on the viscosity of nanofluids. Moreover,
it is observed from the evaluated data that the relative viscosity variation can be almost
40% either upwards or downwards by only altering the particle size.
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Keywords: Nanofluid; Nanoparticles; Size effect; Viscosity; Surfactant; Temperature.
* Corresponding author: Alpaslan Turgut
E-mail addresses: alpaslan.turgut@deu.edu.tr
Nomenclature
T temperature (
o
C)
d diameter (m)
H inter particle space (m)
C correction factor
V velocity (m/s)
m constant of system properties
h hydrodynamic
r thickness of the capping layer (m)
N Avogadro’s number (6.022x10
23
mol
-1
)
M molecular weight (kg/mol)
Subscripts
nf nanofluid
bf base fluid
p particle
o reference
f fluid
Abbreviations
SDBS sodium dodecylbenzenesulfonate
PG propylene glycol
EG ethylene glycol
3
vol. volumetric
wt. weight
A.T. ambient temperature
Greek letters
viscosity (Pa.s)
φ volume concentration (%)
density (kg/m
3
)
δ distance between the centers of particles (m)
α, ω, γ empirical constants
λ, σ exponents
1. Introduction
Technology driven world enforces the researchers to explore more and more in
thermal engineering. Currently, one of the most crucial pursuits of thermal engineers
is to provide efforts on new types of heat transfer fluids. Thermal engineers found that
the addition of solid particles to a base fluid can provide the fluid a better heat transfer
capability. Based on this concept, a new generation fluid named as “nanofluid” has
occurred in the field for the last two decades. Typically, water, ethylene glycol, oil, etc.
are employed as base fluids, which have naturally poor thermal conductivities.
Supplementation of nano-scaled metals, metal oxides or carbon based materials to
these base fluids brings out the nanofluids. Although the idea was first conceived by
Masuda et al. [1], Choi [2] was the one who had named it as nanofluid. Just after their
inventions, a number of nanofluid related papers have increased expeditiously [3] as
can be seen in Fig. 1.
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Fig. 1. Number of publications containing the term nanofluid in literature.
Recent literature reveals that nanofluid based systems have an extensive
potential area such as, solar collectors [3,4], electronics cooling [6-8], automotive
[9,10], nuclear reactor cooling [11], refrigerators [12-14], heat exchangers [15,16]. The
potential utilization of such a colloidal mixture for many divergent systems exposed the
requisiteness of meticulous investigation on thermal properties of the nanofluids. One
of the pioneering studies on the thermal conductivity by Lee et al. [17] concluded that
the presence of nanoparticles provides substantially higher thermal conductivity than
the same liquid without particle addition. Eastman et al. [18] prepared a nanofluid by
adding copper nanoparticles into the ethylene glycol and they observed a thermal
conductivity increment up to 40%. Xie et al. [19] studied the thermal conductivity of
Al2O3–ethylene glycol nanofluid. The conclusion, which is in accordance with the
former one, included that the thermal conductivity of the suspension was much higher
than the base fluid. Although the goal of adding nano sized particles to a base fluid is
to achieve higher thermal conductivity values, the thermal conductivity is not the sole
property that influences the heat transfer. It is also viscosity that is playing a key role
on characterizing the heat transfer behavior of a nanofluid [20]. Li et al. [21] was one
of the first groups who investigated the transport properties of nanofluids and observed
that the viscosity was not only affected by the volume concentration, but also the size
of nanoparticles.
Ascending of nanofluid researches on convective heat transfer brought out many
viscosity based studies. Pozhar [22] made theoretical and simulation efforts to predict
the nanofluids’ viscosity at the very beginning. Then, Wang et al. [23] proposed a
modified viscosity equation from Einstein’s classical viscosity model for suspensions
by considering spatial distribution of nanoparticle clusters and adsorption liquid
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molecules on nanofluid surfaces. Many experimental studies followed them and it is
exposed that viscosity of nanofluids is fairly dependent on several parameters such as
temperature, volume concentration, aggregation, particle shape, surfactant, particle
size, etc. [24,25]. It is also influenced by the ultrasonication period used during
nanofluid preparation [26]. Turgut et al. [27] made an experimental effort on measuring
the viscosity of TiO2–water nanofluid. They observed a decrement in viscosity values
by increasing temperature. Sundar et al. [28] prepared a review paper on viscosity,
which affirms the results of Turgut et al. [27] in terms of temperature. Bahiraei et al.
[29] and Sundar et al. [28] conducted experiments to interpret the effect of vol.
concentration on viscosity by using Fe3O4–water (0.1–1%) and TiO2–water (0–2%)
nanofluids, respectively. They both concluded that the viscosity was increased by the
increase of vol. concentration. Aggregation also plays an important role on the viscosity
of nanofluids [30]. Gaganpreet et al. [31] focused on fractal aggregates and interfacial
layer around the nanoparticle to determine the rheological behavior of nanofluids. It
was found that the increase in effective radius of the aggregates lead to a substantial
viscosity increment. In addition to the aggregation, viscosity of nanofluids depends on
the particle shape [32]. Timofeeva et al. [32] recommended the use of nanoparticles
with spherical shape for lower viscosity values. In accordance with the
recommendation of Timofeeva et al. [32], the study of Jeong et al. [33] concluded that
the viscosity for the nanofluid with nearly rectangular shape particles were 7.7% higher
than that of the one with spherical shape. Stability is an essential point to be ensured
for a colloidal mixture. Use of a surfactant can be an effective way to achieve a stable
behavior for nanofluids [34]. Li et al. [35] used sodium dodecylbenzenesulfonate
(SDBS) as a surfactant for Cu–water suspension. They found out that viscosity of the
Cu–water nanofluid increased slightly by increasing the mass concentration of SDBS