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

A Einstein

A Hatch

CT Nguyen

DP Kulkarni

DS Wen

HC Brinkman

HQ Xie

HT Zhu

Hua-Qing Xie

J Garg

J Philip

JF Zhao

Ji-Sen Jiang

JS Jiang

LF Chen

NR Karthikeyan

PK Namburu

Q Li

SEB Maiga

Shou-Zhu Guo

SZ Heris

TK Hong

W Yu

Y Yang

Yang Li

Z Bhimani

ZF Gan

Nanoscale Research Letters

English

PubMed

Crossref

Nanofluids Containing γ-Fe2O3 Nanoparticles and Their Heat Transfer Enhancements

Springer - Publisher Connector

NANO EXPRESS
Nanofluids Containing c-Fe2O3 Nanoparticles and Their Heat
Transfer Enhancements
Shou-Zhu Guo • Yang Li • Ji-Sen Jiang •
Hua-Qing Xie
Received: 5 March 2010 / Accepted: 4 May 2010 / Published online: 20 May 2010
 The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Homogeneous and stable magnetic nanofluids
containing c-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 vis-
cosity 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 Rey-
nolds number and the volume fraction.
Keywords c-Fe2O3 nanoparticle  Magnetic nanofluid 
Thermal conductivity  Viscosity  Heat transfer coefficient
Introduction
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 con-
ductivity of the nanofluids can be enhanced obviously
when nanoparticles, such as CNTs [1], Fe [2], Cu [3], and
Al2O3 [4], are dispersed into the base fluids. Viscosity of
the fluids also increases with the augment of the nanopar-
ticles concentrations [5, 6] when nanoparticles are dis-
persed into the base fluids as well. At the same time,
temperature and nanoparticles size [6] 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 con-
ventional 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 nano-
particles, show both magnetic and fluid properties and have
important applications in industrial [10, 11] and biomate-
rial 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 [15]. What’s
more, with the development of the industry and the tech-
nology, the performance elevation of the traditional heat
transfer medium using mixture of water and ethylene gly-
col (EG) is necessary. Kulkarni et al. [16] 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, c-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, vis-
cosity, and convective heat transfer coefficient of the
nanofluids were further investigated.
S.-Z. Guo  J.-S. Jiang (&)
Department of Physics, Center of Functional Nanomaterials
and Devices, East China Normal University, 200241 Shanghai,
China
e-mail: jsjiang@phy.ecnu.edu.cn
Y. Li  H.-Q. Xie (&)
School of Urban Development and Environmental Engineering,
Shanghai Second Polytechnic University, 201209 Shanghai,
China
e-mail: hqxie@eed.sspu.cn
123
Nanoscale Res Lett (2010) 5:1222–1227
DOI 10.1007/s11671-010-9630-1
Experiment Details
Preparation of Nanofluids
Two-step method was used to prepare nanofluids. Com-
mercial spherical-shaped c-Fe2O3 nanoparticles with
diameter of 20 nm were selected as additives, and the
mixture of ethylene glycol and deionized water with vol-
ume ratio of 45:55 was selected as a base fluid. In a typical
procedure, adequate surfactant (sodium oleate) was dis-
solved 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 (u, u = 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 ultraso-
nicated 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 (knf) as a
function of volume fraction of the nanofluids was
measured using a transient short hot-wire method. Eth-
ylene glycol was used to calibrate measurement appara-
tus. 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, g (mPas),
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 60C in 5C increments in chamber, was con-
trolled by water bath.
The convective heat transfer coefficient measurement
setup shown in Fig. 1 is self-established, the convective
heat transfer coefficient, h (W/m2K), was measured as a
function of volume fraction of nanofluids in laminar flow
region. The base fluids or the nanofluids was pumped to
flow along the tube from reservoir containing fluids
and back to the reservoir by a peristaltic pump (MasterFlex
L/S, MODEL 77202-50). At the same time, the temperature
of the heating bath and the cooling bath was controlled at
60C and -15C, respectively, to form a constant wall
temperature boundary condition. Four T-Type thermocou-
ples were used to measure the temperature at the outlet of
the heating bath (Tw), the entrance of the cooling bath (T1),
25 cm behind the entrance of the cooling bath (T2), and
inside the cooling bath, respectively. At last, the convective
Fig. 1 Schematic of
experimental setup
Nanoscale Res Lett (2010) 5:1222–1227 1223
123
heat transfer coefficients can be easily determined
according to the formula,
h ¼  _mCp
2prDL
ln
Tw  T2ð Þ
Tw  T1ð Þ ð1Þ
where _m,Cp, r, and DL are the mass flow rates, the specific
heat capacity, the radius of tube, and the length of tested
region, respectively. Reynolds number, Re, was derived
from
Re ¼ quD
g
ð2Þ
where q,u, D, and g are the density of the fluid, the flow
rate of the fluid, the diameter of the tube, and the viscosity
of the fluids, respectively.
Results and Discussion
The XRD pattern in Fig. 2 shows peaks at 30.272,
35.684, 43.34, 53.852, 57.4, and 63.011, which are
corresponding to the diffraction peaks of c-Fe2O3 (JCPDS
25-1402), indicating that nanoparticles are single phase
with tetragonal structure. Figure 3 shows the TEM micro-
graph of c-Fe2O3 nanoparticles. The average size of
nanoparticles is estimated to be about 20 nm.
Figure 4 shows the particle size distributions in the
magnetic nanofluids with and without surfactant, respec-
tively. From Fig. 4, we can see that the average size is
about 1,200 nm without surfactant (Fig. 4a) and about
150 nm with surfactant (Fig. 4b), respectively. When
sodium oleate, a kind of organic salt, was dissolved into
solution, the ionization of C18H33O2
- and Na? happens.
One end of C18H33O2
- plunges into the solution, and
another end was absorbed on the surface of nanoparticles.
With the addition of dispersant in the base fluids, the sus-
pensions can keep stability for a long time, while sedi-
mentation happened immediately in the suspensions
without surfactant. Sodium oleate is effective for improv-
ing the stability of c-Fe2O3 nanofluids.
20 30 40 50 60 70
♦
♦
♦
♦
♦
♦
2 theta (deg.)
In
te
ns
it
y(
a.
u.
)
JCPDS 25-1402
 γ−Fe
2
O
3
♦
Fig. 2 XRD pattern of c-Fe2O3 nanoparticles
Fig. 3 TEM micrograph of the c-Fe2O3 nanoparticles
(a)
(b)
Fig. 4 Particle size distribution in the nanofluids a without dispersant
b with dispersant
1224 Nanoscale Res Lett (2010) 5:1222–1227
123
Figure 5 represents enhanced ratios (knf - k0)/k0 of the
thermal conductivity of the nanofluids as a function of
volume fraction (u). It can be seen from Fig. 5 that the
enhanced ratios of the thermal conductivity increase with
the volume fraction of the nanoparticles. Unexpectedly, the
conductivity of c-Fe2O3 nanofluids was not so encouraging
as previous investigation results for other kind of metal
oxide nanofluids. For example, Zhu et al. [17] measured the
thermal conductivity of Fe3O4/water nanofluids. They
found that the ratios of the thermal conductivity enhanced
by more than 15.0% even at the volume fraction of 0.005.
Karthikeyan et al. [18] reported that the ratios of the
thermal conductivity of CuO/water nanofluid enhancement
were 31.6% with 1.0% CuO nanoparticles loading. The
species of the dispersant may be the main reason to these
differences. Sodium oleate that contains a long carbon-
chain could improve the stability of c-Fe2O3 nanoparticles
suspending in the solution; however, it may also reduce the
efficiency of heat transfer between the particles.
Viscosity is an important parameter in the pipeline flow.
Figure 6 presents the results of the viscosity of the nano-
fluids with different volume fractions as a function of
temperature. It is shown that the viscosity of the nanofluids
strongly depends on both temperature and volume. We
observed that the viscosity of the nanofluids increases with
the augment of the volume fraction but decreases with an
increase in the temperature. Nguyen et al. [6] measured the
viscosity of Al2O3/water nanofluids, which showed the
same trend as well. Figure 7 depicts the rheological
behaviors of the nanofluids with different particle loadings.
The behaviors of the nanofluids from the Fig. 7 are close to
the typical Newtonian fluids. Yu et al. [19] measured the
viscosity of ZnO/EG nanofluids against shear rate. The
results also showed that the viscosity of nanofluids
increased with the increasing of particle concentrations
and decreased with the augment of temperature. And
they observed that the nanofluid demonstrated Newtonian
behaviors and non-Newtonian behaviors at lower (u\ 0.02)
and higher (u[ 0.03) volume fractions, respectively. Some
theoretical predictions of viscosity (Einstein model [20],
Brinkman model [21]) about the fluid were employed to
compare with experiments of the nanofluids in Fig. 8. It is
found that the experiment data of the nanofluids is much
larger than the theoretical predictions values. The result may
ascribe to the specific surface areas of the nanoparticles. The
enhancement of viscosity may due to the very large surface
area of the nanoparticles in the nanofluids [22]. Furthermore,
the reason of the discrepancy may due to the nanoparticles
size, which has an important effect on viscosity of nanofl-
uids. When the diameter of the nanoparticles is less then
20 nm, the viscosity of nanofluids will increase rapidly as the
diameters decrease [23].
0.000 0.005 0.010 0.015 0.020
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
(k
nf
-k
0
)/
k 0
%
Volume Fraction (ϕ )
Fig. 5 Dependence of the enhanced ratios of the thermal conductiv-
ity on the volume fraction of c-Fe2O3 nanoparticles
10 20 30 40 50 60
1
2
3
4
5
6
7
8
9
V
is
co
si
ty
( 
m
Pa
•s
)
Temperature(
Base fluid
ϕ =0.005
ϕ =0.01
ϕ =0.015
ϕ =0.02
°
Fig. 6 Viscosity as a function of the volume fraction of c-Fe2O3
nanoparticles and temperature
0 20 40 60 80 100 120 140
3.00
3.25
3.50
3.75
V
is
co
si
ty
( 
m
Pa
•s
)
Shear Rate (s-1)
ϕ =0.005
ϕ =0.01
ϕ =0.015
ϕ =0.02
Fig. 7 Viscosity as a function of shear rate
Nanoscale Res Lett (2010) 5:1222–1227 1225
123
The convective heat transfer coefficient of the nanofl-
uids with different volume fractions as a function of Rey-
nolds number (Re) was shown in Fig. 9. It is seen that an
augmentation of volume fraction or Reynolds number can
make the heat transfer coefficient increase. It should be
noted that though the conductivity of c-Fe2O3 nanofluids
was not encouraging in our experiment, the convective heat
transfer coefficient increases with Reynolds number and
volume fraction. The behaviors of convective heat transfer
are similar to the base fluids at the volume fraction of
0.005, which may be due to the adhesion of nanoparticles
on tube wall. Obviously, from the volume fraction of 0.01,
the convective heat transfer coefficient enhanced quickly.
For the nanofluid with a volume fraction of 0.02, the
convective heat transfer coefficient can be enhanced by
more than 60.0% at Reynolds number of 1,000. The vis-
cosity value of the nanofluids in our experiment was nearly
close to the base fluid especially at a higher temperature.
The higher viscosity of nanofluids may suppress flow
turbulence [24]. Furthermore, the nanofluids with homo-
geneous and stable property may be also a critical factor to
this result. Heris et al. [8] reported that the nanoparticles in
a fluid changed the flow structure. Heat transfer enhance-
ment of the nanofluids is not only related to the conduc-
tivity of nanofluids but also related to chaotic movements,
dispersions, and so on.
Conclusions
We presented a technical route for preparing stable
nanofluids composed of c-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 c-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.
Acknowledgments 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).
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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V i s c o s i t y ( m P a • s ) Experiment Data Einstein Model Brinkman Model Volume Fraction (ϕ)

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Nanofluids Containing γ-Fe2O3 Nanoparticles and Their Heat Transfer Enhancements

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