18-01-2013, 12:40 PM
Viscosity and thermal conductivity of copper oxide nanofluid dispersed in ethylene glycol
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Abstract
Nanofluid is a novel heat transfer fluid prepared by dispersing nanometer-sized solid particles in traditional
heat transfer fluid to increase thermal conductivity and heat transfer performance. In this research we have
considered the rheological properties of nanofluids made of CuO particles of 10-30 nm in length and ethylene
glycol in conjunction with the thermal conductivity enhancement. When examined using TEM, individual
CuO particles have the shape of prolate spheroid of the aspect ratio of 3 and most of the particles
are under aggregated states even after sonication for a prolonged period. From the rheological property it
has been found that the volume fraction at the dilute limit is 0.002, which is much smaller than the value
based on the shape and size of individual particles due to aggregation of particles. At the semi-dilute regime,
the zero shear viscosity follows the Doi-Edwards theory on rodlike particles. The thermal conductivity measurement
shows that substantial enhancement in thermal conductivity with respect to particle concentration
is attainable only when particle concentration is below the dilute limit.
Introduction
Nanofluid is a novel heat transfer fluid prepared by dispersing
nanometer-sized solid particles in traditional heat
transfer fluid such as water or ethylene glycol to increase
thermal conductivity and therefore heat transfer performance.
For example, when 0.3 volume percent of copper
nano-particles are dispersed in ethylene glycol, one can
observe about 40% of increase in thermal conductivity
(Eastman and Choi, 1995). Metal oxides such as aluminum
oxide or titanium oxide are also feasible even though the
amount of heat transfer increase is not as large as metal
particles (Masuda et al., 1993). The effectiveness of heat
transfer enhancement is known to be dependent on the
amount of dispersed particles, material type, particle shape,
and so on. It is expected that nanofluid can be utilized in
airplanes, cars, micro machines in MEMS, micro reactors
among others. Before the introduction of nanofluid, it was
expected that heat transfer could be enhanced by dispersing
micron-sized particles. But the fluid with micron-sized particles
caused problems due to sedimentation and clogging
(Xuan and Li, 2000).
Measurement of rheological properties
Rheological properties were measured using an ARESLS
(Rheometrics Inc.) with the Couette fixture and
AR2000 (TA Instrument) with an acrylic parallel plate.
The inner and outer diameters of the Couette fixture are 32
and 34 mm, respectively and the height is 32 mm. The
diameter of the acrylic plate is 60 mm and the lower plate
is the constant temperature peltier with a sufficiently larger
diameter compared with the acrylic plate. All the measurements
were done at 298 K. To measure the zero shear
viscosity, a creep test was used with the AR2000 (A stress
rheometer). Steady rate-sweep tests were performed
between 0.01 and 100 s−1 in shear rate.
Thermal conductivity of CuO-Ethylene Glycol
nanofluid
The thermal conductivity of nanofluid was measured by
the hot-wire method. To exclude the effect of natural convection,
data were collected only from 100-300 ms. As
expected the thermal conductivity of nanofluid increases as
concentration of particles increases as shown in Fig. 9.
Even at a very low concentration of 0.001, about 2.6%
increase is observed which is very high compared to the
volume fraction of particles. Therefore the effectiveness of
adding particles is sufficiently large when the volume fraction
is less than 0.001. However, if volume fraction is
larger than 0.002, the increase is not conspicuous as in the
case of low volume fraction.
Concluding remarks
In this research we have considered the rheological
properties of CuO-based nanofluid in conjunction with
the thermal conductivity enhancement. Individual CuO
particles have the shape of prolate spheroids and most of
the particles are under aggregated states. From the rheological
property it has been found that the volume fraction
at the dilute limit is 0.002, which is much smaller
than the value based on the shape of individual particles
due to the aggregation of particles. The result shows that
substantial enhancement in thermal conductivity is
attainable only when particle concentration is below the
dilute limit.