29-11-2012, 04:51 PM
Effective thermal conductivity and thermal diffusivity of nanofluids containing spherical and cylindrical nanoparticles
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Abstract
The effective thermal conductivity and thermal diffusivity of Au/toluene, Al2O3/water, TiO2/water, CuO/water and CNT/water
nanofluids have been measured by using the transient short-hot-wire technique. The average diameters of Au, Al2O3, TiO2 and CuO
spherical particles are 1.65, 20, 40 and 33 nm, respectively. The average length and diameter of CNFs are 10 lm and 150 nm, respectively.
The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity.
The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancements and can
be predicted accurately by the model equation of Hamilton and Crosser for the spherical nanoparticles, and by the unit-cell model equation
of Yamada and Ota for carbon nanofibers.
Introduction
The thermophysical properties of fluids containing
various solid particles have been investigated for several
decades [1–5], and excellent predicting formulae [1,3] on
the effective thermal conductivity of dispersed materials
have been proposed theoretically and experimentally. Since
a large enhanced thermal conductivity for a dispersion of
metallic or non-metallic nanoparticles or nanotubes in conventional
fluids was reported and the term of nanofluids
was coined by Choi [6] in 1995, many researchers [7–10]
have reported their theoretical, numerical, and experimental
results on the thermophysical properties of nanofluids.
Recently, Kumar et al. [11] reported an enhanced thermal
conductivity of about 20% for a nanofluid of only
0.00013% Au nanoparticles in water. Since such an anomalous
enhancement is expected to have wide applications in
thermal engineering, nanofluids have received considerable
attention in thermal science and engineering. However, it is
very difficult to understand why nanofluids would have
such a high thermal conductivity. Meanwhile, there are
large differences among the thermal conductivities reported
by different researchers. Keblinski et al. [12] further pointed
out that the most exciting experimental results have not
been reproducible. Therefore, it is necessary to reconsider
the reliability of the measurements reported so far.
Experiments
The effective thermal conductivity and thermal diffusivity
of nanofluids are simultaneously measured by the transient
SHW technique. Since the principle and procedures of
the SHW technique have been described in detail previously
[13–15], only a brief description is given here. The
SHW technique was developed from the conventional transient
hot-wire (THW) technique and was based on the
numerical solution of two-dimensional transient heat conduction
for a short wire with the same length-to-diameter
ratio and boundary conditions as those used in the actual
measurements.
Results and discussion
The samples of Au/toluene nanofluids were directly
made by chemical reaction method. The Al2O3/water and
TiO2/water nanofluids were made by Sigma-Aldrich Co.,
where the powders were directly dispersed into the deionized
water with a sonic method. The CuO particles were
made by Sigma-Aldrich Co., but the CuO/water nanofluids
were made by the present authors with the sonic method,
where the surfactants were not used. The CNF/water nanofluids
were also made by the present authors with the sonic
method, where the CNFs were made by Showa Denko Co.
and a small amount of surfactants (SDS 1.5 mass%) were
used to disperse the CNFs into pure water. Generally,
the nanoparticles have an inherent static charge. The absolute
value is very small, but due to the extremely small size
of the nanoparticles, the charge is large enough to keep
them in suspension without settling and aggregation.
Conclusions
The effective thermal conductivity and thermal diffusivity
of Au/toluene, Al2O3/water, TiO2/water, CuO/water
and CNT/water nanofluids have been measured accurately
for various volume fractions and temperatures. The present
results show that the effective thermal conductivity and
thermal diffusivity increase with an increase in the particle
concentration, in the particle thermal conductivity and in
the ratio of the length-to-diameter of CNFs. The effective
thermal conductivities of nanofluids have not shown any
anomalous enhancements. All of the measured values at
lower volume fractions agree well with those predicted
by the H–C equation for the spherical particles, and by
the unit-cell model equation of Yamada and Ota for the
CNFs.