Phase Change Material Particles and Their Application in Heat Transfer Fluids - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 460

0.2

1.2

Experiment
Maxwell Model

0.18

1.15

0.16

1.1

0.14

1.05

0.12

PAO
Nanofluids

0.1 1

20 40 60 80 100

Temperature ( o C)

Fig. 9 Thermal conductivity of the pure PAO and the indium-in-PAO nanofluid (left x-axis) and
relative conductivity of the nanofluid (right x-axis) versus temperature. The relative thermal
conductivity estimated from the Maxwell model (solid line) is also shown for comparison. A
temperature-independent interfacial resistivity R b ¼ 3 : 22 10 ffi 8

m 2 k/W is used in this calcu-

lation [ 5 ]

temperature is experimentally found to be 0.143 W/(mK), which compares well
with the literature values [ 35 ].
It is evident in Fig. 9 that the thermal conductivity enhancement increases
slightly with increasing temperature in the Indium-In-PAO PCM fluids about
10.7 % at 30 C and 12.9 % at 90 C. This weak dependence on temperature was
measured with the 3x-wire technique, which is very different from the very strong
dependence observed in some other nanofluids obtained with the hot-wire tech-
nique. For example, the data of Hong et al. showed a factor of about three increase
in thermal conductivity enhancement for a temperature increase of 50 C[ 36 ]. As
also seen in Fig. 9 , the solid Indium nanoparticles increase thermal conductivity of
the pure PAO by about 12 % for volume fraction of 8 %. The experimental data
are well captured in the temperature range of 30-90 C by the Maxwell Model,

k nf ¼ ð 1 þ 3U a ffi 1

a þ 2 Þ k o

ð 5 Þ

when a constant interfacial resistivity R b ¼ 3 : 22 10 ffi 8 m 2 k/W is used [ 37 ]. Here,

/ is the volume fraction of the particles, a ¼ r p R b k o , r p is the particle radius, and
R b is the thermal resistance per unit area of the particle/fluid interface. The
interface resistance appears to play a significant role in thermal transport in the
metallic PCM fluid. The interface resistance plays a significant role in thermal
transport in the nanofluid, as predicted by the Maxwell Model. For example, for a
negligible interface resistance, R b = 0, the conductivity enhancement would
increase to 24 % in the PAO-based metallic PCM fluids containing 8 vol% of
nanoparticles. These experimental data suggest that the thermal conductivity