Biomedical Engineering Reference
In-Depth Information
Table 10.1 (cont.)
Average
particle
size
(nm)
Average
viscosity
ratio
(
Shear
rate
(s
1
)
Volume
fraction
(%)
Temperature
(
Authors
Nanofluid
8
C)
η
eff
/
η
f
)
Shima
et al
CuO-EG
10
5-55
—
0.18
1.15-1.23
.
(2010)
0.54
1.23-1.39
0.92
1.42-1.54
1.31
1.52-1.84
summarized in Fig. 10.4. It is observed that the viscosity of the nanofluids is
greater than that of the base fluid, the ratio being greater than 1 in most
cases. In general, most studies reported a gradual decrease in the viscosity of
nanofluids as well as that of base fluid with increasing temperature.
However, the slope of the curve remains almost identical in the concerned
temperature range. In other words, Fig. 10.4 shows that relative viscosity
remains nearly constant as a function of temperature for most of the cases
reported in the literature. The majority of studies were conducted from
ambient to higher temperatures, but Namburu et al. (2007a, 2007b) reported
viscosity values of a nano-SiO
2
or CuO dispersed EG and water mixture
(60:40 by weight) based nanofluid in the temperature range of
35
8
Cto
50
C. In some of the studies, the viscosity of the nanofluid was observed to
increase by more than twice that of the viscosity of base fluid.
8
10.5.3 Effect of particle size
Although very limited data are available, some studies have reported the
effect of particle size on viscosity of nanofluids. The viscosity ratio of several
nanofluids (with respect to the base fluid) at different particle sizes is
summarized from the data available in literature (in Fig. 10.5). From Fig.
10.5 it can be observed that, in general, the viscosity ratio of nanofluids
decreases with an increase in the size of dispersed particles. However, there
are some variations in the literature, where the viscosity ratio has been
reported to increase with an increase in particle size (Nguyen et al., 2007b),
and, in another study, viscosity first increases and then decreases with an
increase in particle size for 3.0 vol% Al
2
O
3
-propylene glycol nanofluid
(Prasher et al., 2006). The mechanism for such contradictory behavior needs
to be analyzed, which requires in-depth theoretical study of the phenom-
enon.
