Biomedical Engineering Reference
In-Depth Information
size. The general trend is a decrease in thermal conductivity ratio with an
increase in particle size (Chopkar et al., 2006; He et al, 2007; Kim et al, 2007).
But contradicting the trend, Shima et al. (2009) and Beck et al. (2009) report
that for magnetite-water nanofluids, the thermal conductivity ratio increases
considerably with an increase in particle size. In most of these cases, particle
agglomeration being a key factor that cannot be determined from experiment
or theory may contribute largely to the ambiguity of results produced. The
accuracy of the particle size reported is also questionable as in many cases the
researchers report the data available from the manufacturers.
11.3.4 Effect of particle shape
A few articles concerning the effect of particle shape on the thermal
conductivity ratio of nanofluids have been published. The thermal
conductivity ratio for different particle shapes as a function of volume
concentration is shown in Fig. 11.5. From all of the studies in Fig. 11.5 (a)-
(c) it is quite clear that cylindrical particles show the maximum thermal
conductivity ratio enhancement irrespective of the kind of particle
dispersion. It may be assumed that cylinders form a mesh of elongated
particles that conducts heat through the base fluid. This fact is further
validated by the study by Jiang et al. (2009) which shows that by increasing
the aspect ratio of the cylinders, even for the smallest diameter, the thermal
conductivity enhancement is the highest. However, cylindrical particles
often tend to form agglomerates, are difficult to produce, and are liable to
disintegrate into smaller parts during ultrasonic vibration. Hence nanofluids
with spherical nanoparticle dispersion are the easiest to prepare and most
widely exploited and reported.
11.3.5 Effect of type of base fluid
Studies involving the use of several base fluid media, namely water, ethylene
glycol, pump oil, ethanol, refrigerant, and toluene (see Fig. 11.1) for the
preparation of nanofluids have been reported. Among these fluids, water
and ethylene glycol are the most extensively used. The influence of base fluid
medium on the thermal conductivity ratio of nanofluids can be seen in Fig.
11.4(a). It may be pointed out that, in general, the ethylene glycol based
nanofluids show higher thermal conductivity ratio compared to water based
nanofluids, all other parameters being kept constant. Though water has the
highest thermal conductivity among fluids, nanofluids with other base fluid
media show a higher thermal conductivity ratio. This result is encouraging
because heat transfer enhancement is often most desired when fluids with
poorer heat transfer properties are utilized. Ethylene glycol alone is a
relatively poor heat transfer fluid compared to water, and mixtures of
