Biomedical Engineering Reference
In-Depth Information
11.3.1 Effect of particle concentration
As reported in the Appendix (Section 11.7), it is evident that the most
extensive research has concerned the effect of nanoparticle loading
concentration on the thermal conductivity of nanofluids. Different research
groups have reported thermal conductivity ratio data for different
nanofluids measured by different techniques at different temperatures and
other conditions such as particle size, pH level of the solution, and with/
without addition of surfactants. Most of the thermal conductivity
enhancement ratio data for different nanofluids reported by different
research groups as a function of particle concentration have been
graphically represented in Fig. 11.1. The references in the legends of the
graph represent the first few letters of the first author of the article followed
by the last two digits of the year of publication. The same convention has
been followed for other figures in this chapter.
As is evident from Fig. 11.1, the general trend shows a gradual increase in
the thermal conductivity ratio of nanofluids with an increase in nanoparticle
loading concentration. Most of the studies report the investigation of
thermal conductivity of nanofluids for a volume fraction less than 1%
(shown as the magnified view of Fig. 11.1) since loading the nanofluids with
higher concentrations shows an adverse effect on the stability of nanofluids.
Many results show the thermal conductivity of the nanofluid to be almost
2.8 or 3 times that of the base fluid. The thermal conductivity ratio for a
particular particle-base fluid combination has been observed to vary as a
function of concentration for different reports by different research groups.
A comparative study of Al
2
O
3
-water nanofluids (shown in Fig. 11.2)
validates the fact that although some results show some identical trends
(Wen and Ding, 2005; Yoo et al., 2007; Sundar and Sharma, 2008), large
variations can be observed in the results. This variation might be attributed
to the different conditions and measurement techniques and differences in
particle size, purity, surrounding conditions, and nanoparticle synthesis
process.
11.3.2 Effect of nature of dispersed particles
The effect of different nature/type of particles on the thermal conductivity of
nanofluids is shown in Fig. 11.3. It can be observed that the metal particles
(Au and Cu) dispersed nanofluids show a higher thermal conductivity ratio
than the metal oxide particles (Al
2
O
3
, TiO
2
, SiC, CuO) dispersed nanofluids.
This is an expected result since metals possess a higher thermal conductivity
than metal oxides. It can also be seen that the thermal conductivity ratio
achieved by the metal oxide dispersed nanofluids at a much higher
concentration can be achieved by the metal dispersed nanofluids at a much
