Biomedical Engineering Reference
In-Depth Information
sterically bulky, thus keeping the nanoparticles separated from each other
resulting in the formation of stable nanofluids. A detailed summary of
nanofluids prepared by a two-step process with and without the use of
surfactants is given in the Appendix (Section 11.7).
11.2.3 Other processes
While most nanofluid productions to date have used one of the above
(single-step or two-step) techniques, other techniques are also available
depending on the particular combination of nanoparticle material and fluid.
For example, nanoparticles with specific geometries, densities, porosities,
charge, and surface chemistries can be synthesized by templating,
electrolytic metal deposition, layer-by-layer assembly, microdroplet drying,
and other colloid chemistry techniques. Another process is the shape- and
size-controlled synthesis of nanoparticles at room temperature (Cao et al.,
2006). The structural characteristics of nanoparticles such as mean particle
size, particle size distribution, and shape depend on the synthesis method,
which can provide an opportunity to ensure good control over such physical
characteristics. These characteristics for nanoparticles in suspensions cannot
be easily measured. This fact could account for some of the discrepancies in
thermal properties reported in the literature among different experimenters
(Yu et al., 2007).
11.3 The thermal conductivity of nanofluids
Thermal conductivity is the most important intrinsic parameter to
demonstrate the enhancement potential of heat transfer in nanofluids. It
has been shown that the thermal conductivity of a nanofluid is influenced by
the heat transfer properties of the base fluid and identity/composition,
volume fraction, size, shape of the nanoparticles suspended in the liquid
(Xuan and Roetzel, 2000). It is also intuitive to anticipate that spatial and
temporal distribution and uniformity of dispersed nanoparticles should
affect the thermal conductivity. Until now, the development of a
comprehensive theory to predict the thermal conductivity of nanofluids
has remained elusive, although some attempts and propositions have been
made to calculate the apparent conductivity of a two-phase mixture (Xuan
and Li, 2000). Most of the data related to the thermal conductivity of
nanofluids are consolidated in the Appendix. The volume of experimental
research carried out on different particle material and base fluid combina-
tions, the thermal conductivity enhancement of nanofluids for these
particle-fluid combinations, and the techniques utilized for the measure-
ment of thermal conductivity by several researchers are provided in the
Appendix as extensively as possible.
