Biomedical Engineering Reference
In-Depth Information
nanofluids; most nanofluid studies focus on heat transfer behavior including
thermal conduction, phase change, and convective heat transfer issues. The
relationship between thermal conductivity and viscosity and the role of
viscosity on the thermal behavior of nanofluids warrant extensive studies to
understand the underlying mechanism of heat conduction in nanofluids.
Nanofluid viscosity is a critical parameter that governs the amount of
particles suspended in the fluid. It is suggested that the viscosity of
nanofluids depends on a number of parameters, such as temperature and
concentration and size of nanoparticles.
Efforts have been made to analyze the effect of the above-mentioned
parameters on the viscosity of nanofluids. An overview of the relevant
studies is summarized in Table 10.1 where the variation of relative viscosity
is listed as a function of relevant parameters for several nanofluids. The
effect of these parameters is discussed in the following sections. Table 10.1
indicates that the most common nanomaterial used as a dispersoid in studies
analyzing the viscosity of nanofluids is Al
2
O
3
, although some studies have
utilized TiO
2
and SiO
2
as the dispersed particles.
10.5.1 Effect of particle volume concentration
Similarly to thermal conductivity, viscosity is also strongly dependent on the
concentration of particle dispersion in the base fluid. In the literature (Table
10.1), it has been reported that nanofluid viscosity in most cases is higher than
that of the base fluid. The ratio of viscosity of the nanofluid to that of the base
fluid increases with an increase in particle concentration. The available
literature data (Table 10.1) on the relative viscosity (
η
f
) of nanofluids
(with respect to the base fluid) as a function of particle concentration are
summarized in Fig. 10.3. In the figure (and others in this chapter), the
references in the legend represent the first three letters of the first author
followed by the last two digits of the year of publication. It is evident that
relative viscosity of the nanofluid in most cases gradually increases as a
function of particle concentration, although Williams et al. (2008) report an
exponential increase in the relative viscosity at a very low concentration of
dispersed particles. It is interesting to note that the degree of increase of
viscosity is more than three-fold compared to the viscosity of base fluid,
which cannot be explained by the classical equation of Einstein (1906). There
is thus an urgent need to investigate the underlying mechanisms for the
increase in viscosity of nanofluids with increasing particle concentration.
η
eff
/
10.5.2 Effect of temperature
The variation of viscosity of nanofluids (with respect to the base fluid) as a
function of
temperature as reported in the literature (Table 10.1)
is
