12
D. P. Barai et al.
viscosity of the basefluid will determine the rheology of the nanofluid. Even though
nanoparticles tend to alter the rheological behaviour of the nanofluid, the inherent
flow property of the basefluid will rule the major part of the nanofluids’ rheology. The
effect of basefluid on the thermal properties of the nanofluid is well studied by Syam
Sundar et al. (2017). They have prepared graphene oxide/Co 3 O 4 nanocompositebased nanofluid and used water, ethylene glycol and mixtures of both in the ratios of
EG/water as 20:80, 40:60 and 60:40 as basefluids. Although there is enhancement in
thermal conductivity of all the nanofluids, the thermal conductivity of water being
higher than that of ethylene glycol, the thermal conductivity of the nanofluid also
shows the same trend. Also, in the mixtures of ethylene glycol and water, the thermal
conductivity is found to be 0.619, 0.496 and 0.402 for EG/water mixture basefluid
having ratios as 20:80, 40:60 and 60:40, respectively. The thermal conductivity of the
nanofluid is bound by the thermal conductivity of the mixture of the two fluids. This
shows how the basefluid composition alters the thermal conductivity of the nanofluid
to a larger extent.
Agglomeration of nanoparticles has been a common observation as well as a
serious problem in dealing with nanofluids and must be avoided so as to obtain
a stable nanofluid. Nanoparticles do agglomerate and form aggregates that try to
settle down, thus degrading the thermal properties of a nanofluid. The method of
dispersion does affect the agglomerating property of the nanofluid but there is a limit
after which the nanoparticles do not stay uniformly dispersed in the nanofluid. Use
of surfactants or some dispersing agents is the most common method which is used
to decrease agglomeration of nanoparticles in the fluids (Xuan et al. 2013). They are
amphiphilic compounds having a tail and polar head group which are hydrophobic
and hydrophilic, respectively (Schramm et al. 2003). This hydrophobic tail gets
attached to the nanoparticles which are hydrophobic in nature. And the hydrophilic
group interacts with the surrounding fluid. Thus, the wettability of the nanoparticle
is improved by the surfactant. This reduces the surface tension and assists fluid
continuity. On the other hand, it has also been found out by Xuan et al. (2013) that the
use of surfactants in the nanofluids affects the thermal properties of the nanofluid and
deteriorates heat transfer. They studied the effect of sodium dodecyl benzoic sulphate
(SDBS) as surfactant on nanofluid and found out that the heat transfer coefficient
offered by 0.34 vol.% Cu nanofluid decreases from 21,000 to 20,000 W/m
2 K as the
amount of SDBS increases from 0.05 to 0.1 wt% in the nanofluid. Although this is
the case for using surfactants, proper selection of surfactant must be done in order
to control its effect over the crucial properties of a nanofluid. Xia et al. (2014) have
studied the effect of two different surfactants on Al 2 O 3 /water nanofluids. They found
out that polyvinylpyrrolidone (PVP), being a non-ionic surfactant has positive effects
on the thermal conductivity of the nanofluid than sodium dodecyl sulphate (SDS)
which is an anionic surfactant.
D. P. Barai et al.
viscosity of the basefluid will determine the rheology of the nanofluid. Even though
nanoparticles tend to alter the rheological behaviour of the nanofluid, the inherent
flow property of the basefluid will rule the major part of the nanofluids’ rheology. The
effect of basefluid on the thermal properties of the nanofluid is well studied by Syam
Sundar et al. (2017). They have prepared graphene oxide/Co 3 O 4 nanocompositebased nanofluid and used water, ethylene glycol and mixtures of both in the ratios of
EG/water as 20:80, 40:60 and 60:40 as basefluids. Although there is enhancement in
thermal conductivity of all the nanofluids, the thermal conductivity of water being
higher than that of ethylene glycol, the thermal conductivity of the nanofluid also
shows the same trend. Also, in the mixtures of ethylene glycol and water, the thermal
conductivity is found to be 0.619, 0.496 and 0.402 for EG/water mixture basefluid
having ratios as 20:80, 40:60 and 60:40, respectively. The thermal conductivity of the
nanofluid is bound by the thermal conductivity of the mixture of the two fluids. This
shows how the basefluid composition alters the thermal conductivity of the nanofluid
to a larger extent.
Agglomeration of nanoparticles has been a common observation as well as a
serious problem in dealing with nanofluids and must be avoided so as to obtain
a stable nanofluid. Nanoparticles do agglomerate and form aggregates that try to
settle down, thus degrading the thermal properties of a nanofluid. The method of
dispersion does affect the agglomerating property of the nanofluid but there is a limit
after which the nanoparticles do not stay uniformly dispersed in the nanofluid. Use
of surfactants or some dispersing agents is the most common method which is used
to decrease agglomeration of nanoparticles in the fluids (Xuan et al. 2013). They are
amphiphilic compounds having a tail and polar head group which are hydrophobic
and hydrophilic, respectively (Schramm et al. 2003). This hydrophobic tail gets
attached to the nanoparticles which are hydrophobic in nature. And the hydrophilic
group interacts with the surrounding fluid. Thus, the wettability of the nanoparticle
is improved by the surfactant. This reduces the surface tension and assists fluid
continuity. On the other hand, it has also been found out by Xuan et al. (2013) that the
use of surfactants in the nanofluids affects the thermal properties of the nanofluid and
deteriorates heat transfer. They studied the effect of sodium dodecyl benzoic sulphate
(SDBS) as surfactant on nanofluid and found out that the heat transfer coefficient
offered by 0.34 vol.% Cu nanofluid decreases from 21,000 to 20,000 W/m
2 K as the
amount of SDBS increases from 0.05 to 0.1 wt% in the nanofluid. Although this is
the case for using surfactants, proper selection of surfactant must be done in order
to control its effect over the crucial properties of a nanofluid. Xia et al. (2014) have
studied the effect of two different surfactants on Al 2 O 3 /water nanofluids. They found
out that polyvinylpyrrolidone (PVP), being a non-ionic surfactant has positive effects
on the thermal conductivity of the nanofluid than sodium dodecyl sulphate (SDS)
which is an anionic surfactant.