Synthesis and Characterization of Nanofluids …
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3.3.2 Composition of Nanoparticle
It has been already known that the nanoparticle nature influences the thermal conductivity of the nanofluid. Nanoparticles contained in a nanofluid may also be a composite of two or more nano-sized materials, which is known as nanocomposite. The
components of a nanocomposite may have different properties and thus may impart
their individual properties to the whole nanoparticle. So, there are chances that the
amount of these components will determine the properties variation of a nanocomposite. Trinh et al. (2016) investigated the thermal conductivity of Cu/graphene
nanocomposite-based nanofluid by changing the nanocomposite ratio using ethylene
glycol as basefluid. They synthesized nanofluids containing Cu/graphene nanocomposite having graphene/Cu ratio of 7:1, 5:1, 3:1 and 1:1 by weight and found out that
the thermal conductivity of Cu/graphene-based nanofluid is higher than that compared to graphene-based nanofluid. The thermal conductivity of Cu/graphene-based
nanofluids containing nanoparticles of graphene/Cu ratio as 7:1 and 5:1 is 0.48 and
0.5 W/mK, respectively, at 60 °C. This is because of the combined effect of graphene
sheets and Cu particles decorated over them, both having higher thermal conductivity.
The decoration of graphene sheet with Cu particles decreases stacking of graphene
sheets, thus elevating thermal properties of the nanofluid. Further, the nanofluid containing Cu/graphene nanoparticles having graphene/Cu ratio of 3:1 and 1:1 shows
a decreasing trend of thermal conductivity values, that is, 0.42 and 0.415 W/mK,
respectively. This is reported to be happening due to the formation of clusters of Cu
particles as their amount is greater than that required to get attached to the functional
groups over the graphene sheet. This is how the composition of nanoparticles plays
an important role in altering the thermal conductivity of the nanofluid.
3.3.3 Volume Fraction of Nanoparticles in the Nanofluid
The increase in the thermal conductivity of a basefluid due to addition of nanoparticles
is well known. But increasing the amount of nanoparticles in the basefluid also
affects the thermal conductivity, as shown by many researchers (Alawi et al. 2018;
Gupta et al. 2011; Khedkar et al. 2012; Kumar et al. 2018; Tijani and Sudirman
2018). The values of thermal conductivity for different ranges of volume fractions of
nanoparticles in different nanofluids recorded in the past can be seen in Table 1. The
nanoparticles acting as heat boats carry the heat through the nanofluid. Increasing
the number of these heat boats ultimately leads to the increase in transport of the
heat energy, thus augmenting the thermal conductivity of the nanofluid. This is due
to the intensification of Brownian motion as the number of particles is high. Thermal
conductivity of the nanofluid is reported to be having linear relationship with the
concentration of the nanoparticles in it (Ali et al. 2010). Figure 5 depicts a trend of
thermal conductivity of Al 2 O 3 /ethylene glycol nanofluids as a function of volume
fraction at various temperatures as studied by Esfe et al. (2015a).
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3.3.2 Composition of Nanoparticle
It has been already known that the nanoparticle nature influences the thermal conductivity of the nanofluid. Nanoparticles contained in a nanofluid may also be a composite of two or more nano-sized materials, which is known as nanocomposite. The
components of a nanocomposite may have different properties and thus may impart
their individual properties to the whole nanoparticle. So, there are chances that the
amount of these components will determine the properties variation of a nanocomposite. Trinh et al. (2016) investigated the thermal conductivity of Cu/graphene
nanocomposite-based nanofluid by changing the nanocomposite ratio using ethylene
glycol as basefluid. They synthesized nanofluids containing Cu/graphene nanocomposite having graphene/Cu ratio of 7:1, 5:1, 3:1 and 1:1 by weight and found out that
the thermal conductivity of Cu/graphene-based nanofluid is higher than that compared to graphene-based nanofluid. The thermal conductivity of Cu/graphene-based
nanofluids containing nanoparticles of graphene/Cu ratio as 7:1 and 5:1 is 0.48 and
0.5 W/mK, respectively, at 60 °C. This is because of the combined effect of graphene
sheets and Cu particles decorated over them, both having higher thermal conductivity.
The decoration of graphene sheet with Cu particles decreases stacking of graphene
sheets, thus elevating thermal properties of the nanofluid. Further, the nanofluid containing Cu/graphene nanoparticles having graphene/Cu ratio of 3:1 and 1:1 shows
a decreasing trend of thermal conductivity values, that is, 0.42 and 0.415 W/mK,
respectively. This is reported to be happening due to the formation of clusters of Cu
particles as their amount is greater than that required to get attached to the functional
groups over the graphene sheet. This is how the composition of nanoparticles plays
an important role in altering the thermal conductivity of the nanofluid.
3.3.3 Volume Fraction of Nanoparticles in the Nanofluid
The increase in the thermal conductivity of a basefluid due to addition of nanoparticles
is well known. But increasing the amount of nanoparticles in the basefluid also
affects the thermal conductivity, as shown by many researchers (Alawi et al. 2018;
Gupta et al. 2011; Khedkar et al. 2012; Kumar et al. 2018; Tijani and Sudirman
2018). The values of thermal conductivity for different ranges of volume fractions of
nanoparticles in different nanofluids recorded in the past can be seen in Table 1. The
nanoparticles acting as heat boats carry the heat through the nanofluid. Increasing
the number of these heat boats ultimately leads to the increase in transport of the
heat energy, thus augmenting the thermal conductivity of the nanofluid. This is due
to the intensification of Brownian motion as the number of particles is high. Thermal
conductivity of the nanofluid is reported to be having linear relationship with the
concentration of the nanoparticles in it (Ali et al. 2010). Figure 5 depicts a trend of
thermal conductivity of Al 2 O 3 /ethylene glycol nanofluids as a function of volume
fraction at various temperatures as studied by Esfe et al. (2015a).