26
D. P. Barai et al.
Different types of nanofluids were prepared by Konakanchi et al. (2011) to study
the effect of different parameters affecting the electrical conductivity of the nanofluids. A decrease in the electrical conductivity has been reported with an increase in
the particle size of Al 2 O 3 nanoparticles. 1% Al 2 O 3 nanofluid showed electrical conductivity of 165, 80 and 15 μS/cm when 10, 20 and 45 nm-sized Al 2 O 3 nanoparticles
were dispersed in propylene glycol and water mixture at 60:40 mass ratio, respectively, at around 40 °C. Further, 1% ZnO nanofluid showed electrical conductivity
of 36 and 24 μS/cm, when 36 and 70 nm-sized Al 2 O 3 nanoparticles were dispersed
in the same basefluid, respectively, at around 40 °C. ZnO nanofluids containing different sizes of ZnO nanoparticles prepared using propylene glycol as basefluid have
been studied by White et al. (2011). Also, a decrease in the electrical conductivity
from 9.6 to 1.2 μS/cm with an increase in the ZnO nanoparticle size from 20 to
60 nm in 7 vol.% concentrated nanofluid has been reported. The effect of increasing
the size of the nanoparticles is opposite to the effect of increasing volume fraction of
the nanoparticles in the nanofluids, leading to a decrease in the electrical conductivity
of the nanofluids.
Azimi and Taheri (2015) investigated the effect of particle size of CuO nanoparticles dispersed in water on the electrical conductivity of the water-based nanofluids.
An optimum particle size of the CuO nanoparticles, which is 95 nm, has been determined that exhibits maximum electrical conductivity of 0.108 μS/cm for 0.18 g/l
concentration of CuO nanofluid at 25 °C. A decrease of diameter below 95 nm or
increase beyond 95 nm of the CuO nanoparticles in the nanofluid leads to a decrease
in the electrical conductivity of their nanofluid.
4.2.3 Temperature
The electrical conductivity of nanofluids relies on the efficiency of electron transfer
through the nanofluids due to the nanoparticles. With an increase in temperature, the
electrons can transfer through the energy barriers very easily as found out by Liu
et al. (2004) or multi-walled carbon nanotube nanofluids. Several researchers have
reported an increase in electrical conductivity with an increase in temperature (Hadadian et al. 2014; Konakanchi et al. 2011; Shen et al. 2012). However, it has also been
known that the influence of temperature on electrical conductivity is lesser than that of
the concentration (Mehrali et al. 2015, Goharshadi and Azizi-Toupkanloo 2013). The
increase in electrical conductivity of nanofluids becomes more pronounced at higher
temperatures (Adio et al. 2015a; Heyhat and Irannezhad 2018). Also, the mechanism
for an enhancement in the electrical conductivity of nanofluids is different from the
mechanism of enhancement of their thermal conductivity (Sarojini et al. 2013). Ganguly et al. (2009) found an increase of electrical conductivity of 0.03 volume fraction
of alumina/water nanofluids from 258 to 351 μS/cm for an increase in temperature
from 24 °C to 45 °C. Baby and Ramaprabhu (2010) also studied the effect of temperature on the graphene-based nanofluids using water as well as ethylene glycol as
basefluids and reported an increase in electrical conductivity if the temperature of
the nanofluid was increased. Konakanchi et al. (2011) also showed almost a linear
D. P. Barai et al.
Different types of nanofluids were prepared by Konakanchi et al. (2011) to study
the effect of different parameters affecting the electrical conductivity of the nanofluids. A decrease in the electrical conductivity has been reported with an increase in
the particle size of Al 2 O 3 nanoparticles. 1% Al 2 O 3 nanofluid showed electrical conductivity of 165, 80 and 15 μS/cm when 10, 20 and 45 nm-sized Al 2 O 3 nanoparticles
were dispersed in propylene glycol and water mixture at 60:40 mass ratio, respectively, at around 40 °C. Further, 1% ZnO nanofluid showed electrical conductivity
of 36 and 24 μS/cm, when 36 and 70 nm-sized Al 2 O 3 nanoparticles were dispersed
in the same basefluid, respectively, at around 40 °C. ZnO nanofluids containing different sizes of ZnO nanoparticles prepared using propylene glycol as basefluid have
been studied by White et al. (2011). Also, a decrease in the electrical conductivity
from 9.6 to 1.2 μS/cm with an increase in the ZnO nanoparticle size from 20 to
60 nm in 7 vol.% concentrated nanofluid has been reported. The effect of increasing
the size of the nanoparticles is opposite to the effect of increasing volume fraction of
the nanoparticles in the nanofluids, leading to a decrease in the electrical conductivity
of the nanofluids.
Azimi and Taheri (2015) investigated the effect of particle size of CuO nanoparticles dispersed in water on the electrical conductivity of the water-based nanofluids.
An optimum particle size of the CuO nanoparticles, which is 95 nm, has been determined that exhibits maximum electrical conductivity of 0.108 μS/cm for 0.18 g/l
concentration of CuO nanofluid at 25 °C. A decrease of diameter below 95 nm or
increase beyond 95 nm of the CuO nanoparticles in the nanofluid leads to a decrease
in the electrical conductivity of their nanofluid.
4.2.3 Temperature
The electrical conductivity of nanofluids relies on the efficiency of electron transfer
through the nanofluids due to the nanoparticles. With an increase in temperature, the
electrons can transfer through the energy barriers very easily as found out by Liu
et al. (2004) or multi-walled carbon nanotube nanofluids. Several researchers have
reported an increase in electrical conductivity with an increase in temperature (Hadadian et al. 2014; Konakanchi et al. 2011; Shen et al. 2012). However, it has also been
known that the influence of temperature on electrical conductivity is lesser than that of
the concentration (Mehrali et al. 2015, Goharshadi and Azizi-Toupkanloo 2013). The
increase in electrical conductivity of nanofluids becomes more pronounced at higher
temperatures (Adio et al. 2015a; Heyhat and Irannezhad 2018). Also, the mechanism
for an enhancement in the electrical conductivity of nanofluids is different from the
mechanism of enhancement of their thermal conductivity (Sarojini et al. 2013). Ganguly et al. (2009) found an increase of electrical conductivity of 0.03 volume fraction
of alumina/water nanofluids from 258 to 351 μS/cm for an increase in temperature
from 24 °C to 45 °C. Baby and Ramaprabhu (2010) also studied the effect of temperature on the graphene-based nanofluids using water as well as ethylene glycol as
basefluids and reported an increase in electrical conductivity if the temperature of
the nanofluid was increased. Konakanchi et al. (2011) also showed almost a linear