10
D. P. Barai et al.
operation of the instrument, it gained a lot of attention and utilization by researchers
working in the field of nanofluid (Esfe et al. 2015a; Leong et al. 2018; Zadkhast et al.
2017). The transient hot wire method involves the use of dynamic technique that
measures the rise in temperature in a specific distance from a linear source of heat,
that is, a hot wire immersed in the test material. Thus, if the heat source has constant
heat output along the test material, the thermal conductivity is known directly from
the effect of change in temperature over a period of time. An instrument that uses this
method consists of a heating wire along with a temperature sensor together making
up the probe that electrically insulates the probe from the test material.
3.3 Factors Affecting Thermal Conductivity of Nanofluids
Thermal conductivity of the nanofluid, being a characteristic of thermo-physical
property of its components, is bound to vary with a lot of conditions. It includes nature
of the nanoparticle and the basefluid, composition of the nanoparticle, concentration
of the nanofluid, that is, its volume fraction in the nanofluid, temperature and pH.
Following are the various factors explained along with some findings in the literature.
3.3.1 Nature of Nanoparticle and Basefluid
The nanofluid comprises two basic components, that is, the nanoparticle and basefluid, the properties of which shall definitely affect the properties of the prepared
nanofluid. In fact, the nature of both these components has a big impact on the
behaviour of the nanofluid. First of all, we know that the thermal conductivities of
different nanomaterials vary over a wide range, right from polymeric materials having
very low thermal conductivities to some carbon allotropes having very high thermal
conductivities. Few of the thermal conductivity data found for different nanoparticles dispersed in water have been plotted in Fig. 4 (Ahammed et al. 2016; Minea and
Manca 2017; Sundar et al. 2013).
Nanoparticles of different materials can be of different sizes and shapes. Chopkar
et al. (2008) found that the relative thermal conductivity of the nanofluid increases
nonlinearly with decrease in diameter of the nanoparticles dispersed in it. Similar
outcome was obtained by Esfe et al. (2015a) for metal-based nanofluid and by Teng
et al. (2010) for metal oxide-based nanofluid. For silica-ethanol nanofluid, it was
found by Darvanjooghi and Esfahany (2016) that there are –OH groups on the surface
of silica nanoparticles and that the hydrophilicity of the surface of nanoparticles
and restricted movement of molecules of basefluid at the interface increases the
intramolecular force field, thus enhancing the heat conductance through the interface.
Increase in the nanoparticle size increases the amount of –OH groups on its surface
and ultimately increasing the thermal conductivity of the nanofluid. At nanofluid
concentration of 0.15 vol.%, the value of relative thermal conductivity was found to
D. P. Barai et al.
operation of the instrument, it gained a lot of attention and utilization by researchers
working in the field of nanofluid (Esfe et al. 2015a; Leong et al. 2018; Zadkhast et al.
2017). The transient hot wire method involves the use of dynamic technique that
measures the rise in temperature in a specific distance from a linear source of heat,
that is, a hot wire immersed in the test material. Thus, if the heat source has constant
heat output along the test material, the thermal conductivity is known directly from
the effect of change in temperature over a period of time. An instrument that uses this
method consists of a heating wire along with a temperature sensor together making
up the probe that electrically insulates the probe from the test material.
3.3 Factors Affecting Thermal Conductivity of Nanofluids
Thermal conductivity of the nanofluid, being a characteristic of thermo-physical
property of its components, is bound to vary with a lot of conditions. It includes nature
of the nanoparticle and the basefluid, composition of the nanoparticle, concentration
of the nanofluid, that is, its volume fraction in the nanofluid, temperature and pH.
Following are the various factors explained along with some findings in the literature.
3.3.1 Nature of Nanoparticle and Basefluid
The nanofluid comprises two basic components, that is, the nanoparticle and basefluid, the properties of which shall definitely affect the properties of the prepared
nanofluid. In fact, the nature of both these components has a big impact on the
behaviour of the nanofluid. First of all, we know that the thermal conductivities of
different nanomaterials vary over a wide range, right from polymeric materials having
very low thermal conductivities to some carbon allotropes having very high thermal
conductivities. Few of the thermal conductivity data found for different nanoparticles dispersed in water have been plotted in Fig. 4 (Ahammed et al. 2016; Minea and
Manca 2017; Sundar et al. 2013).
Nanoparticles of different materials can be of different sizes and shapes. Chopkar
et al. (2008) found that the relative thermal conductivity of the nanofluid increases
nonlinearly with decrease in diameter of the nanoparticles dispersed in it. Similar
outcome was obtained by Esfe et al. (2015a) for metal-based nanofluid and by Teng
et al. (2010) for metal oxide-based nanofluid. For silica-ethanol nanofluid, it was
found by Darvanjooghi and Esfahany (2016) that there are –OH groups on the surface
of silica nanoparticles and that the hydrophilicity of the surface of nanoparticles
and restricted movement of molecules of basefluid at the interface increases the
intramolecular force field, thus enhancing the heat conductance through the interface.
Increase in the nanoparticle size increases the amount of –OH groups on its surface
and ultimately increasing the thermal conductivity of the nanofluid. At nanofluid
concentration of 0.15 vol.%, the value of relative thermal conductivity was found to