8
D. P. Barai et al.
3 Thermal Conductivity of Nanofluids
Thermal conductivity is the ability of the nanofluids to transfer heat through them.
There have been several studies to identify and study the thermal conductivity of
fluids by incorporating various kinds of nanoparticles in them and to find out the
enhancement in the thermal properties and the mechanism behind it.
3.1 Reason Behind Improved Thermal Performance
of Nanofluids
Nanofluids as discussed earlier are suspensions of nanoparticles in a particular basefluid. We already know that the solids, due to the collision of their vibrating molecules,
propagating phonons and diffused-free electrons transfer heat through them efficiently. This does not seem to happen in liquids and gases, because of their loosely
packed molecules. On the other hand, the molecules of a solid are tightly packed,
which makes it a good conductor of thermal energy. These solids, if incorporated into
liquids affect the thermal properties as explained by Maxwell (1881). The solids act
as “heat boats” that carry the thermal energy through the liquid and also as “stirrers”
that generate convection currents, thus providing chances of collisions of molecules
and augmentation of the thermal conductivity (Yang and Han 2006). Nanofluid can
be called as a pseudo-homogenous suspension of solids into specific liquids, because
of the very small size of the solid particles that are distributed throughout the base
liquid called as the basefluid. They impart homogeneity to the mixture due to their
size, which is between 1 and 100 nm.
There are many reasons which elevate the thermal properties of the basefluid
when nanoparticles are contained in it. One of the reasons is the higher surface area
of the nano-sized particles that is available to gain and distribute heat throughout
their body. Major reasons include the Brownian motion of the nanoparticles in the
fluid and the interfacial layer around the nanoparticles (Jang and Choi 2004). The
Brownian motion is the movement that naturally occurs due to the movement of
molecules of fluid around the particle. It gives rise to micro-mixing of the nanofluid,
thus producing localized convection throughout the fluid. The thermal conductivity
of the nanofluid is looked upon as a combined effect of the static and dynamic
mechanisms which involve the thermal properties and interfacial layer phenomena
and the Brownian motion phenomena, respectively (Sohel Murshed and Nieto de
Castro 2011).
Another phenomenon that contributes to the enhanced thermal properties is the
interfacial layer of liquid around the particle at the solid–liquid interface. The layer
of liquid formed is an ordered molecule layer whose thickness plays a significant role
in transportation of heat from solid surface to the bulk liquid (Yu and Choi 2003).
It has been known that this interfacial layer has higher thermal conductivity than
the bulk basefluid (Kotia et al. 2017). It has been found out by Yu and Choi (2003)
D. P. Barai et al.
3 Thermal Conductivity of Nanofluids
Thermal conductivity is the ability of the nanofluids to transfer heat through them.
There have been several studies to identify and study the thermal conductivity of
fluids by incorporating various kinds of nanoparticles in them and to find out the
enhancement in the thermal properties and the mechanism behind it.
3.1 Reason Behind Improved Thermal Performance
of Nanofluids
Nanofluids as discussed earlier are suspensions of nanoparticles in a particular basefluid. We already know that the solids, due to the collision of their vibrating molecules,
propagating phonons and diffused-free electrons transfer heat through them efficiently. This does not seem to happen in liquids and gases, because of their loosely
packed molecules. On the other hand, the molecules of a solid are tightly packed,
which makes it a good conductor of thermal energy. These solids, if incorporated into
liquids affect the thermal properties as explained by Maxwell (1881). The solids act
as “heat boats” that carry the thermal energy through the liquid and also as “stirrers”
that generate convection currents, thus providing chances of collisions of molecules
and augmentation of the thermal conductivity (Yang and Han 2006). Nanofluid can
be called as a pseudo-homogenous suspension of solids into specific liquids, because
of the very small size of the solid particles that are distributed throughout the base
liquid called as the basefluid. They impart homogeneity to the mixture due to their
size, which is between 1 and 100 nm.
There are many reasons which elevate the thermal properties of the basefluid
when nanoparticles are contained in it. One of the reasons is the higher surface area
of the nano-sized particles that is available to gain and distribute heat throughout
their body. Major reasons include the Brownian motion of the nanoparticles in the
fluid and the interfacial layer around the nanoparticles (Jang and Choi 2004). The
Brownian motion is the movement that naturally occurs due to the movement of
molecules of fluid around the particle. It gives rise to micro-mixing of the nanofluid,
thus producing localized convection throughout the fluid. The thermal conductivity
of the nanofluid is looked upon as a combined effect of the static and dynamic
mechanisms which involve the thermal properties and interfacial layer phenomena
and the Brownian motion phenomena, respectively (Sohel Murshed and Nieto de
Castro 2011).
Another phenomenon that contributes to the enhanced thermal properties is the
interfacial layer of liquid around the particle at the solid–liquid interface. The layer
of liquid formed is an ordered molecule layer whose thickness plays a significant role
in transportation of heat from solid surface to the bulk liquid (Yu and Choi 2003).
It has been known that this interfacial layer has higher thermal conductivity than
the bulk basefluid (Kotia et al. 2017). It has been found out by Yu and Choi (2003)