Figure 4. Raman spectra of the graphene that solves the problems
regarding insufficient exfoliation of the graphite powder and
deterioration in the electrical conductivity and thermal conductivity.
The effect of temperature on thermal conductivity of the graphene in
purified water is illustrated in Figure 5 for which the material
consists of stacks of graphene sheets having a platelet shape. The quest
for thermal systems that are more efficient in removing heat is never
ending, with design improvements of heat exchangers being one of the
main areas of focus. Water is the working fluid of choice for years for
several reasons; it is clean, readily available and has fairly good
thermal properties for heat removal. Over a century ago, micro-sized
particles with high thermal conductivity are used as a way to increase
the thermal characteristics of working fluids [57, 58]. However,
micro-sized particles can be abrasive and can precipitate out due to
their higher density. Once the nanoparticles settle, the nanofluid loses
its property enhancement. Nano-sized particles introduced into a base
liquid are also used and called nanofluids. This concept of using
copper, aluminum, or carbon nanoparticles to create colloidal suspension
fluids has been accepted as a new avenue for enhancing the thermal
characteristics [59, 60]. Since heat transfer can be enhanced due to
particle size and dispersion isometry, the key technical challenge in
implementing this technology is to understand the fundamental physics
responsible for enhancing the transport of heat, which can lead to the
knowledge-based development of a stable nanofluid with maximum thermal
conductivity. Due to the necessity of compact thermal management
systems, many researchers have begun to investigate the benefits of the
nanofluids on the heat transfer in the thermal management system [61,
62]. Scientists have reported varying degrees of increase in thermal
performance with the addition of the nanoparticles to the thermal fluid
[63, 64]. The earlier studies are primarily on the enhancements of
the thermal conductivity [65, 66]. Most studies report very good
enhancements of the thermal conductivity even with small volume percent
concentrations [67, 68]. Researchers have investigated the addition
of nanoparticles made of highly conductive materials such as aluminum,
carbon, diamond, and copper with varying positive results [69, 70].
Many prior researchers have focused on the use of copper oxide
nanoparticles to form the nanofluid due to the favorable thermal
properties of copper oxide powders [71, 72]. However, nanofluids
formed with copper oxide suffer from several drawbacks that can impede
its commercial use in a thermal management system [73, 74]. For
example, fluids containing copper oxide nanoparticles have a tendency to
mix with and retain air and oxygen within the fluid, which adversely
affects the thermal properties of the fluid and can create problems in
the thermal system [75, 76]. Additionally, the copper oxide
nanoparticles tend to agglomerate and stick to the container of the
fluid in the thermal system, which can lead to impairment and fouling of
fluid flow in the system [77, 78]. In the present study, the
nanofluid includes graphene nanoparticles suspended in a base liquid at
a nanoparticle concentration in the nanofluid of about 0.008 percent by
volume. Graphene nanoparticles are an excellent method to improve
thermal conductivity in water. Graphene-containing nanofluids provide
several advantages over the conventional fluids, including thermal
conductivities far above those of traditional solid-liquid suspensions,
a nonlinear relationship between thermal conductivity and concentration,
strongly temperature-dependent thermal conductivity, and a significant
increase in critical heat flux. Each of these features is highly
desirable for thermal systems and together makes nanofluids strong
candidates for the next generation of heat transfer fluids. The observed
substantial increases in the thermal conductivities of nanofluids can
have broad industrial applications and can also potentially generate
numerous economic and environmental benefits. Enhancement in the heat
transfer ability could translate into high energy efficiency, better
performance, and low operating costs. The need for maintenance and
repair can also be minimized by developing a nanofluid with a better
wear and load-carrying capacity. Consequently, classical heat
dissipating systems widely used today can become smaller and lighter,
thus resulting in better fuel efficiency, less emission, and a cleaner
environment. Nanoparticles of various materials can be used to make heat
transfer nanofluids, including copper, aluminum, copper oxide, alumina,
titania, and graphene. Of these nanoparticles, graphene shows greatest
promise due to their excellent chemical stability and extraordinary
thermal conductivity.