Figure 6. Thermal conductivity enhancement ratio of the nanofluid with
suspended graphene nanoparticles at different nanoparticle volume
fractions.
The effect of nanoparticle volume fraction on the viscosity enhancement
ratio is illustrated in Figure 7 for the nanofluid with suspended
graphene nanoparticles.
Production of metal containing nanofluids faces
some major challenges, such as stability towards agglomeration and
surface oxidation, availability, cost of materials and manufacturing
issues. Dramatic increases in thermal conductivity of nanofluids are
most likely due to the unique nature of such highly anisotropic graphene
nanomaterials that allows engaging multiple heat transfer mechanisms in
suspensions, arising from the effective medium theory, percolation, and
plasmon resonances. The drawback of carbonaceous nanofluids with high
aspect ratio particles is very high viscosity, up to about 14 percent
higher than viscosity of the base fluid. Such viscosity increases result
in pumping power penalties that are much higher than the benefits in
thermal conductivity of suspensions. Industrial applications for
nanofluid technology are in an embryonic state. However, today, the
nanofluid field has developed to the point where it is appropriate to
look to the next level, namely nanofluids that show substantial heat
transfer enhancement over their base fluids and are candidates for use
in a variety of industrial and commercial systems. For example,
potential use of nanofluids for cooling systems such as power
electronics and also for radiators in vehicles, will require not only
enhanced thermal properties, but also minimal negative mechanical
effects of the nanofluids in a closed system. In this regard, reduced
viscosity of the nanofluid for instance is a contributing factor to
reducing pumping power needed for the circulation of the nanofluid.
Since the cooling efficiency of the heat transfer fluids is the main
consideration in the current nanofluid development, the ratio of heat
transfer coefficients for the suspensions and the base fluid is
estimated for fully developed, laminar and turbulent flow regimes using
conventional fluid dynamic equations. The ratio of heat transfer
coefficients is a convenient measure for comparison of two fluids
flowing in the same geometry and at the same flow rates. In a laminar
flow regime, the heat transfer coefficients are proportional to the
thermal conductivity, but in a turbulent flow regime the heat transfer
coefficients depend on a set of thermo-physical properties. Introduction
of nanoparticles to the fluid changes density, thermal conductivity
viscosity, and specific heat of the fluid. In the case of
hydrodynamically and thermally fully developed laminar flow, the heat
transfer coefficient is proportional to the thermal conductivity, and
within the acceptable range of inlet and outlet temperature difference
is independent of the flow velocity. The comparison of two liquids
flowing in fully developed turbulent flow regime over or through a given
geometry at a fixed velocity reduces to the ratio of changes in the
thermo-physical properties. Suspensions with larger diameter and
thickness of nanoparticles provide the highest increase in thermal
conductivity; however, viscosity increase of up to about 14 percent
makes this fluid impractical for heat transfer applications. The
optimization of viscosity and thermal conductivity increases in
nanofluids is required for development of practical nanofluid with
advanced heat transfer. Segregation of surfactants at
liquid-nanoparticle interface creates additional thermal resistance for
heat flow. Thus, organic surfactants are detrimental for the thermal
conductivity of water-based suspensions. Use on non-surfactant approach
to stabilizing dispersions of nanoparticles involves an additional
surface functionalization step as described hereinafter.