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.