Characterization and Convective Heat Transfer With Nanofluids

Abstract

Heat transfer and flow dynamics of nanofluids are investigated in developing laminar pipe flows. Characterization of nanofluids is examined by measuring resultant effective particle size, thermal conductivity and viscosity for various values of particle concentrations and temperatures. Nanofluids considered in this study are diamond-graphene (ND-50) nanoparticle in silicone oil (Syltherm 800), and Al2O3 nanoparticles in DI water with and without dispersers/stabilizers. The particle size of various nanofluids is determined quantitatively from measurements using Dynamic Light Scattering device (DLS) and also determined qualitatively from SEM images. Thermal conductivity measurements are conducted by using nano-flash LFA447 device for particle volume fractions ranging from 0.8% to 5.1%. Measured values of thermal conductivity of all fluids at low concentrations agree well with the results predicted by Maxwell model. Viscosity measurements are conducted using parallel plate geometry Rheometrics viscometer at different concentration and temperature as a function of shear rate. At low shear rates the fluid behaves as a Newtonian fluid while it becomes a shear thinning fluid at higher particle concentration of the same nanofluid. There is a significant increase in the viscosity at even low concentrations. Viscosity of nanofluids is also a strong function of temperature at all values of concentration considered in this study. The significant increase in viscosity may diminish nanofluids’ application as an advanced heat transfer fluid. The effects of nanofluid on the drag reduction and heat transfer enhancement are determined and compared with the pressure drop and heat transfer coefficient measurements with the base fluids at the same flow conditions. Our experimental measurements indicate that the pumping power to flow nanofluids is nearly the same as the pumping power required to flow the same amount of base fluid although the viscosity of nanofluids are significantly higher. Convective heat transfer enhancement with the nanofluids is limited to 5% or slightly higher as has also been reported by other workers. Hence addition of nanoparticles into heat transfer fluids could have the potential for heat transfer enhancement in pipe flow without paying the penalty of increasing pumping power.