Heat transfer and flow behavior of carbon based nanofluids

Thermal and rheological behaviors of the graphene-water nanofluids are investigated experimentally. Nanofluids with particle mass fraction from 0.025 to 2.0% are prepared by using PVP as surface active material and ultrasonication. Morphological, material and stability characterization are carried out by STEM imaging, Raman spectroscopy, and zeta potential measurements. Rheological behavior of the prepared samples is analyzed at different temperatures and shear rates. Maximum viscosity increase is observed to be 45% for the 2.0% mass concentration at 25oC. The relative viscosity does not change with temperature, unlike the absolute viscosity that decreases with temperature due to the viscosity change of the base fluid. Moreover, the relative viscosity is nearly constant at low concentrations for all shear rates, and it decreases with increasing shear rate for the mass fractions higher than 1.0%. Hence, graphene-water nanofluids exhibit Newtonian behavior for particle mass fractions below 1.0% and shear thinning behavior at higher concentrations. Hysteresis is observed when increasing and reducing the shear rates within the same speed ranges for the samples higher than 1% particle mass fraction and above 40oC. Thermal conductivity is enhanced with increasing concentration and maximum augmentation is observed to be 96% for a mass fraction of 2.0%. Forced convection is investigated for graphene-water nanofluids of 0.025, 0.1 and 0.2% mass fractions at a Reynolds numbers from 1400 to 4000. Transition to turbulence is observed at lower Reynolds numbers for 0.1 and 0.2% concentrations. Maximum heat transfer enhancement is measured as 36% for 0.2% nanofluid for a Reynolds number of 3950. Besides, pressure drop and friction factor measurements are carried out. Maximum pressure loss is observed to increase 30% at transition region.