  The convective heat transfer and flow behavior of graphene-water nanofluids
are studied experimentally by focusing on transitional flow. Graphene-water
nanofluids with different particle mass fractions (0.025, 0.1 and 0.2%) are
produced following two-step method and using PVP as a surfactant.
Thermo-physical characterization is performed by measuring viscosity and
thermal conductivity of the nanofluids. Convection characteristics are
experimentally studied from laminar to turbulent flow regimes. It is seen that
pressure drop increases dramatically in the transition region, and laminar to
turbulent transition shifts to lower Reynolds numbers with increasing
nanoparticle concentration. The transition initiates at a Reynolds number of
2475 for water, while it initiates at 2315 for the nanofluid with 0.2% particle
mass fraction. Increase in mean heat transfer coefficient and Nusselt numbers
are nearly identical at different Reynolds numbers and axial positions along
the test tube in the laminar flow for nanofluids and water due to dominance of
conduction enhancement mechanisms on the heat transfer increase in laminar
flow. Beyond laminar flow regime, enhancement of Nusselt number is observed
indicating that thermophoresis and Brownian motion are more effective heat
transfer augmentation mechanisms. The maximum heat transfer enhancement is
observed as 36% for a Reynolds number of 3950.
