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The convective heat transfer and flow behavior of graphene-water nanofluidsare studied experimentally by focusing on transitional flow. Graphene-waternanofluids with different particle mass fractions (0.025, 0.1 and 0.2%) areproduced following two-step method and using PVP as a surfactant.Thermo-physical characterization is performed by measuring viscosity andthermal conductivity of the nanofluids. Convection characteristics areexperimentally studied from laminar to turbulent flow regimes. It is seen thatpressure drop increases dramatically in the transition region, and laminar toturbulent transition shifts to lower Reynolds numbers with increasingnanoparticle concentration. The transition initiates at a Reynolds number of2475 for water, while it initiates at 2315 for the nanofluid with 0.2% particlemass fraction. Increase in mean heat transfer coefficient and Nusselt numbersare nearly identical at different Reynolds numbers and axial positions alongthe test tube in the laminar flow for nanofluids and water due to dominance ofconduction enhancement mechanisms on the heat transfer increase in laminarflow. Beyond laminar flow regime, enhancement of Nusselt number is observedindicating that thermophoresis and Brownian motion are more effective heattransfer augmentation mechanisms. The maximum heat transfer enhancement isobserved as 36% for a Reynolds number of 3950.

Convective Heat Transfer and Pressure Drop Characteristics of Graphene-Water Nanofluids in Transitional Flow