Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications. A consensus is now lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. An experimental investigation was conducted to study single-phase forced convection of Al2O3-water nanofluid in a circular minichannel. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared to these of the base fluid. The Reynolds number varied from 600 to 4500, covering the laminar, transition and early fully developed turbulent regions. It was found that the nanofluids exhibit pronounced entrance region behaviors in the laminar region. In the transition and turbulent regions, the onset of transition to turbulence is delayed in nanofluids. Further, both the friction factor and convective heat transfer coefficient are below these of water at the same Re in the transition flow. Once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A scaling analysis showed these behaviors may be attributed to the variation in the relative size of nanoparticle with respect to the turbulent microscales. This work suggests that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, nanofluids should be used in either the laminar flow or fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.

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