Single-Phase Thermal Transport of Nanofluids in a Minichannel

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage and electronics cooling to thermal processing of materials. In spite of the extensive studies in the literature, a consensus is lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. In this work, an experimental investigation was conducted to study single-phase forced convection of $Al2O3$-water nanofluid in a circular minichannel with a 1.09 mm inner diameter. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared with those of the base fluid. The Reynolds number (Re) varied from 600 to 4500, covering the laminar, transition, and early fully developed turbulent regions. It was found that in the laminar region, the nanofluids exhibit pronounced entrance region behaviors possibly due to the flattening of the velocity profile caused by the flow-induced particle migration. Three new observations were made for nanofluids in the transition and turbulent regions: (1) The onset of transition to turbulence is delayed; (2) both the friction factor and the convective heat transfer coefficient are below those of water at the same Re in the transition flow; and (3) once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A simple scaling analysis was used to show that these behaviors may be attributed to the variation in the relative size of the nanoparticle with respect to the turbulent microscales at different Re. The results from this work suggest that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, as a heat transfer fluid, nanofluids should be used in either the laminar flow or the fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.