Single-phase flow and heat transfer characteristics of ethanol/polyalphaolefin nanoemulsion fluids in circular minichannels

This work experimentally studied the single-phase flow and heat transfer characteristics of a novel nanostructured heat transfer fluid: Ethanol/Polyalphaolefin nanoemulsion fluid flowing through twelve circular minichannels of 1 mm diameter each. In this study, ethanol and polyalphaolefin (PAO) fluids were used to prepare Ethanol/PAO nanoemulsion fluid: it is a thermodynamically stable system formed by dispersing ethanol into a mixture of PAO and surfactants, in which ethanol forms self-assembled micelles of tens of nanometers in diameter. The formed Ethanol/PAO nanoemulsion fluids were used as the working fluids to study the effect of ethanol nanodroplets on its single-phase flow and heat transfer characteristics. In addition, the effect of flow regime on its heat transfer characteristics was examined as well. In this experiment, the Reynolds number was varied between 140 and 3400 to cover the range of flow regime from laminar to transitional. The friction factor and convective heat transfer coefficients were measured for nanoemulsion fluids of two different mass concentrations, and then compared with those of the base fluid. It is found that using Ethanol/PAO nanoemulsion fluids can improve single-phase convective heat transfer compared to that of pure PAO: under laminar flow, there is no significant difference in Nusselt number between Ethanol/PAO nanoemulsion fluids and pure PAO; however, Ethanol/PAO nanoemulsion fluids showed a substantial increase in Nusselt number when entering into transitional flow. Meanwhile, there is an increase in pressure drop and early onset of transition to turbulence for Ethanol/PAO nanoemulsion fluids compared to pure PAO. The results from this work suggest that as a single-phase heat transfer fluid, nanoemulsion fluids should be used in either transitional or fully developed turbulent flow in order to yield enhanced heat transfer performance. While the mechanisms behind are not clear yet, the stronger interaction and interfacial thermal transport between nanodroplets and base fluid at transitional flow regime is believed to be the contributing factor. (C) 2017 Elsevier Ltd. All rights reserved.