Computational analysis of nanofluid flow in microchannels with applications to micro-heat sinks and bio-MEMS.

摘要

Nanofluids, i.e., dilute suspensions of nanoparticles in liquids, may exhibit quite different thermal properties than the pure carrier fluids. For example, numerous experiments with nanofluids have shown that the effective thermal conductivities keff for such mixtures are measurably elevated, and hence beneficial applications to (micro-scale) cooling are obvious. A very different application of nanofluids could be in modern medicine, where for example, nanodrugs are mixed in microchannels for controlled delivery with bio-MEMS.;In general, to optimize nanofluid flow in microchannels, best possible conduit geometries, mixing units, and device operational conditions have to be found for specific applications. Specifically, a suitable k eff -- model of common nanofluids, performance as well as cost effective mixers, and entropy minimizing channel designs are the prerequisites for achieving these project objectives.;Two effective thermal conductivity models for nanofluids were compared in detail, where the new KKL (Koo-Kleinstreuer-Li) model, based on Brownian-motion induced micro-mixing, achieved good agreements with the currently available experimental data sets. The thermal performance of nanofluid flow in a trapezoidal microchannel was analyzed using pure water as well as a nanofluid, i.e., CuO-water, with volume fractions of 1% and 4% CuO-particles with dp = 28.6nm. It was found that nanofluids do measurably enhance the thermal performance of microchannel mixture flow with a small increase in pumping power. Specifically, the thermal performance increases with volume fraction; but, the extra pressure drop, or pumping power, will somewhat decrease the beneficial effects. Microchannel heat sinks with nanofluids are expected to be good candidates for the next generation of cooling devices. Microcooling device design aspects in light of minimization of entropy generation were investigated numerically. The influence of the Reynolds number (inlet velocity), fluid inlet temperature, and channel geometry on frictional and heat transfer entropy generation was investigated. It was found that the employment of nanofluids can help achieving entropy minimization due to their high thermal properties. The heat transfer induced entropy generation is dominant for the micro-cooling device. The frictional entropy generation becomes more important for high aspect ratio geometries.;A bio-MEMS application in terms of nanofluid flow in microchannels is presented. Specifically, the transient 3-D problem of controlled nanodrug delivery in a heated microchannel has been numerically solved to gain new physical insight and to determine suitable geometric and operational system parameters. Computer model accuracy was verified via numerical tests and comparisons with benchmark experimental data sets. The overall design goals of near-uniform nanodrug concentration at the microchannel exit plane and desired mixture fluid temperature were achieved with computer experiments considering different microchannel lengths, nanoparticle diameters, channel flow rates, wall heat flux areas, and nanofluid supply rates. Such micro-systems, featuring controlled transport processes for optimal nanodrug delivery, are important in laboratory-testing of predecessors of implantable smart devices as well as in the development of pharmaceuticals and for performing biomedical precision tasks.;As a sample application, the microfluidics of controlled nanodrug delivery to living cells in a representative, partially heated microchannel has been analyzed. The objective was to achieve uniform nanoparticle exit concentrations at a minimum microchannel length with the aid of simple static mixers, e.g., a multi-baffle-slit or perforated injection micro-mixer. A variable wall heat flux, which influences the local nanofluid properties and carrier fluid velocities, was added to ensure that mixture delivery to the living cells occurs at the required (body) temperature of 37°C. The results show that both the baffle-slit micro-mixer and the perforated injection micro-mixer aid in decreasing the microchannel length while achieving uniform nanoparticle exit concentrations. The injection micro-mixer not only decreases best the system's dimension, but also reduces the system power requirement. The baffle-slit micro-mixer also decreases the microchannel length; however, it may add to the power requirement. The imposed wall heat flux aids in enhanced nanoparticle and base-fluid mixing as well.