Heat transfer of nanofluids in turbulent pipe flow :: Technology, Nanofluids

Heat change over of nanoparticle suspensions in turbulent pipe flow is studied theoretically.The main whim upon which this work is establish is that nano fluids behave more like singlephasefluids than like conventional solidliquid mixtures. This assumption implies thatall the convective heat transfer correlations available in the literature for single-phaseflows can be extended to nanoparticle suspensions, provided that the thermophysicalproperties appearing in them are the nanofluid effective properties calculated at thereference temperature. In this regard, two empirical equations, based on a wide varietyof experimental data reported in the literature, are used for the evaluation of thenanofluid effective thermal conductivity and driving viscosity. Conversely, the othereffective properties are computed by the traditional mixing theory. The novelty of thepresent study is that the merits of nanofluids with respect to the corresponding baseliquid are evaluated in terms of global e nergetic performance, and not simply by thecommon point of view of the heat transfer enhancement. Both cases of constant quantitypumping power and constant heat transfer rate are investigated for different operatingconditions, nanoparticle diameters, and solidliquid combinations. The fundamentalresult obtained is the existence of an optimal particle loading for all maximum heattransfer at constant driving power or minimum cost of operation at constant heattransfer rate. In particular, for any assigned combination of solid and liquid phases, it isfound that the optimal concentration of suspended nanoparticles increases as thenanofluid bulk temperature is increased, the Reynolds number of the base fluid isincreased, and the length-to-diameter ratio of the pipe is decreased, while it ispractically independent of the nanoparticle diameter.The usual design requirements for modern heat transfer equipment are reduced size andhigh thermal performance. In this connection, in the past decade s a considerableresearch effort has been dedicated to the development of advanced methods for heattransfer enhancement, such as those relying on new geometries and configurations, andthose based on the use of extended surfaces and/or turbulators. On the other hand,according to a number of studies executed in recent times, a further importantcontribution may derive by the replacement of traditional heat transfer fluids, such aswater, ethylene glycol and mineral oils, with nanofluids, i.e., colloidal suspensions ofnano-sized solid particles, whose effective thermal conductivity has been demonstratedto be higher than that of the corresponding pure base liquid.The main results of prior work on pipe flow, that is undoubtedly one of the mostinvestigated topics in the field of convection in nanofluids, clearly show thatnanoparticle suspensions offer better thermal performance than the base liquids at sameReynolds number, and that heat transfer increases with increasing the nanoparticle