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

Heat transfer of nanoparticle suspensions in turbulent pipe flow is studied theoretically. The main idea upon which this work is based is that nanofluids behave more like singlephase fluids than like conventional solidï€ ­liquid mixtures. This assumption implies that all the convective heat transfer correlations available in the literature for single-phase flows can be extended to nanoparticle suspensions, provided that the thermophysical properties appearing in them are the nanofluid effective properties calculated at the reference temperature. In this regard, two empirical equations, based on a wide variety of experimental data reported in the literature, are used for the evaluation of the nanofluid effective thermal conductivity and dynamic viscosity. Conversely, the other effective properties are computed by the traditional mixing theory. The novelty of the present study is that the merits of nanofluids with respect to the corresponding base liquid are evaluated in terms of global energetic performance, and not simply by the common point of view of the heat transfer enhancement. Both cases of constant pumping power and constant heat transfer rate are investigated for different operating conditions, nanoparticle diameters, and solidï€ ­liquid combinations. The fundamental result obtained is the existence of an optimal particle loading for either maximum heat transfer at constant driving power or minimum cost of operation at constant heat transfer rate. In particular, for any assigned combination of solid and liquid phases, it is found that the optimal concentration of suspended nanoparticles increases as the nanofluid bulk temperature is increased, the Reynolds number of the base fluid is increased, and the length-to-diameter ratio of the pipe is decreased, while it is practically independent of the nanoparticle diameter. The usual design requirements for modern heat transfer equipment are reduced size and high thermal performance. In this connection, in the past decades a considerable research effort has been dedicated to the development of advanced methods for heat transfer enhancement, such as those relying on new geometries and configurations, and those 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 important contribution may derive by the replacement of traditional heat transfer fluids, such as water, ethylene glycol and mineral oils, with nanofluids, i.e., colloidal suspensions of nano-sized solid particles, whose effective thermal conductivity has been demonstrated to 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 most investigated topics in the field of convection in nanofluids, clearly show that nanoparticle suspensions offer better thermal performance than the base liquids at same Reynolds number, and that heat transfer increases with increasing the nanoparticle