Heat transfer behaviours of nanofluids in a uniformly heated tube

Abstract

In the present work, we consider the problem of the forced convection flow of water– γAl₂O₃ and ethylene glycol– γAl₂O₃ nanofluids inside a uniformly heated tube that is submitted to a constant and uniform heat flux at the wall. In general, it is observed that the inclusion of nanoparticles has increased considerably the heat transfer at the tube wall for both the laminar and turbulent regimes. Such improvement of heat transfer becomes more pronounced with the increase of the particle concentration. On the other hand, the presence of particles has produced adverse effects on the wall friction that also increases with the particle volume concentration. Results have also shown that the ethylene glycol– γAl₂O₃ mixture gives a far better heat transfer enhancement than the water– γAl₂O₃ mixture.

Introduction

The thermal properties of heating or cooling fluids play a vital role in the development of new energy-efficient heat transfer equipment. However, conventional heat transfer fluids such as water, ethylene glycol and engine oils have, in general, poor heat transfer properties compared to those of most solids. In spite of considerable research and efforts deployed until today, major improvements in heat transfer capabilities have suffered a major lacking. As a result, an important need still exists to develop new strategies in order to improve the effective heat transfer behaviours of conventional heat transfer fluids.

The term ‘nanofluid’ refers to a two-phase mixture with its continuous phase being generally a liquid and the dispersed phase constituted of ‘nanoparticles’ i.e. extremely fine metallic particles of size below 100 nm. It has been shown that the thermal properties of such a nanofluid appear to be well above those of the base-fluid. In fact, some available experimental data [1], [2], [3] have shown that even with a relatively low concentration of particles, say from 1 to 5% in volume, the effective thermal conductivity of the mixture has increased by almost 20% compared to that of the base-fluid. Such an increase depends mainly on several factors such as the form and size of the particles and their concentration, the thermal properties of the base-fluid as well as those of the particles. Hence, the nanofluids can constitute an interesting alternative for advanced applications in heat transfer in the future, especially those in micro scale, see for example [8]. In spite of their potentials and advantages, these very special fluids are still in their earlier development. In fact, the first experimental works were concerned only with the measuring and the determination of the effective thermal conductivity [1], [2], [3], [4], [5], [6], [7], [9]; some also provided the effective viscosity of the nanofluids [1], [4], [9] showing that the inclusion of the nanoparticles can appreciably increase the viscosity of the resulting mixture. These works considered some current fluids, water, ethylene glycol and engine oil, and metallic particles such as γAl₂O₃, SiO₂, TiO₂ and Cu particles. It is very important to note that the amount of experimental data resulting from these studies remains, surprisingly, very limited. Furthermore, there are no available data regarding the effects of the temperature on the effective thermal conductivity and viscosity of nanofluids. It appears obvious, at the present stage, that much more works will be needed on this important issue in the future.

With regard to the thermal performance of nanofluids in confined flows, the only recent experimental works [9], [10] have provided the first empirical correlation for computing the Nusselt number in laminar and turbulent tube flows using nanofluids that are composed of water and Cu, TiO₂ and γAl₂O₃ particles. The data as obtained from these studies have clearly shown that the suspended nanoparticles remarkably increase the heat transfer performance of the base-fluid and the suspensions have larger heat transfer coefficients than the base-fluid (pure water) under the same Reynolds number. Such an improvement becomes more important with an augmentation of the particle volume fraction. In spite of their great potentials and advantages, these new, special yet rather challenging fluids still remain in their earlier development state and much more research work will be needed in order to better understand their fluid dynamics and thermal characteristics, in particular for confined flows. In this paper, we are interested to study numerically the beneficial influence due to the inclusion of nanoparticles on the flow behaviours and the temperature field inside a uniformly heated tube.