Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions

https://doi.org/10.1016/j.ijheatmasstransfer.2004.07.012Get rights and content

Abstract

This paper reports an experimental work on the convective heat transfer of nanofluids, made of γ-Al2O3 nanoparticles and de-ionized water, flowing through a copper tube in the laminar flow regime. The results showed considerable enhancement of convective heat transfer using the nanofluids. The enhancement was particularly significant in the entrance region, and was much higher than that solely due to the enhancement on thermal conduction. It was also shown that the classical Shah equation failed to predict the heat transfer behaviour of nanofluids. Possible reasons for the enhancement were discussed. Migration of nanoparticles, and the resulting disturbance of the boundary layer were proposed to be the main reasons.

Introduction

Heat transfer fluids such as water, minerals oil and ethylene glycol play an important role in many industrial sectors including power generation, chemical production, air-conditioning, transportation and microelectronics. The performance of these conventional heat transfer fluids is often limited by their low thermal conductivities. Driven by industrial needs of process intensification and device miniaturization, development of high performance heat transfer fluids has been a subject of numerous investigations in the past few decades. As solids materials in particular metals can have very high thermal conductivities, lots of studies have been carried out in the past on the thermal behaviour of suspensions of particulate solids in liquids; see for example [1], [2], [3], [4]. These early studies, however, used suspensions of millimeter or micrometer sized particles, which, although showed some enhancement, experienced problems such as poor suspension stability and hence channel clogging, which are particularly serious for systems using mini- and/or micro-channels.

Recent advances in nanotechnology have allowed development of a new category of fluids termed nanofluids. Such fluids are liquid suspensions containing particles that are significantly smaller than 100 nm, and have a bulk solids thermal conductivity of orders of magnitudes higher than the base liquids. The term of nanofluids was first used by the group at the Argonne National Laboratory, USA, about a decade ago [5], but equal credit should also be given to some earlier workers such as Masuda et al. [6]. The potential advantages of properly engineered nanofluids include (a) higher thermal conductivities than that predicted by currently available macroscopic models, (b) excellent stability, and (c) little penalty due to an increase in pressure drop and pipe wall abrasion experienced by suspensions of millimeter or micrometer particles. As a consequence, a number of experimental studies have been performed to investigate the transport properties of nanofluids in the past decade; see for examples [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Most of these studies are on the effective thermal conductivity under macroscopically stationery conditions. There are very few studies on other aspects related to nanofluids such as phase change behaviour [15], [16], [17], [18], [19], [20] and convective heat transfer [21], [22], [23], [24], [25], [26]. Even of these few studies, there are some controversial results. For example, Pak and Cho [22] investigated convective heat transfer in the turbulent flow regime using water–Al2O3 and water–TiO2 nanofluids, and found that the Nusselt number of the nanofluids increased with increasing volume fraction of the suspended nanoparticles, and the Reynolds number. However, for a given average fluid velocity, the convective heat transfer coefficient of a nanofluid with 3 vol.% nanoparticles was 12% lower than that of pure water. This appears to disagree with the observation of Lee and Choi [21] and Xuan and Li [25]. Lee and Choi [21] studied convective heat transfer of laminar flows of an unspecified nanofluid in microchannels, and observed a reduction in thermal resistance by a factor of 2. Nanofluids were also observed to be able to dissipate a heat power three times more than pure water could do. Xuan and Li [25] measured convective heat transfer coefficient of water–Cu nanofluids, and found substantial heat transfer enhancement. For a given Reynolds number, heat transfer coefficient of nanofluids contained ∼2 vol.% Cu nanoparticles was shown to be approximately 60% higher than that of pure water.

This work aims at more fundamental understanding of the convective heat transfer behaviour of nanofluids. The focus will be on the entrance region under the laminar flow conditions, for which no previous studies have been found in the literature. Aqueous based nanofluids containing γ-Al2O3 nanoparticles of various concentrations will be tested under the constant heat flux boundary condition. The results will be compared with conventional theories, and the mechanisms for the enhancement of convective heat transfer will be discussed.

Section snippets

Experimental system

The experimental system constructed for this work is shown schematically in Fig. 1. It consisted of a flow loop, a heating unit, a cooling part, and a measuring and control unit. The flow loop included a pump with a built-in flowmeter, a reservoir, a collection tank and a test section. A straight copper tube with 970 mm length, 4.5 ± 0.02 mm inner diameter, and 6.4 ± 0.05 mm outer diameter was used as the test section. The whole test section was heated by a silicon rubber flexible heater (Watlow, UK)

Experimental results

Having built-up confidence in the experimental system, systematic experiments were performed over a Reynolds number of 500–2100. The results are presented and discussed in this section.

Conclusions

This paper is concerned with the convective heat transfer of nanofluids made of water and γ-Al2O3 nanoparticles. Experiments were carried out in the laminar flow regime. The following conclusions were obtained:

  • The use of Al2O3 nanoparticles as the dispersed phase in water can significantly enhance the convective heat transfer in the laminar flow regime, and the enhancement increases with Reynolds number, as well as particle concentration under the conditions of this work.

  • The enhancement is

Acknowledgments

The work is partly supported by EPSRC under Grants GR/S524985.

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