An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of multi-wall carbon nanotube-based aqueous nanofluids

doi:10.1016/j.ijheatmasstransfer.2009.04.029

International Journal of Heat and Mass Transfer

Volume 52, Issues 21–22, October 2009, Pages 5090-5101

https://doi.org/10.1016/j.ijheatmasstransfer.2009.04.029 Get rights and content

Abstract

Four samples of 1 wt% multi-walled carbon nanotube-based (MWCNT) aqueous nanofluids prepared via ultrasonication were thermally characterized. Direct imaging was done using a newly developed wet-TEM technique to assess the dispersion state of carbon nanotubes (CNT) in suspension. The effect of dispersing energy (ultrasonication) on viscosity, thermal conductivity, and the laminar convective heat transfer was studied. Results indicate that thermal conductivity and heat transfer enhancement increased until an optimum ultrasonication time was reached, and decreased on further ultrasonication. The suspensions exhibited a shear thinning behavior, which followed the Power Law viscosity model. The maximum enhancements in thermal conductivity and convective heat transfer were found to be 20% and 32%, respectively. The thermal conductivity enhancement increased considerably at temperatures greater than 24 °C. The enhancement in convective heat transfer was found to increase with axial distance. A number of mechanisms related to boundary layer thickness, micro-convective effect, particle rearrangement, possible induced convective effects due to temperature and viscosity variations in the radial direction, and the non-Newtonian nature of the samples are discussed.

Introduction

Managing high thermal loads has become very critical in the rapid developed infrastructure, industrial, transportation, defense, space sectors. Several cooling technologies have been researched recently. Conventional heat transfer techniques that rely on fluids like water, ethylene glycol, and mineral oils continue to be popular due to its simple nature. Conventional heat transfer systems used in applications like petrochemical, refining, and power generation are rather large and involve significant amount of heat transfer fluids. In certain cooling applications, small heat transfer systems are required. These applications have a critical relationship between size of a mechanical system and the cost associated with manufacturing and operation. Improvements could be made in the existing heat transfer systems by enhancing the performance of heat transfer fluids resulting in lesser heat exchanger surface area, lower capital costs, and higher energy efficiencies. In this pursuit, numerous techniques to enhance the thermal performance of heat transfer fluids have been investigated. One of the methods used is to add nanoparticles of highly thermally conductive materials like carbon, metal, metal oxides into heat transfer fluids to improve the overall thermal conductivity. Nanoparticles could be either spherical or cylindrical. Carbon nanoparticles of cylindrical form are called carbon nanotubes (CNT). One type of carbon nanotube is called multi-walled carbon nanotubes (MWCNT) because they have multiple concentric tubes in a single configuration.

This study is concerned with nanofluids prepared by dispersing MWCNT in water, and their potential use as heat transfer fluids in a host of applications. It has now been established that when carbon nanotubes are suspended in conventional heat transfer fluids, enhancements in thermal conductivity and convective heat transfer performance are observed [1], [2], [3], [4], [5]. The motivations behind the current study are as follows. Firstly, there is limited information about the effect of preparation and processing conditions on the physical properties and thermal performance of CNT aqueous nanofluids. Secondly, limited experimental data is currently available for CNT-based nanofluids particularly in the area of viscosity. Lastly, only two studies have been reported to date on convective heat transfer of aqueous CNT nanofluids [4], [6]. Convective heat transfer is an area which still needs to be completely explored and understood. In this study, an effort has been made to consider all the above factors in a way that would move forward nanofluid research to the next phase.

Section snippets

Past research in CNT nanofluids preparation

One of the critical steps in preparing carbon nanofluids is dispersing carbon nanotubes in the base fluid. Due to the high aspect ratio of carbon nanotubes and strong Van der Waal’s forces between carbon surfaces, dispersion of CNT in aqueous medium can be challenging. CNTs are hydrophobic in nature and thus cannot be dispersed in water under normal conditions. There are usually two methods to disperse carbon nanotubes in base fluids: mechanical and chemical [7]. Mechanical methods generally

Sample preparation

De-ionized (DI) water, Gum Arabic (GA) and multi-walled carbon nanotubes (MWCNT) were used to produce aqueous suspensions. The nanotubes were procured from Helix Material Solutions Inc., USA. The nanotubes had a specified average outside diameter of 10–20 nm, length of 0.5–40 μm and purity of 95%, produced by chemical vapor deposition (CVD) process. GA fine powder was supplied by Biochemika. Four 500 g samples were prepared with mass fraction of GA and MWCNT as 0.25% and 1%, respectively, however,

Imaging data

Fig. 2a–d shows the pictures of samples A–D at a scale of 0.5 μm, under in-situ conditions, using wet-TEM technique [24]. It can be seen that samples A and B exhibit a good three-dimensional network of carbon nanotubes. Samples C and D show shorter carbon nanotubes which can be attributed to the additional ultrasonication time. From Fig. 2e and f (scale of 200 nm), it appears that the length of the nanotubes is relatively less in sample D than in sample B. The images provide only snapshots of CNT

Conclusions

Through this work, it has been confirmed once again that CNT nanofluids have potential as heat transfer fluids. The aqueous suspensions of multi-walled carbon nanotubes prepared by using Gum Arabic as surfactant were found to be stable for months. However, the preparation method of the nanofluids is an important step affecting the overall heat transfer performance. It was found that at a given CNT composition, there are certain optimum processing conditions (ultrasonication time in this case)

Acknowledgements

The authors express their deepest thanks to Alex Hays and Ryan Franks of the U.S. Army Corps of Engineers Construction Engineering Research Laboratory; Guillermo Soriano and Landon Sommer from Texas A&M University; Professor B. Jones of the Nuclear, Plasma, and Radiological Engineering Department, and Jianguo Wen, Dongxiang Liao of the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign for their support. The project was supported by the National

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An experimental study on the effect of ultrasonication on viscosity and heat transfer performance of multi-wall carbon nanotube-based aqueous nanofluids

Abstract

Introduction

Section snippets

Past research in CNT nanofluids preparation

Sample preparation

Imaging data

Conclusions

Acknowledgements

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Food Hydrocolloids

Int. J. Heat Mass Transfer

Thermal conductivity of suspensions of carbon nanotubes in water

Int. J. Thermophys.

Effective thermal conductivity of aqueous suspensions of carbon nanotubes (carbon nanotube nanofluids)

J. Thermophys. Heat Transfer

Thermal conductivity enhancement in aqueous suspensions of carbon multi-walled and double-walled nanotubes in the presence of two different dispersants

Int. J. Thermophys.

Anomalous thermal conductivity enhancement in nanotube suspensions

Appl. Phys. Lett.

Dispersion of carbon nanotubes in liquids

J. Dispersion Sci. Technol.

Thermal conductivity of heterogeneous two-component systems

IEC Fundam.

Stabilization of individual carbon nanotubes in aqueous solutions

Nano Lett.

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Exp. Heat Transfer

Relationship between water mobility and viscosity of nanometric alumina suspensions

J. Am. Ceram. Soc.