Research Papers: Conduction

Influence of Controlled Aggregation on Thermal Conductivity of Nanofluids

[+] Author and Article Information
Reza Azizian

Nuclear Science and Engineering Department,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: Azizian@mit.edu

Elham Doroodchi

Center for Advanced Particle Processing,
Chemical Engineering Department,
The University of Newcastle,
Callaghan, New South Wales 2308, Australia
e-mail: Elham.Doroodchi@Newcastle.edu.au

Behdad Moghtaderi

Center for Energy,
Chemical Engineering Department,
The University of Newcastle,
Callaghan, New South Wales 2308, Australia
e-mail: Behdad.Moghtaderi@Newcastle.edu.au

1Corresponding authors.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 14, 2014; final manuscript received September 24, 2015; published online October 27, 2015. Assoc. Editor: Leslie Phinney.

J. Heat Transfer 138(2), 021301 (Oct 27, 2015) (6 pages) Paper No: HT-14-1190; doi: 10.1115/1.4031730 History: Received April 14, 2014; Revised September 24, 2015

Nanoparticles aggregation is considered, by the heat transfer community, as one of the main factors responsible for the observed enhancement in the thermal conductivity of nanofluids. To gain a better insight into the veracity of this claim, we experimentally investigated the influence of nanoparticles aggregation induced by changing the pH value or imposing a magnetic field on the thermal conductivity of water-based nanofluids. The results showed that the enhancement in thermal conductivity of TiO2–water nanofluid, due to pH-induced aggregation of TiO2 nanoparticles, fell within the ±10% of the mixture theory, while applying an external magnetic force on Fe3O4–water nanofluid led to thermal conductivity enhancements of up to 167%. It is believed that the observed low enhancement in thermal conductivity of TiO2–water nanofluid is because, near the isoelectric point (IEP), the nanoparticles could settle out of the suspension in the form of large aggregates making the suspension rather unstable. The magnetic field however could provide a finer control over the aggregate size and growth direction without compromising the stability of the nanofluid, and hence significantly enhancing the thermal conductivity of the nanofluid.

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Fig. 1

Schematic diagram of (a) electromagnet and (b) solenoid

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Fig. 2

(a) DLS size distribution of TiO2 (rutile) and Fe3O4 nanoparticles and (b) TEM image of Fe3O4 nanoparticles

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Fig. 3

XRD pattern of synthesized magnetite nanoparticles, inset: reference XRD pattern for Fe3O4 [15]

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Fig. 4

(a) Zeta potential (mV) measurement of different TiO2 (rutile)–water nanofluid samples at different pH values in comparison to two reported values, and (b) zeta potential as well as particle size for TiO2 (rutile)–water nanofluid samples prepared in the present study

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Fig. 5

Thermal conductivity and zeta potential of TiO2 (rutile)–water nanofluid measured at different pH values

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Fig. 6

Aggregation effect on effective thermal conductivity of nanofluids [25]

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Fig. 7

Thermal conductivity of the magnetite nanofluid under application of an external magnetic field parallel and perpendicular to the temperature gradient, in comparison to the thermal conductivity of DI water and TiO2 (rutile, 1 vol. %)–water nanofluid

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Fig. 8

Schematic diagram of aggregate formation in the direction of an external magnetic field (a) parallel and (b) perpendicular to the temperature gradient




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