Experimental Investigation of Heat Conduction Mechanisms in Nanofluids. Clue on Clustering

Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510641, People's Republic of China.
Nano Letters (Impact Factor: 12.94). 12/2009; 9(12):4128-32. DOI: 10.1021/nl902358m
Source: PubMed

ABSTRACT Heat conduction mechanisms in nanofluids, fluids seeded with nanoparticles, have been extensively scrutinized in the past decades to explain some experimental observations of their enhanced thermal conductivity beyond the effective medium theory. Although many mechanisms such as Brownian motion, clustering, ballistic transport, and internanoparticle potential are speculated, experimental proof of any of the mechanisms has been difficult. Here, we investigate the mechanisms experimentally by thermal conductivity measurements and structural analysis for the same materials in both liquid and solid states. These studies strongly suggest that clustering holds the key to the thermal conductivity enhancement of nanofluids.


Available from: Jinwei Gao, Jun 02, 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Different cupric oxide (CuO) structures have attracted intensive interest because of their promising applications in various fields. In this study, three kinds of CuO structures, namely, CuO microdisks, CuO nanoblocks, and CuO microspheres, are synthesized by solution-based synthetic methods. The morphologies and crystal structures of these CuO structures are characterized by field-emission scanning electron microscope and X-ray diffractometer, respectively. They are used as thermal conductive fillers to prepare silicone-based thermal greases, giving rise to great enhancement in thermal conductivity. Compared with pure silicone base, the thermal conductivities of thermal greases with CuO microdisks, CuO nanoblocks, and CuO microspheres are 0.283, 0256, and 0.239 W/mK, respectively, at filler loading of 9 vol.%, which increases 139%, 116%, and 99%, respectively. These thermal greases present a slight descendent tendency in thermal conductivity at elevated temperatures. These experimental data are compared with Nan's model prediction, indicating that the shape factor has a great influence on thermal conductivity improvement of thermal greases with different CuO structures. Meanwhile, due to large aspect ratio of CuO microdisks, they can form thermal networks more effectively than the other two structures, resulting in higher thermal conductivity enhancement.
    Nanoscale Research Letters 12/2015; 10(1):113. DOI:10.1186/s11671-015-0822-6 · 2.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Switchable electrical and thermal conductivities are desirable in many applications such as automatic regulation of building temperature, circuit protection, etc. In this paper, we study the electrical and thermal conductivities of multi-walled carbon nanotube (MWCNT)/DI-water composites via first order phase transition. We demonstrate that, with SDBS functionalized MWCNTs, the composites show unique positive temperature coefficient (PTC) electrical switching and negative temperature coefficient (NTC) thermal switching properties. Around 0 °C, the corresponding contrast ratio of electrical and thermal conductivities reaches 1250 and 3.58 times respectively.
    02/2015; 3(10). DOI:10.1039/C5TA00165J
  • [Show abstract] [Hide abstract]
    ABSTRACT: Nanofluids are produced by dispersing nanoparticles in basefluid. Given its superior thermo-physical properties, nanofluids are gaining increasing attention and are showing promising potential in various applications. Numerous studies have been conducted in the past decade to experimentally and theoretically investigate thermal conductivity. The experimental finding is briefly summarized in this study; however, we do not intend to present a systematic summary of the available references from the literature. The primary objective of this study is to review and summarize the most debated mechanisms for heat conduction in nanofluids, such as the effects of a nanolayer, the Brownian motion of nanoparticles and aggregation, as well as induced convection. Finally, at a low concentration of nanoparticles, nanoconvection is the leading contributor to thermal conductivity enhancement, whereas at a higher concentration, the natural thermal transport along the backbone would aggregate, and the effects of the nanolayer would become significant and become ineligible.
    Journal of Mechanical Science and Technology 07/2014; 28(7):2925-2936. DOI:10.1007/s12206-014-0645-x · 0.70 Impact Factor