Investigation on two abnormal phenomena about thermal conductivity enhancement of BN/EG nanofluids
© Li et al; licensee Springer. 2011
Received: 8 November 2010
Accepted: 9 July 2011
Published: 9 July 2011
The thermal conductivity of boron nitride/ethylene glycol (BN/EG) nanofluids was investigated by transient hot-wire method and two abnormal phenomena was reported. One is the abnormal higher thermal conductivity enhancement for BN/EG nanofluids at very low-volume fraction of particles, and the other is the thermal conductivity enhancement of BN/EG nanofluids synthesized with large BN nanoparticles (140 nm) which is higher than that synthesized with small BN nanoparticles (70 nm). The chain-like loose aggregation of nanoparticles is responsible for the abnormal increment of thermal conductivity enhancement for the BN/EG nanofluids at very low particles volume fraction. And the difference in specific surface area and aspect ratio of BN nanoparticles may be the main reasons for the abnormal difference between thermal conductivity enhancements for BN/EG nanofluids prepared with 140- and 70-nm BN nanoparticles, respectively.
The concept "nanofluids" was proposed by Choi  in 1995. Roughly speaking, nanofluids are solid-liquid composite materials consisting of solid nanoparticles or nanofibers with typically of 1-100 nm suspended in base liquid. Nanofluids provide a promising technical selection for enhancing heat transfer because of its anomalous high thermal conductivity and appear to be ideally suited for practical application with excellent stability and little or no penalty in pressure drop. As a result, nanofluids attract more and more interests theoretically and experimentally.
In the past decades, many investigations on thermal conductivity enhancement of nanofluids have been reported. These papers mainly focused on factors influencing thermal conductivity enhancement [2–16], mechanism for thermal conductivity enhancement [17–22], model for predicting the enhancement of thermal conductivity [23–29]. Recently, controversy about whether the dramatic increase of thermal conductivity with small nanoparticle loading in nanofluids is true was reported [30, 31]. Some researches showed that no anomalous enhancement of thermal conductivity with small nanoparticle loading was achieved in the nanofluids and the thermal conductivity enhancement is moderate and can be predicted by effective medium theories. Besides, the mechanism of thermal conductivity enhancement is a hotly debated topic now, and many researchers pay attention to the influence of aggregation, morphology, and size of nanoparticles on thermal conductivity enhancement of nanofluids [32–39].
Focus on the current research interest, boron nitride/ethylene glycol (BN/EG) nanofluid was synthesized by a two-step method. The effect of particles volume fraction and size of nanoparticles on thermal conductivity enhancement were investigated and two abnormal phenomena were observed. In present paper, the two abnormal phenomena are reported and the mechanism of thermal conductivity enhancement is discussed.
Apparatus for preparing nanofluids
Magnetic force stirring
Results and discussion
Thermal conductivity enhancement of the BN/EG nanofluids
Volume fraction (vol.%)
where k p is the thermal conductivity of the dispersed particles, k f is the thermal conductivity of the dispersion liquid, and ϕ is the particle volume concentration of the suspension.
α11, α22 and α33 are, respectively, radii of the ellipsoid along the , and axes of this ellipsoidal composite unit cell, L ii are well-known geometrical factors dependent on the particle shape, p is the aspect ratio of the ellipsoide, k m is the thermal conductivity of the matrix phase, is quivalent thermal conductivities along the symmetric axis of this ellipsoidal composite unit cell, and R bd is the Kapitza interfacial thermal resistance.
The conventional Maxwell model and Nan's model severely underestimates the enhancement of thermal conductivity for BN/EG nanofluids. It may be ascribed to that Maxwell model only takes the effect of particle volume fraction into account for thermal conductivity enhancement of nanofluids without considering the effect of particle shape, nanolayers at solid/liquid interface, and Brownian motion of nanoparticles and others. Nan's model is for particulate composites not for nanofluids. Although Nan's model considered the effect of nanoparticle shape and finite interfacial resistance, the effects of Brownian motion and aggregation of nanoparticles on thermal conductivity of nanofluids cannot be ignored. Now, no suitable model proposed by other researchers can fit well with the data we got. It is necessary to develop a new model considering all important factors influencing the thermal conductivity enhancement of BN/EG nanofluids. The work about this issue is being done by our group and will be reported later.
The volume fraction of this 0.025 vol.% BN/EG nanofluids was measured after sedimentation for 120 days and the value of it is 0.017 vol.%. This phenomenon indicated that the stability of the nanofluid is excellent. And the long-term stability of this nanofluid may be ascribed to the flake-like morphology and incompact aggregation of the BN nanoparticles, as showed in Figure 4a. It can be expected that the stability of this nanofluid can be improved further when some appropriate dispersant was used. The phenomenon mentioned above indicates that nanofluids with high thermal conductivity and long-term stability can be obtained by adding relatively lower volume fraction of nanoparticles when the nanoparticles suspended in base liquid with proper morphology and aggregation. This kind of nanofluid is promising for engineering application.
In summary, two abnormal phenomena about thermal conductivity enhancement of BN/EG nanofluids was investigated. One is the abnormal increment of thermal conductivity for BN/EG nanofluids at very low volume fraction, and the other is the abnormal thermal conductivity enhancement for BN/EG nanofluids synthesized with different size of BN nanoparticles. The chain-like loose aggregation of nanoparticles is responsible for the abnormal increment of thermal conductivity in the BN/EG nanofluids with very low particles volume fraction. And the difference in specific surface area and aspect ratio of BN nanoparticles may be the main reason for the abnormal difference between thermal conductivity enhancements for BN/EG nanofluids prepared with 140 and 70-nm BN nanoparticles, respectively.
The authors acknowledge the financial support from GM Corporation for this work.
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