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.
- Choi SUS: Developments and Applications of Non-Newtonian Flows. New York: ASME; 1995. (FED-vol.231/MD-vol 66:99) (FED-vol.231/MD-vol 66:99)
- Eastman JA, Choi SUS: Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles. Appl Phys Lett 2001, 78: 718. 10.1063/1.1341218View Article
- Liu MS, Lin MCC, Tsai CY, Wang CC: Enhancement of thermal conductivity with cu for nanofluids using chemical reduction method. Int J Heat Mass Transf 2006, 49: 3028. 10.1016/j.ijheatmasstransfer.2006.02.012View Article
- Hong TK, Yang HS, Choi CJ: Study of the enhanced thermal conductivity of Fe nanofluids. J Appl Phys 2005, 97: 064311. 10.1063/1.1861145View Article
- Patel HE, Das SK, Sundararagan T, Nair AS, Geoge B, Pradeep T: Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: manifestation of anomalous enhancement and chemical effects. Appl Phys Lett 2003, 83: 2931. 10.1063/1.1602578View Article
- Putnam SA, Cahill DG, Braun PV: Thermal conductivity of nanoparticle suspensions. J Appl Phys 2006, 99: 084308. 10.1063/1.2189933View Article
- Zhu H, Zhang C, Liu S: Effects of nanoparticle clustering and alignment on thermal conductivities of Fe3O4aqueous nanofluids. Appl Phys Lett 2006, 89: 23123. 10.1063/1.2221905View Article
- Murshed SMS, Leong KC, Yang C: Enhanced thermal conductivity of TiO2- water based nanofluid. Int J Therm Sci 2005, 44: 367. 10.1016/j.ijthermalsci.2004.12.005View Article
- Xie H, Wang J, Xi T, Liu Y, Ai F: Thermal conductivity enhancement of suspensions containing nanosized alumina particle. J Appl Phys 2002, 91: 4568. 10.1063/1.1454184View Article
- Choi SUS, Zhang ZG, Yu W, Lockwood FE, Grulke EA: Anomalous thermal conductivity enhancement in nanotube suspension. Appl Phys Lett 2001, 79: 2252. 10.1063/1.1408272View Article
- Xie HQ, Lee H, Youn W, Choi M: Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities. J ApplPhys 2003, 94: 4967.View Article
- Yang B, Han ZH: Thermal conductivity enhancement in water-in-FC72 nanoemulsion fluid. Appl Phys Lett 2006, 88: 261914. 10.1063/1.2218325View Article
- Ma KQ, Liu J: Nano liquid-metal fluid as ultimate coolant. Phys Lett 2007, A361: 252.View Article
- Wei X, Wang L: Synthesis and thermal conductivity of microfluidic copper nanofluids. Particuology 2010, 8: 262. 10.1016/j.partic.2010.03.001View Article
- Wang X, Zhu D, Yang S: Investigation of pH and SDBS on enhancement of thermal conductivity in nanofluids. Chem Phys Lett 2009, 470: 107. 10.1016/j.cplett.2009.01.035View Article
- Wei X, Zhu H, Kong T, Wang L: Synthesis and thermal conductivity of Cu2O nanofluids. Int J Heat Mass Transf 2009, 52: 4371. 10.1016/j.ijheatmasstransfer.2009.03.073View Article
- Xuan Y, Li Q, Zhang X, Fujii M: Stochastic thermal transport of nanoparticle suspensions. J Appli Phys 2006, 100: 043507. 10.1063/1.2245203View Article
- Keblinski P, Phillpot SR, Choi SUS: Mechanisms of heat flow in suspensions of nano-sized particles (nanofluids). Int J Heat Mass Transf 2002, 45: 855. 10.1016/S0017-9310(01)00175-2View Article
- Eastman JA, Phillpot SR, Choi SUS, Keblinski P: Thermal transport in nanofluids. Annu Rev Mater Res 2004, 34: 219. 10.1146/annurev.matsci.34.052803.090621View Article
- Xie HQ, Xi TG, Wang JC: Study on the mechanism of heat conduction in nanofluid medium. Acta Phys Sin 2003, 52: 1444.
- Prasher R, Phelan PE, Bhattacharya P: Effect of aggregation kinetics on the thermal conductivity of nanoscale colloidal solutions (nanofluid). Nano Lett 2006, 6: 1529. 10.1021/nl060992sView Article
- Evans W, Prasher R, Fish J, Meakin P, Phelan P, Keblinski P: Effect of aggregation and interfacial thermal resistance on thermal conductivity of nanocomposite and colloidal nanofluids. Int J Heat Mass Transf 2008, 51: 1431. 10.1016/j.ijheatmasstransfer.2007.10.017View Article
- Leong KC, Yang C, Murshed SMS: A model for the thermal conductivity of nanofluids--the effect of interfacial layer. J Nanopart Res 2006, 8: 245. 10.1007/s11051-005-9018-9View Article
- Yu W, Choi SUS: The role of interfacial layers in the enhanced thermal of nanofluids: a renovated maxwell model. J Nanopart Res 2003, 5: 167.View Article
- Xue QZ: Model for effective thermal conductivity of nanofluids. Phys Lett 2003, A307: 313.View Article
- Xue Q, Xu WM: A model of thermal conductivity of nanofluids with interfacial shells. Mater Chemist Phys 2005, 90: 298. 10.1016/j.matchemphys.2004.05.029View Article
- Nan CW, Birringer R, Clarke DR, Gleiter H: Effective thermal conductivity of particulate composites with interfacial thermal resistance. J Appl Phys 1997, 81: 6692. 10.1063/1.365209View Article
- Nan CW, Shi Z, Lin Y: A simple model for thermal conductivity of carbon nanotube-based composites. Chem Phys Lett 2003, 375: 666. 10.1016/S0009-2614(03)00956-4View Article
- Murshed SMS, Leong KC, Yang C: A combined model for the effective thermal conductivity of nanofluids. Appli Therm Eng 2009, 29: 2477. 10.1016/j.applthermaleng.2008.12.018View Article
- Buongiorno J, Venerus DC, Prabhat N, McKrell T, Townsend J, Christianson R, Tolmachev YV, Keblinski P, Hu LW, Alvarado JL, Bang IC, Bishnoi SW, Bonetti M, Botz F, Cecere A, Chang Y, Chen G, Chen H, Chung SJ, Chyu MK, Das SK, Di Paola R, Ding Y, Dubois F, Dzido G, Eapen J, Escher W, Funfschilling D, Galand Q, Gao J, Gharagozloo PE, Goodson KE, Gutierrez J G, Hong H, et al.: A benchmark study on the thermal conductivity of nanofluids. J Appl Phys 2009, 106: 094312. 10.1063/1.3245330View Article
- Keblinski P, Prasher R, Eapen J: Thermal conductance of nanofluids: is the controversy over? J Nanopart Res 2008, 10: 1089. 10.1007/s11051-007-9352-1View Article
- Peasher R, Phelan PE, Bhattacharya P: Effect of Aggregation Kinetics on the Thermal Conductivity of Nanoscale Colloidal Solutions (Nanofluid). Nano Lett 2006, 6(7):1529. 10.1021/nl060992sView Article
- Karthikeyan NR, Philip J, Raj B: Effect of clustering on the thermal conductivity of nanofluids. Mater Chemist Phys 2008, 109: 50. 10.1016/j.matchemphys.2007.10.029View Article
- Chen G, Yu W, Singh D, Cookson D, Routbort J: Application of SAXS to the study of particle-size-dependent thermal conductivity in silica nanofluids. J Nanopart Res 2008, 10: 1109. 10.1007/s11051-007-9347-yView Article
- Jang SP, Choi SUS: Role of Brownian motion in the enhanced thermal conductivity of nanofluids. Appli Physi Lett 2004, 84: 4316. 10.1063/1.1756684View Article
- Chon CH, Kihm KD, Lee SP, Choi SUS: Empirical correlation finding the role of temperature and particle size for nanofluid(Al2O3) thermal conductivity enhancement. Appl Phys Lett 2005, 87: 153107. 10.1063/1.2093936View Article
- Kim SH, Choi SR, Kim DS: Thermal Conductivity of Metal-Oxide Nanofluids: Particle Size Dependence and Effect of Laser Irradiation. ASME J Heat Transfer 2007, 129: 298. 10.1115/1.2427071View Article
- Jang SP, Choi SUS: Effects of Various Parameters on Nanofluid Thermal Conductivity, Journal of Heat Transfer. J of Heat Transfer 2007, 129: 617. 10.1115/1.2712475View Article
- Peng XF, Yu XL, Zhong X, Yu QF: Experiments on thermal conductivity of nanofluids. J Zhejiang University (Engineering Science) (in Chinese) 2007, 41(7):1177.
- Luo ZF: Investigation on Preparation and Properties of Dielectric Nanofluids. In Master's thesis. Xi'an Jiaotong University, School of Materials Science and Engineering; 2009.
- Wu JT, Zheng HF, Qian XH, Li XJ, Assael MJ: Thermal Conductivity of Liquid 1, 2-Dimethoxyethane from 243K to 353K at Pressures up to 30Mpa. Int J Thermophys 2009, 30: 385. 10.1007/s10765-008-0549-zView Article
- Wang YG, Wu JT, Liu ZG: Thermal Conductivity of Gaseous Dimethyl Ether from (263 to 383) K. J Chem Eng Data 2006, 51: 164. 10.1021/je050305zView Article
- Jin XG, Wu JT, Liu ZG, Pan J: The thermal conductivity of dimethyl carbonate in the liquid phase. Fluid Phase Equilibria 2004, 220: 37. 10.1016/j.fluid.2003.10.018View Article
- Maxwell JC: A Treatise on Electricity and Magnetism. 2nd edition. Oxford: Clarendon; 1881.
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