Stability of nanofluids in quiescent and shear flow fields
 Sanjeeva Witharana^{1}Email author,
 Haisheng Chen^{2}Email author and
 Yulong Ding^{1}
DOI: 10.1186/1556276X6231
© Witharana et al; licensee Springer. 2011
Received: 31 October 2010
Accepted: 16 March 2011
Published: 16 March 2011
Abstract
An experimental study was conducted to investigate the structural stability of ethylene glycolbased titanium dioxide nanoparticle suspensions (nanofluids) prepared by twostep method. The effects of particle concentration, fluid temperature, shear rate and shear duration were examined. Particle size and thermal conductivity measurements in quiescent state indicated the existence of aggregates and that they were stable in temperatures up to 60°C. Shear stability tests suggested that the structure of nanoparticle aggregates was stable in a shear interval of 5003000 s^{1} measured over a temperature range of 2060°C. These findings show directions to resolve controversies surrounding the underlying mechanisms of thermal conduction and convective heat transfer of nanofluids.
Introduction
Nanofluids are suspensions of nanosized particles in liquids, where particle sizes are preferably below 100 nm. At modest particle concentrations, the thermal conductivity, forced convective heat transfer, and critical heat flux of nanofluids were reported to be superior to respective base liquids [1–8]. In the backdrop of conventional heat transfer technologies approaching their upper limits, nanofluids are seen as a potential contender for small and largescale thermal applications [9–12]. A number of attempts had been made in the past, and postulates were put forward to explain the underlying mechanisms. Although yet inconclusive, the nanoparticle aggregation in liquids is believed to be one of the principal mechanisms behind the enhanced thermal conductivity and convective heat transfer [13–16]. In either case, the importance of particle aggregation and their stability were underlined.
On the other hand, the aggregation of nanoparticles is found to be the key mechanism behind the increase of nanofluid viscosity and shear thinning behaviour [14, 17, 18]. Recently, it was shown that the high shear viscosity of nanofluids could accurately be predicted by combining the conventional Krieger and Dougherty model and aggregation effects [18–20]. Those postulates were based on the assumption that, in the shear flow field, the aggregates will be stable because the hydrodynamic forces are insufficient to break the aggregates down to primary particles. However, the experimental evidences are insufficient to showcase the stability and particle structuring of nanofluids in flow conditions.
In the present study, the ethylene glycol (EG)based Titania (TiO_{2}) suspensions are selected to investigate the stability of nanofluids in quiescent and shear flow fields. Also their thermal conductivities are measured at various temperatures and compared with theoretical predictions. The experimental conditions were chosen resembling the possible industrial applications for nanofluids. Considering the bounded yet deep focus of the stability of nanofluids under different conditions, this article is reported as a letter without comparing the data with the other literature.
Experimental
Measurements of thermal conductivity (k, W/mK) of TiO_{2}EG nanofluids were conducted using the stateofthe art Lambda meter device acquired from PSL Measurement Systems GMBH of Germany. This instrument works on transient hot wire principle. For calibration with EG, the instrument reproduced the data up to 99% precision.
Shear flow field was applied to the samples using a Bohlin rotational rheometer. The experimental conditions were as follows: shear rates 500, 1000, 2000 and 3000 s^{1}; time durations 5, 10, 20 and 40 min; and temperatures 20, 30, 40, 50 and 60°C. These temperature and flow parameters were so chosen to suit possible industrial applications [19]. The shearing was preceded and followed by particle size measurements using Malvern Zetasizernano. The size measurements were repeated six times, and the reproducibility of data fell within error of 4%. In all instruments, the thermal equilibrium was ensured by leaving the samples at measuring temperature for a sufficient period of time before taking the readings.
Results and discussion
where k, k_{0}, k_{p} are, respectively, the thermal conductivities of the nanofluid, base liquid, and particle material, and n is the shape factor given by n = 3/ψ with ψ the surface areabased sphericity (ψ = 1.0 for spheres).
Here, φ_{a} is the effective particle volume fraction given by φ_{a} = φ(a_{a}/a)^{3}^{ D }according with the fractal theory, and φ_{in} is the solid volume fraction of aggregates given by φ_{in} = (a_{a}/a)^{ D }^{3}. Also a and a_{a} are the radii of primary nanoparticles and aggregates, respectively [27], and D is the fractal index having a typical value of 1.8 for nanofluids [20]. From Figure 3, the conventional HC model underpredicts the measurements by a considerable margin can be seen. However, the modified HC model that takes into account the aggregates of nanoparticles agreed well with the experimental data.
Overall view of Figures 2 and 3 suggests that (i) the aggregation of nanoparticles is a principal mechanism that drives the thermal conductivity enhancement and (ii) the aggregates are stable in quiescent flow fields even at temperature as high as 60°C. Independence of the experimental data on temperature further suggests the weak or negligible effect of particle Brownian motion on reported enhancement.
Conclusions
Experiments were conducted to study the dependence of shear stability of nanofluids on temperature, particle loading and shear rate. Observed weak dependence of thermal conductivity enhancement on temperature supports the claim of particle aggregation as a principal mechanism behind the enhancement. Moreover, the aggregates in quiescent flow fields were stable in temperatures up to 60°C. The data on shear stability show that the aggregates are sufficiently stable over a range of rigorous shear rates and temperatures. The observations of thermal conductivity and particle size complement each other in terms of predicting the former from the latter. A comparison of the present findings with the literature data is currently underway and will be reported in future.
Abbreviations
 EG:

ethylene glycol
 EM:

electron microscopy
 HC:

HamiltonCrosser
 INPBE:

International Nanofluids Property Benchmarking Exercise.
Declarations
Authors’ Affiliations
References
 Eastman JA, Choi SUS, Li S, Yu W, Thompson LJ: Anomalously increased effective thermal conductivities of ethylene glycolbased nanofluids containing copper nanoparticles. Applied Physics Letters 2001, 78: 718–720. 10.1063/1.1341218View ArticleGoogle Scholar
 Murshed SMS, Leong KC, Yang C: Enhanced thermal conductivity of TiO2  water based nanofluids, International. Journal of Thermal Sciences 2005, 44: 367–373. 10.1016/j.ijthermalsci.2004.12.005View ArticleGoogle Scholar
 Wen DS, Ding YL: Effective thermal conductivity of aqueous suspensions of carbon nanotubes (carbon nanotubes nanofluids). Journal of Thermophysics and Heat Transfer 2004, 18: 481–485. 10.2514/1.9934View ArticleGoogle Scholar
 Wensel J, Wright B, Thomas D, Douglas W, Mannhalter B, Cross W, Hong HP, Kellar J, Smith P, Roy W: Enhanced thermal conductivity by aggregation in heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes. Applied Physics Letters 2008, 92: 023110. 10.1063/1.2834370View ArticleGoogle Scholar
 Wen DS, Ding YL: Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions. International Journal of Heat and Mass Transfer 2004, 47: 5181–5188. 10.1016/j.ijheatmasstransfer.2004.07.012View ArticleGoogle Scholar
 Ding Y, Alias H, Wen D, Williams RA: Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids). International Journal of Heat and Mass Transfer 2006, 49: 240–250. 10.1016/j.ijheatmasstransfer.2005.07.009View ArticleGoogle Scholar
 He YR, Jin Y, Chen HS, Ding YL, Cang DQ, Lu HL: Heat transfer and flow behaviour of aqueous suspensions of TiO2 nanoparticles (nanofluids) flowing upward through a vertical pipe. International Journal of Heat and Mass Transfer 2007, 50: 2272–2281. 10.1016/j.ijheatmasstransfer.2006.10.024View ArticleGoogle Scholar
 Xuan Y, Li Q: Heat transfer enhancement of nanofluids. International Journal of Heat and Fluid flow 2000, 21: 58–64. 10.1016/S0142727X(99)000673View ArticleGoogle Scholar
 Kim SJ, McKrell T, Buongiorno J, Hu LW: Enhancement of flow boiling Critical Heat Flux (CHF) in alumina/water nanofluids. Advanced Science Letters 2009, 2: 100–102.View ArticleGoogle Scholar
 Kim SJ, Bang IC, Buongiorno J, Hu LW: Effects of nanoparticle deposition on surface wettability influencing boiling heat transfer in nanofluids. Applied Physics Letters 2006, 89: 153107. 10.1063/1.2360892View ArticleGoogle Scholar
 Fan XL, Chen HS, Ding YL, Plucinski PK, Lapkin AA: Potential of 'nanofluids' to further intensify microreactors. Green Chemistry 2008, 10: 670–677. 10.1039/b717943jView ArticleGoogle Scholar
 Roberts NA, Walker DG: Convective Performance of Nanofluids in Commercial Electronics Cooling Systems. Applied Thermal Engineering 2010, 30: 2499–2504. 10.1016/j.applthermaleng.2010.06.023View ArticleGoogle Scholar
 Buongiorno J: Convective transport in nanofluids. Journal of Heat TransferTransactions of the Asme 2006, 128: 240–250. 10.1115/1.2150834View ArticleGoogle Scholar
 Keblinski P, Prasher R, Eapen J: Thermal conductance of nanofluids: is the controversy over? Journal of Nanoparticle research 2008, 10: 1089–1097. 10.1007/s1105100793521View ArticleGoogle Scholar
 Prasher R, Song D, Wang JL, Phelan P: Measurements of nanofluid viscosity and its implications for thermal applications. Applied Physics Letters 2006, 89: 133108. 10.1063/1.2356113View ArticleGoogle Scholar
 Ding Y, Chen H, Wang L, Yang CY, He Y, Yang W, Lee WP, Zhang L, Huo R: Heat Transfer Intensification Using Nanofluids. KONA 2007, 25: 23–38.View ArticleGoogle Scholar
 Keblinski P, Eastman JA, Cahill DG: Nanofluids for thermal transport. Materials Today 2005, 8: 36–44. 10.1016/S13697021(05)709366View ArticleGoogle Scholar
 Chen H, Ding Y, Tan C: Rheological behaviour of nanofluids. New Journal of Physics 2007, 9: 367. 10.1088/13672630/9/10/367View ArticleGoogle Scholar
 Chen H, Yang W, Y He, Ding Y, Zhang L, C Tan, Lapkin AA, Bavykin DV: Heat transfer and flow behaviour of aqueous suspensions of titanate nanotubes (nanofluids). Powder Technology 2008, 183: 63–72. 10.1016/j.powtec.2007.11.014View ArticleGoogle Scholar
 Chen H, Witharana S, Y Jin, Ding Y, Kim C: Predicting the thermal conductivity of nanofluids based on suspension rheology.
 Wen DS, Ding YL: Formulation of nanofluids for natural convective heat transfer applications. International Journal of Heat and Fluid Flow 2005, 26: 855–864. 10.1016/j.ijheatfluidflow.2005.10.005View ArticleGoogle Scholar
 Buongiorno J, et al.: A benchmark study on the thermal conductivity of nanofluids. Journal of Applied Physics 2009, 106: 094312. 10.1063/1.3245330View ArticleGoogle Scholar
 Ding YL, Chen HS, He YR, Lapkin AX, Yeganeh M, Siller L, Butenko YV: Forced convective heat transfer of nanofluids. Advanced Powder Technology 2007, 18: 813–824. 10.1163/156855207782515021View ArticleGoogle Scholar
 Ding Y, Chen H, Musina Z, Jin Y, Zhang T, Witharana S, Yang W: Relationship between the thermal conductivity and shear viscosity of nanofluids. Physica Scripta 2010, T139: 014078. 10.1088/00318949/2010/T139/014078View ArticleGoogle Scholar
 Hamilton RL, Crosser OK: Thermal conductivity of hetrogeneous twocomponent systems. Industrial & Engineering chemistry fundamentals 1962, 1: 187–191.View ArticleGoogle Scholar
 Bruggeman DAG: Calculation of various physics constants in heterogenous substances I Dielectricity constants and conductivity of mixed bodies from isotropic substances. Annalen der Physik 1935, 24: 636–664. 10.1002/andp.19354160705View ArticleGoogle Scholar
 Goodwin JW, Hughes RW: Rheology for ChemistsAn introduction. The Royal Society of Chemistry, UK; 2000.Google Scholar
Copyright
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.