Journal of Nano Research Vol. 13 (2011) pp 69-74 Online available since 2011/Feb/12 at www.scientific.net (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/jnanor.13.69 EG/CNTs Nanofluids Engineering and Thermo-Rheological Characterization B.C. Lamas 1,a, M.A.L. Fonseca 1,b, F.A.M.M. Gonçalves 1,c, A.G.M. Ferreira 2,d, I.M.A.Fonseca 2,e, S. Kanagaraj 3,f, N. Martins 1,g, M.S.A. Oliveira 1,h 1 Departamento de Engenharia Mecânica, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193, Aveiro, Portugal 2 Departamento de Engenharia Química, Universidade de Coimbra, Pólo II, Rua Sílvio Lima, 3030-790 Coimbra, Portugal 3 Mechanical Engineering Department, IIT Guwahati, Assam, India a brunolamas@ua.pt, b mfonseca@ua.pt, c fgoncalves@ua.pt, d abel@eq.uc.pt, e fonseca@eq.uc.pt, f kanagaraj@iitg.ernet.in, g nmartins@ua.pt, h monica.oliveira@ua.pt Keywords: Nanofluids, thermal conductivity, colloidal stability, viscosity. Abstract. The research work presented here intends to contribute to the overall research effort towards nanofluids engineering and characterization. To accomplish the latter, multiwalled carbon nanotubes (MWCNTs) are added to an ethylene glycol (EG) based fluid. Different aspects concerning the nanofluids preparation and its thermal characterization will be addressed. The study considers and exploits the relative influence of CNTs concentration on EG based fluids, on the suspension effective thermal conductivity and viscosity. In order to guarantee a high-quality dispersion it was performed a chemical treatment on the MWCNTs followed by ultrasonication mixing. Furthermore, the ultrasonication mixing-time is optimized through the UV-vis spectrophotometer to ensure proper colloidal stability. The thermal conductivity is measured via transient hot-wire within a specified temperature range. Viscosity is assessed through a controlled stress rheometer. The results obtained clearly indicate an enhancement in thermal conductivity consistent with carbon nanotube loading. The same trend is observed for the viscosity, which decreases with temperature rise and its effect is nullified at higher shear rates. Introduction The improvement of conventional fluid characteristics is becoming mandatory when economy dematerialization and energy efficiency in industrial processes and systems is concerned. It is a fact that regular fluids (i.e. water, ethylene glycol) used in heat exchangers have low thermal conductivities, which highly contributes to a poor system thermal efficiency. It is also known that convective heat transfer can be improved passively by changing different parameters, namely, flow geometry, boundary conditions, or even by enhancing the fluid thermal properties. The latter one clearly justifies the research around nanofluids, where nanoparticles are suspended in a conventional fluid, allowing for a drastic rise in thermal conductivity of the base fluid [1-3]. The development of new nanofluids through the incorporation of different types of nanoparticles, such as CuO, and Al 2 O 3, in normal base fluids has been reported [4]. The results clearly reveal an enhancement in thermal conductivity of the base fluid, but at the expense of extremely high concentrations, giving rise to problems of colloidal stability. The incorporation of carbon nanotubes (CNTs), which detain exceptional physical and mechanical properties, seems to be the best-suited nanoparticles to be added to a base fluid [5]. It could be possible, with smaller concentrations of CNTs, to achieve a higher enhancement on the effective thermal conductivity than the ones obtained for different nanoparticle based suspensions. The engineering of nanofluids for thermal property enhancement is not problem free and involves the proper usage of several techniques and the analysis of different parameters that are believed to seriously influence these new fluid All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 210.212.8.62-06/07/11,06:28:43)
70 Journal of Nano Research Vol. 13 characteristics. Since nanofluids are heterogeneous solutions, the incorporated nanoparticles tend to aggregate and settle down with time, which may cause the clogging of the channels and the decreasing of the effective thermal conductivity. Therefore, the research on nanofluids stability is believed to be a key factor to the development of stable and effective application. There are different methods to evaluate the nanofluids colloidal stability, however, the most simple and consistent is the sedimentation. In this, the concentration of the nanosized particles is obtained by the use of a specific experimental apparatus, such as UV-vis spectrophotometry. This research work intends to provide a detailed study of the thermo-rheological properties of ethylene glycol/multiwalled carbon nanotube (MWCNT/EG) nanofluids. The colloidal stability of nanofluids with different MWCNT concentrations is studied and suspensions time of mixture is established and optimized. Furthermore, it is studied the thermal conductivity variation with temperature and MWCNTs loading as well as viscosity. Nanofluids preparation and colloidal stability To achieve a stable nanoparticle suspension, suitable for heat transfer applications, it is necessary to guarantee its colloidal stability. To produce a stable and highly conductive nanofluid, a top-down approach (two-step method) has been applied. In this approach, dried nanoparticles, in the form of dry powders, are dispersed into the base fluid. The used nanoparticles were multiwalled carbon nanotubes, with a diameter ranging from 50 to80 nm and a length of 10-20 µm (aspect ratio of 125-400), and ethylene-glycol used as the base fluid. The production of a stable suspension of MWCNTs in EG is not problem free, as the nanosized particles tend to agglomerate and settle down with time. Therefore, a chemical procedure, based on the one suggested by Esumi el al., was applied [6]. The MWCNTs were immersed into concentrated nitric and sulphuric acid in the volume ratio of (1:3) and refluxed at 413 K for 30 minutes. After, the MWCNTs were washed with deionised water until the supernatant attained a ph around 7. Finally, the MWCNTs were dried at 373 K, and milled to become powder. Through this procedure it is possible to change the surface of the MWCNTs with additional functional groups, purifying the tubes and increasing its surface-to-volume ratio, which provides a strong dynamic force to speed up thermodynamic processes. The nanofluids were prepared by ultrasonic mixing combining with a magnetic stirrer in order to assure a good homogeneity of the suspension and increase the colloidal stability. Nanofluids with different volume fractions (0.25, 0.50, 0.75, 1.0 and 1.5%vol.) of MWCNTs were prepared. For the nanofluid with more MWCNT vol. concentration, it was performed an optimization of the mixingtime: 30, 60, 75 and 90 min. In Fig. 1(a), it is shown a graphical representation of the MWCNTs concentration after each mixing-time. From Fig. 1(a), it can be observed that the optimum mixingtime is achieved for 60 min., which corresponds to the higher value of MWCNT concentration, and consequently, to the most stable suspension. For the 75 and 90 min. mixture, the concentration of MWCNTs decreases, suggesting an agglomeration of the nanosized particles followed by settlement. Hong et al. studied the effect of ultrasonication time on the effective thermal conductivity of MWCNTs based nanofluid, obtaining the highest value for 60 min. of mixture [7]. As the studied nanofluid was the one with higher MWCNT concentration, it can be extrapolated the same mixing-time (60 min.) to the preparation of the other nanofluids with smaller MWCNT concentrations: 0.25, 0.5, 0.75 and 1.0% vol. The colloidal stability of the nanofluids was evaluated by UV-visible spectrophotometry, and the absorption/transmittance percentage of the solution was measured. All spectrophotometric methods that measure absorption resides upon two basic rules, which combined are known as the Beer-Lambert law that can be expressed as: (1) The Beer-Lambert law states that the absorbance of a solution (A) is directly proportional to the concentration of the absorbing species in the solution (a), and to the path length (L) [8-10].
Journal of Nano Research Vol. 13 71 A Shimadzu UVmini-1240 was used to evaluate the colloidal stability. The selected wavelength range was 200-500 nm, and the maximum of absorbance for the MWCNTs was observed at the interval [260; 270] nm. Figure 1: (a) Concentration of the 1.5% vol. MWCNT/EG nanofluid for different mixture time; (b) evaluation of the concentration ratio for the 1.5% vol. MWCNT/EG nanofluid (60min.) with time. From Error! Reference source not found.(a) it is clearly observed that the error for the 30 min. mixture is higher than those obtained for the other mixing-time. This can be explained by the nonuniformity of the mixture, due to the presence of agglomerates and settling down of the particles in the suspension. Although, the errors for the 60, 75 and 90 min. mixture are quite large, a fact that may be explained by the correlation obtained from the linear regression made with known concentrations samples (Calibration Curve). Therefore, to avoid the propagation of the correlation error the following mathematical formulation was used, expressing the concentration ratio: Through this formulation, the diluted samples can be placed in the UV-vis spectrophotometer for several hours, preventing measurement errors caused by mechanical motion, in the transportation and/or changing the samples. In Fig. 1(b) it can be depicted the variation of MWCNT concentration ratio with sedimentation time (after ultrasonication for 60 min.), for the 1.5% vol. MWCNT/EG nanofluid. The results show a convergence to 90% of the initial concentration, which is achieved in 100 hours. These results also show that the first 24h are detrimental for a good colloidal stability. Experimental results and discussion Thermal Conductivity (2) Figure 2: (a) Thermal conductivity for the prepared nanofluids and their evaluation with increasing temperature. (b) Thermal conductivity ratio with increasing temperature.
72 Journal of Nano Research Vol. 13 Thermal conductivity is a property of a material that indicates its ability to conduct heat. Atomically, the thermal conductivity of a system is determined by how atoms composing the system interact. The effective thermal conductivity of the nanofluids was measured by the transient hot wire (THW) method, the most widely used in the nanofluids research area. The effective thermal conductivity was measured for a range of temperatures from 283.15 K to 333.15 K. As the thermal conductivity measurements are sensitive to external vibrations that could influence the stability of the nanofluid under study, it was performed at least 20 measurements for each temperature. In order to increase the thermal stability it was used a circulating liquid bath (Kryo 30 solution) and a KD2 Pro thermal analyser (Decagon devices). The sample was inserted in a double jacket box connected to the circulating bath and resting in a Styrofoam box. The thermal conductivity of the pure EG was measured, with a maximum deviation of 1.3% from the theoretical values. Also, the maximum standard error that was obtained for all the nanofluids tested was about 0.91%. In Fig. 2(a) it is shown the effective thermal conductivity for all the tested nanofluids for different temperatures. A comparison is made with pure EG. As can be observed, with the incorporation of the MWCNTs, the thermal conductivity increases, even for the smallest studied MWCNT concentration. The experimental results present a slightly increase of the thermal conductivity with temperature. In Fig. 2(b) it is represented the normalized thermal conductivity of the prepared nanofluids. It can be observed a thermal conductivity improvement of 17% for the highest MWCNT concentration, and for the smaller concentration (0.25%), the enhancement of thermal conductivity is approximately 2%. Fig. 3 shows the evolution of the thermal conductivity with increasing MWCNTs volume fraction. As can be observed, there is an enhancement in thermal conductivity with increasing MWCNT volume fraction. It is also observed a slight increasing of thermal conductivity with temperature. Figure 3: Thermal conductivity evolution with volume fraction enhancement at 283.15, 298.15 and 333.15 K. The thermal conductivity of MWCNT/EG was also studied by Xie et al., and Liu et al. [11-12]. These authors observed a 12% enhancement for 1% vol. at room temperature. Several authors observed that several parameter including the aspect ratio and diameter of the nanotubes influence the effective thermal conductivity of the nanofluids, a fact that may well be responsible for the discrepancy between the published results [1, 5, 11-13]. Several models have been developed to predict the effective thermal conductivity of solid particle suspensions based on both thermal conductivity of solid particles and base fluid, their relative volume fraction, the nanoparticles shape, and working temperature. Hamilton and Crosser developed a model for non-spherical particles, depending on the thermal conductivity of both base fluid and particles and volume fraction [14]. Xue et al. proposed a model based on the Maxwell model and the average polarization theory, considering the effect of the interface among the solid particle and the base fluid [15]. Nan et al. developed a specific model for CNTs based nanofluids [13]. Murshed et al. also proposed a model to predict the effective thermal conductivity of
Journal of Nano Research Vol. 13 73 nanofluids considering the interfacial thermal resistance [16]. Several other models have been proposed, however, there still exists a large number of phenomena about heat transfer in nanofluids that cannot be predicted analytically. A comparison of the previous models to the presented experimental results, at room temperature (298.15K), can be made, and is depicted in Fig. 4. The graph shows that the experimental results present a linear tendency, although the models described do not fit within these results. These theoretical models consider some variables that are empirical, and the shape of the nanoparticles is quite different from the MWCNTs used in this study. Figure 4: Effective thermal conductivity of the nanofluids compared with the prediction models for 298.15 K Viscosity The viscosity of nanofluids is an important key factor for the applications of nanofluids as a new class of heat transfer fluids. The viscosity was measured using a controlled stress rheometer, Haake Model RS1. For accurate results, 5 measurements were made for shear rate ranging from 0 to 600 sec -1 for temperatures from 293.15 to 333.15 K. Figure 5: Dynamic viscosity vs volume fraction at temperature ranging from 293.15 to 333.15 K. All the nanofluids presented a non-newtonian behaviour, with the dynamic viscosity decreasing with the shear rate rise. In Fig. 5 are presented the results of the dynamic viscosity versus volume fraction, for a range of temperature from 293.15 to 333.15 K, and a shear rate of 300 s -1. The results show an increase of the viscosity consistent with CNTs loading. This enhancement of the viscosity could cause a pressure drop, and an increase in pumping power. With increasing temperature the viscosity of the nanofluid decreases which improves the Brownian motion of the nanoparticles. With an intensified Brownian motion, the contribution of microconvection in heat transport increases resulting in an enhancement of the thermal conductivity of nanofluids. However, the Brownian motion is not the only phenomenon responsible for the anomalous enhancement of the effective thermal conductivity of nanofluids, since the viscosity
74 Journal of Nano Research Vol. 13 increases with the increasing volume fraction, which presents the highest effective thermal conductivities. Xuan et al. [17] proposed that the increasing of the surface area due to suspended particles also contribute for the enhancement of the effective thermal conductivity. Keblinski et al. and Eastman et al. [18-19] also proposed that the molecular-level layering of the liquid at the liquid/particle interface influence the effective thermal conductivity. Several other authors proposed other theoretical explanations for the anomalous enhancement of the effective thermal conductivity, however, there are controversial. Thus, more experimental and analytical work needs to be done. Conclusion The colloidal stability was evaluated and results show that the nanofluids are stable after 100h.It was also pointed out that the ultrasonication time of mixture influences the colloidal stability and, it was established an optimum mixing-time of 60 min. for these nanofluids in particular. The thermal conductivity enhancement for the MWCNTs in EG suspension was investigated. The volume fraction, temperature and viscosity impact on the thermal conductivity enhancement was evaluated. It was observed an enhancement on thermal conductivity of about 17% for 1.5% vol. MWCNT/EG. The studies on viscosity have shown a significant rise in viscosity for all the MWCNTs concentrations on the nanofluids tested, proving that the Brownian motion is not the only phenomenon responsible for the anomalous enhancement of the effective thermal conductivity observed. References [1] S. U. S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood, E. A. Grulke: Appl. Phys. Lett. Vol. 79 (2001), p. 2252. [2] J. A. Eastman, U. S. Choi, S. Li, W. Yu, L. J. Thompson: Appl. Phys. Lett. Vol. 78 (2001), p. 718. [3] S. Choi, J. Eastman: paper presented at the ASME International Mechanical Engineering Congress & Exposition, San Francisco, 1995. [4] Y. Li, J. e. Zhou, S. Tung, E. Schneider, S. Xi: Powder Technology Vol. 196 (2009), p. 89. [5] W. Evans et al.: Int. J. Heat Mass Trans. Vol. 51 (2008), p. 1431. [6] K. Esumi, M. Ishigami, A. Nakajima, K. Sawada, H. Honda: Carbon 34 (1996), p. 279. [7] T.-K. Hong, H.-S. Yang, C. J. Choi: J. Appl. Phys. Vol. 97 (2005), p. 064311. [8] J. D. Ingle, S. R. Crouch: Spectrochemical analysis. (Prentice Hall, 1988). [9] W. B. Russel, D. A. Saville, W. R. Showalter (Cambridge University Press, Cambridge, 1989). [10] M. G. Gore, Ed.: Spectrophotometry and Spectrofluorimetry (Oxford University Press, 2000). [11] H. Xie, H. Lee, W. Youn, M. Choi: J Appl Phys Vol. 94 (2003), p. 4967. [12] M.-S. Liu, M. C.-C. Lin, I.-T. Huang, C.-C. Wang: Int. Commun. Heat Mass Vol. 32 (2005), p. 1202. [13] C.-W. Nan, G. Liu, Y. Lin, M. Li: Appl. Phys. Lett. Vol. 85 (2004), p. 3549. [14] R. L. Hamilton, O. K. Crosser: Ind. Eng. Chem. Fund. Vol. 1 (1962), p. 187. [15] Q.-Z. Xue: Physics Letters A Vol. 307 (2003), p. 313. [16] S. M. S. Murshed, K. C. Leong, C. Yang: Int. J. Therm. Sci. Vol. 47 (2008), p. 560. [17] Y. Xuan, Q. Li: Int. J. Heat Fluid Flow Vol. 21 (2000), p. 58. [18] P. Keblinski, S. R. Phillpot, S. U. S. Choi, J. A. Eastman: Int. J. Heat Mass Trans. Vol. 45(2002), p. 855. [19] J. A. Eastman, S. R. Phillpot, S. U. S. Choi, P. Keblinski: Annu. Rev. Mater. Res. Vol. 34 (2004), p. 219.
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