Dispersion behavior and thermal conductivity characteristics of Al2O3–H2O nanofluids
Introduction
Recently, due to the development of nanotechnology and surface science, many researches on nanofluids have been carried out actively. Nanofluid is a kind of new engineering material consisting of solid nanoparticles with sizes typically of 1–100 nm suspended in base fluids. It cannot only solve the problems such as sedimentation, cohesion and corrosion which happen conventionally in heterogeneous solid/liquid mixture with millimeter or micrometer particles, but also increase the thermal performance of base fluids remarkably [1], [2], [3], [4], [5], [6], [7], [8], [9]. Choi [1], Das et al. [4], Xuan et al. [5], Eastman et al. [6], [7] and Lee et al. [8] who found great enhancement of thermal conductivity (5–60%) over the volume fraction range of 0.1–5%. Patel et al. [10] concluded that 5–21% enhancement of the thermal conductivity of nanofluids for water with citrate in the temperature range 30–60 °C at a very low loading of 0.00026 vol% of Ag particles. For a loading of 0.011% of Au particles, the improvement of thermal conductivity was around 7–14%. Kumar et al. [11] reported an enhanced thermal conductivity of about 20% for a nanofluid of only 0.00013% Au nanoparticles in water. Since such an anomalous enhancement is expected to have wide applications in thermal engineering, nanofluids have received considerable attention in thermal science and engineering. However, it is very difficult to understand why nanofluids would have such a high thermal conductivity. Meanwhile, there are large differences among the thermal conductivities reported by different researchers. Keblinski et al. [12] further pointed out that the most exciting experimental results have not been reproducible. Therefore, it is necessary to reconsider the reliability of the measurements reported so far.
To explain the experimental results, Keblinski et al. [13] suggested the potential mechanisms for thermal conductivity enhancement such as Brownian motion, liquid layering and nanoparticle clustering. Koo and Kleinstreuer [14] found that the role of Brownian motion is much more important than the thermo-phoretic and osmo-phoretic motions. Lee et al. [15] experimentally investigated the effect of surface charge state of the nanoparticle in suspension on the thermal conductivity. They showed that the pH value of the nanofluid strongly affected the thermal performance of the fluid. With farther diverged pH value from the isoelectric point of the particles, the nanoparticles in the suspension got more stable so to change the thermal conductivity. That may partially explain the disparities between different experimental data since many researchers used surfactants in nanofluids. Vadasz [16] demonstrated that the transient heat conduction process in nanofluids may provide a valid explanation for the apparent heat transfer enhancement.
Many researches are conducted to enhance the thermal conductivity of nanofluid and also to produce more stable suspensions. Studies to date have shown that the amount and the charge of nanoparticles in the nanofluid, and the interaction between the particles and the dispersant directly affect the stability of the suspension. Agitation [17], [18], changing pH values of the suspension [19], [20] and adding surfactants [21], [22] have been applied to reduce the coagulation of nanoparticles in the nanofluid. Although the stability of nanofluid is very important for its application, there is a little study on estimating the stability of suspension. Zeta potential and UV–vis spectrophotometric measurements have been used to quantitatively characterize colloidal stability of the dispersions [23], [24], [25]. Lisunova et al. [26] studied the stability of the aqueous suspensions of nanotubes in the presence of nonionic surfactant, and found the addition of surfactant exerts a stabilizing effect at surfactant concentration Cs proportional to the weight concentration C of multiwalled carbon nanotubes (MWNTs), Cs ≈ C mol/dm3. These studies have given a good foundation for highly homogenous, industrial enhanced heat transfer suspension fluids. Karimian et al. [27] investigated the dispersion behavior of colloidal Al2O3 aqueous suspensions in the presence of highly charged CeO2 nanoparticles and concluded the stability of these bidispersed systems reaches an optimum condition at pH 10 by increasing ceria nanoparticle concentration. The calculated Derjaguin–Landau–Verwey–Overbeek (DLVO) interparticle interaction potentials verified the experimental results of the pH effect on the stability behavior.
In order to study the stability and thermal conductivity of Al2O3 nanoparticles in aqueous solutions, during this work, Al2O3 nanoparticle suspensions were synthesized, their dispersion behaviors and thermal conductivity in water were investigated under different pH values and different sodium dodecylbenzenesulfonate (SDBS) concentration. It is expected to provide guidance to design nanofluids with excellent performance.
Section snippets
Chemical
The Al2O3 powder (Alfa Aesar, Ward Hill, MA, USA) with alumina content >99.9% was used in the study. It has a BET surface area of 100 m2/g, and a median particle size of 15–50 nm, and a density of 3965 kg/m3. The surface area was determined using the Micromeretic Tristar 3000. The particle size was measured using the X-ray disk centrifuge particle size analyser ver. 3.49. The transmission electron micrograph (TEM) of alumina powder is shown in Fig. 1. In Fig. 1, there are a few larger particles,
Transient plane source (TPS) theory
Thermal conductivity of nanofluids is measured by means of the TPS method [28]. In this method, the TPS element behaves both as temperature sensor and heat source. This novel method offers some advantages such as fast and easy experiments, wide range of thermal conductivities (from 0.02 to 200 W/m K), no sample preparation and flexible sample size.
The TPS element consists of an electrical conducting pattern of thin nickel foil (10 μm) in the form of double spiral, which resembles a hot disk,
Preparation of nanofluids
Ultrasonication was used for preparation of mixed aqueous nano-suspensions, which is an accepted technique for dispersing the highly entangled or aggregated nanoparticle samples [29], [30], but longer time of high-energy sonication can introduce defects. In the study, alumina nanoparticle (0.1 g) and a water solution (99.8 g) with SDBS surfactant (0.1 g) were directly mixed in a 150 ml beaker. The suspension was transferred into an ultrasonic vibrator and sonicated for 1 h at a frequency of 40 kHz
Conclusions
This paper is concerned with the dispersion behaviors and thermal conductivity of Al2O3–H2O nanofluid under different pH values and different sodium dodecylbenzenesulfonate (SDBS) concentration. Key conclusions can be summarized as follows:
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Zeta potential and absorbency are important basis for selecting conditions for dispersing particles. There is a good correlation between absorbency and zeta potential. The higher the absolute value of zeta potential is, the greater the absorbency is, and the
Acknowledgments
The authors like to acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 20346001), Program for New Century Excellent Talents in University (Grant No. NCET-04-0826), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20050561017), Post-doctor Foundation of China (Grant No. 20060400219) for the research work.
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