Nanofluids thermal conductivity

W.H. Yu, D M. France, J.L. Routbort and S.U.S. Choi, Review and comparison of nanofluid thermal conductivity and heat transfer enhancements. Heat... 

Koo and Kleinstreuer [8] investigated laminar nanofluid flow in micro-heat sinks using the effective nanofluid thermal conductivity model they had established [24]. For the effective viscosity due to micromixing in suspensions, they proposed ... 

Carbon nanotubes (CNTs) Diamond nanoparticles Nanofluids Thermal conductivity... 

Stability of the Resulting Nanofluids We have recently reported that plasma treatment of diamond nanoparticles significantly improved their suspension stability in water [10]. After 45-day sediment test, the nanofluids using plasma-treated diamond nanoparticles did not give any phase separation, while the nanofluids using untreated controls showed clear phase separation of water. In this study, the settling time dependence of the nanofluid thermal conductivity... 

As shown in Fig. 10, for plasma-treated diamond nanofluids, about 20 % thermal conductivity increase was observed with addition of 0.15 vol.% of plasma-treated diamond nanoparticle. Despite of the higher thermal conductivity of CNTs than diamond materials, no significant increase in thermal conductivity was observed fi om stable plasma-treated CNT nanofluids. This finding suggests that nanofluid thermal conductivity is affected more by the dispersion stability of nanomaterials than thermal conductivities of nanomaterials themselves. 

Evans, W., Fish, J., and Keblinski, P. 2006. Role of Brownian motion hydrodynamics on nanofluid thermal conductivity , Appl. Phys. Lett. Vol. 88 Article ID 093116. 

The concept of nanofluids to further intensify microreactors has been discussed by Fan et al. [17]. The nano-fluids are suspensions of solid nano-partides with sizes typically of 1-100 nm in traditional liquids such as water, glycol and oils. These solid-liquid composites are very stable and show higher thermal conductivity and higher convective heat transfer performance than traditional liquids. They can thus be used to enhance the heat transfer in nanofluids in compact multifunctional reactors. A nanofluid based on Ti02 material dispersed in ethylene glycol showed an up to 35% increase in the overall heat transfer coefficient and a... 

Abstract. In this study we report a literature review on the research and development work concerning thermal conductivity of nanofluids as well as their viscosity. Different techniques used for the measurement of thermal conductivity of nanofluids are explained, especially the 3co method which was used in our measurements. The models used to predict the thermal conductivity of nanofluids are presented. Our experimental results on the effective thermal conductivity by using 3co method and effective viscosity by vibro-viscometer for Si02-water, Ti02-water and A Os-water nanofluids at different particle concentrations and temperatures are presented. Measured results showed that the effective thermal conductivity of nanofluids increase as the concentration of the particles increase but not anomalously as indicated in the some publications and this enhancement is very close to Hamilton-Crosser model, also this increase is independent of the temperature. The effective viscosities of these nanofluids increased by the increasing particle concentration and decrease by the increase in temperature, and cannot be predicted by Einstein model. 

There are many publications on predictive models for effective thermal conductivity of nanofluids [5, 11, 23, 25-27], some of these publications make an overview of the existing models, and some drives their own model and compares with experimental data. None of the models is able to explain and predict an effective thermal conductivity value for the nanofluids. 

Many theoretical and empirical models have been proposed to predict the effective thermal conductivity of two phase mixtures. Comprehensive review articles have discussed the applicability of many of these models that appear to be more promising [34-36]. First, using potential theory. Maxwell [20] obtained a simple relationship for the conductivity of randomly distributed and non-interacting homogeneous spheres in a homogeneous medium. Maxwell model is good for low solid concentrations. Relative thermal conductivity enhancement (ratio of the effective thermal conductivity keffO nanofluid to base fluid kj) is. 

Yu and Choi [38] derived a model for the effective thermal conductivity of nanofluid by assuming that there is no agglomeration by nanoparticles in nanofluids. They assumed that the nanolayer surrounding each particle could combine with the particle to form an equivalent particle and obtained the equivalent thermal conductivity kpe of equivalent particles as fallows. 

Xie et al. [40] derived an expression for calculating enhanced thermal conductivity of nanofluid by considering The effects of nanolayer thickness, nanoparticle size, volume fraction, and thermal conductivity ratio of particle to fluid. The expression is ... 

Besides these models there are many others models, but no single model explains the effective thermal conductivity in all cases. Besides the thermal conductivities of the base fluid and nanoparticles and the volume fraction of the particles, there are many other factors influencing the effective thermal conductivity of the nanofluids. Some of these factors are the size and shape of nanoparticles, the agglomeration of particle, the mode of preparation of nanofluids, the degree of purity of the particles, surface resistance between the particles and the fluid. Some of these factors may not be predicted adequately and may be changing with time. This situation emphasizes the importance of having experimental results for each special nanofluid produced. 

Properties of nanoparticles and base fluid used in this study are shown in Table 1. De-ionized water was used as a base fluid. In the nanofluid, nanoparticles tend to cluster and form agglomerates which reduce the effective thermal conductivity. It is known that ultrasonication break the nanoclusters into smaller clusters. Hong et al. [41] investigated the role of sonication time on thermal conductivity of iron (Fe) nanofluids. The thermal conductivity of each nanofluid showed saturation after a gradual increase as the sonication time was increased. The thermal conductivity of 0.2 vol% Fe nanofluid exhibited 18% enhancement with a 30 min sonication and was saturated after 30 min. So, in order to obtain good quality nanofluids, it is essential that the solid-liquid mixture be exposed to ultrasonication. 

Figure 4. Relative thermal conductivity of (1% vol.) Ti02-water nanofluid as a function of the sonication time.

Figure 5. Effect of SDBS surfactant on relative thermal conductivity of 1% by volume Al203-water nanofluids at different mass ratio of SDBS/AI2O3.

The transient hot wire (THW) method has been well developed and widely used for measurements of the thermal conductivities and, in some cases, the thermal diffusivities of fluids with a high degree of accuracy [6, 42]. More than 80% of the thermal conductivity measurements on nanofluids were performed by transient hot wire method [6, 8, 18, 19, 45-47]. 

Another method for measuring thermal diffusivity is the flash method developed by Parker et al. [48] and successfully used for the thermal diffusivity measurement of solid materials [49]. A high intensity short duration heat pulse is absorbed in the front surface of a thermally insulated sample of a few millimeters thick. The sample is coated with absorbing black paint if the sample is transparent to the heat pulse. The resulting temperature of the rear surface is measured by a thermocouple or infrared detector, as a function of time and is recorded either by an oscilloscope or a computer having a data acquisition system. The thermal diffusivity is calculated from this time-temperature curve and the thickness of the sample. This method is commercialized now, and there are ready made apparatus with sample holders for fluids. There is only one publication on nanofluids with this method. Shaikh et al. [50] measured thermal conductivity of carbon nanoparticle doped PAO oil. 

Figure 10. Relative thermal conductivity versus particle volume fraction of Xi02, AI2O3 and Si02 nanofluids.

S.K. Das, N. Putra, P. Thiesen and W. Roetzel, Temperature dependence of thermal conductivity enhancement for nanofluids, Journal of Heat Transfer, 125, 567-574 (2003). 

S.M.S. Murshed, K.C. Leong and C. Yang, Enhanced thermal conductivity of Ti02-water based nanofluids, International Journal of Thermal Science, 44, 367-373 (2005). 

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