Showing papers on "Nanofluid published in 2007"

Investigation into the antibacterial behaviour of suspensions of ZnO nanoparticles (ZnO nanofluids)

Abstract: The antibacterial behaviour of suspensions of zinc oxide nanoparticles (ZnO nanofluids) against E. Coli has been investigated. ZnO nanoparticles from two sources are used to formulate nanofluids. The effects of particle size, concentration and the use of dispersants on the antibacterial behaviour are examined. The results show that the ZnO nanofluids have bacteriostatic activity against E. coli. The antibacterial activity increases with increasing nanoparticle concentration and increases with decreasing particle size. Particle concentration is observed to be more important than particle size under the conditions of this work. The results also show that the use of two types of dispersants (Polyethylene Glycol (PEG) and Polyvinylpyrolidone (PVP)) does not affect much the antibacterial activity of ZnO nanofluids but enhances the stability of the suspensions. SEM analyses of the bacteria before and after treatment with ZnO nanofluids show that the presence of ZnO nanoparticles damages the membrane wall of the bacteria. Electrochemical measurements using a model DOPC monolayer suggest some direct interaction between ZnO nanoparticles and the bacteria membrane at high ZnO concentrations.

Critical review of heat transfer characteristics of nanofluids

Abstract: Researches in heat transfer have been carried out over the previous several decades, leading to the development of the currently used heat transfer enhancement techniques. The use of additives is a technique applied to enhance the heat transfer performance of base fluids. Recently, as an innovative material, nanometer-sized particles have been used in suspension in conventional heat transfer fluids. The fluids with these solid-particle suspended in them are called ‘nanofluids’. The suspended metallic or nonmetallic nanoparticles change the transport properties and heat transfer characteristics of the base fluid. The aim of this review is to summarize recent developments in research on the heat transfer characteristics of nanofluids for the purpose of suggesting some possible reasons why the suspended nanoparticles can enhance the heat transfer of conventional fluids and to provide a guide line or perspective for future research.

Thermal Conductivity and Particle Agglomeration in Alumina Nanofluids: Experiment and Theory

Abstract: In recent years many experimentalists have reported an anomalously enhanced thermal conductivity in liquid suspensions of nanoparticles. Despite the importance of this effect for heat transfer applications, no agreement has emerged about the mechanism of this phenomenon, or even about the experimentally observed magnitude of the enhancement. To address these issues, this paper presents a combined experimental and theoretical study of heat conduction and particle agglomeration in nanofluids. On the experimental side, nanofluids of alumina particles in water and ethylene glycol are characterized using thermal conductivity measurements, viscosity measurements, dynamic light scattering, and other techniques. The results show that the particles are agglomerated, with an agglomeration state that evolves in time. The data also show that the thermal conductivity enhancement is within the range predicted by effective medium theory. On the theoretical side, a model is developed for heat conduction through a fluid containing nanoparticles and agglomerates of various geometries. The calculations show that elongated and dendritic structures are more efficient in enhancing the thermal conductivity than compact spherical structures of the same volume fraction, and that surface (Kapitza) resistance is the major factor resulting in the lower than effective medium conductivities measured in our experiments. Together, these results imply that the geometry, agglomeration state, and surface resistance of nanoparticles are the main variables controlling thermal conductivity enhancement in nanofluids.

A critical review of convective heat transfer of nanofluids

Abstract: A nanofluid is a suspension of ultrafine particles in a conventional base fluid which tremendously enhances the heat transfer characteristics of the original fluid. Furthermore, nanofluids are expected to be ideally suited in practical applications as their use incurs little or no penalty in pressure drop because the nanoparticles are ultrafine, therefore, appearing to behave more like a single-phase fluid than a solid–liquid mixture. About a decade ago, several published articles focused on measuring and determining the effective thermal conductivity of nanofluids, some also evaluated the effective viscosity. There are only a few published articles on deriving the forced convective heat transfer of nanofluids. The purpose of this article is to summarize the published subjects respect to the forced convective heat transfer of the nanofluids both of experimental and numerical investigation.

Rheological behaviour of nanofluids

Abstract: This work aims at a more fundamental understanding of the rheological behaviour of nanofluids and the interpretation of the discrepancy in the recent literature. Both experiments and theoretical analyses are carried out with the experimental work on ethylene glycol (EG)-based nanofluids containing 0.5–8.0 wt% spherical TiO2 nanoparticles at 20–60 °C and the theoretical analyses on the high shear viscosity, shear thinning behaviour and temperature dependence. The experimental results show that the EG-based nanofluids are Newtonian under the conditions of this work with the shear viscosity as a strong function of temperature and particle concentration. The relative viscosity of the nanofluids is, however, independent of temperature. The theoretical analyses show that the high shear viscosity of nanofluids can be predicted by the Krieger–Dougherty equation if the effective nanoparticle concentration is used. For spherical nanoparticles, an aggregate size of approximately 3 times the primary nanoparticle size gives the best prediction of experimental data of both this work and those from the literature. The shear thinning behaviour of nanofluids depends on the effective particle concentration, the range of shear rate and viscosity of the base liquid. Such non-Newtonian behaviour can be characterized by a characteristic shear rate, which decreases with increasing volume fraction, increasing base liquid viscosity, or increasing aggregate size. These findings explain the reported controversy of the rheological behaviour of nanofluids in the literature. At temperatures not very far from the ambient temperature, the relative high shear viscosity is independent of temperature due to negligible Brownian diffusion in comparison to convection in high shear flows, in agreement with the experimental results. However, the characteristic shear rate can have strong temperature dependence, thus affecting the shear thinning behaviour. The theoretical analyses also lead to a classification of nanofluids into dilute, semi-dilute, semi-concentrated and concentrated nanofluids depending on particle concentration and particle structuring.

Effects of Various Parameters on Nanofluid Thermal Conductivity

Abstract: The addition of a small amount of nanoparticles in heat transfer fluids results in the new thermal phenomena of nanofluids (nanoparticle-fluid suspensions) reported in many investigations. However, traditional conductivity theories such as the Maxwell or other macroscale approaches cannot explain the thermal behavior of nanofluids. Recently, Jang and Choi proposed and modeled for the first time the Brownian-motion-induced nanoconvection as a key nanoscale mechanism governing the thermal behavior of nanofluids, but did not clearly explain this and other new concepts used in the model. This paper explains in detail the new concepts and simplifying assumptions and reports the effects of various parameters such as the ratio of the thermal conductivity of nanoparticles to that of a base fluid, volume fraction, nanoparticle size, and temperature on the effective thermal conductivity of nanofluids. Comparison of model predictions with published experimental data shows good agreement for nanofluids containing oxide, metallic, and carbon nanotubes.

Thermal Conductivity of Metal-Oxide Nanofluids: Particle Size Dependence and Effect of Laser Irradiation

Abstract: The thermal conductivity of water- and ethylene glycol-based nanofluids containing alumina, zinc-oxide, and titanium-dioxide nanoparticles is measured using the transient hot-wire method. Measurements are performed by varying the particle size and volume fraction, providing a set of consistent experimental data over a wide range of colloidal conditions. Emphasis is placed on the effect of the suspended particle size on the effective thermal conductivity. Also, the effect of laser-pulse irradiation, i.e., the particle size change by laser ablation, is examined for ZnO nanofluids. The results show that the thermal-conductivity enhancement ratio relative to the base fluid increases linearly with decreasing the particle size but no existing empirical or theoretical correlation can explain the behavior. It is also demonstrated that high-power laser irradiation can lead to substantial enhancement in the effective thermal conductivity although only a small fraction of the particles are fragmented.

Experimental investigation of viscosity and specific heat of silicon dioxide nanofluids

Abstract: Results of an experimental investigation into the viscosity and specific heat of silicon dioxide (SiO 2 ) nanoparticles with various diameters (20, 50 and 100 nm) suspended in a 60:40 (by weight) ethylene glycol and water mixture are presented. Nanofluids with particle volume percentages ranging from 0 to 10% were examined. Viscosity experiments were carried out over wide temperature ranges, from -35 to 50degC, to demonstrate their applicability in cold regions. The nano- particle diameter effect on the rheology of the SiO 2 nanofluid is explored. Non-Newtonian behaviour was observed for the particle volume concentrations of these nanofluids at sub-zero temperatures. A new correlation was developed from experimental data, which related viscosity with particle volume percent and nanofluid temperature. The specific heats of the SiO 2 nanofluids for various particle volume concentrations are presented.

Enhancement of thermal conductivity in magnetite based nanofluid due to chainlike structures

Abstract: We observe a dramatic enhancement of thermal conductivity in a nanofluid containing magnetite particles of average diameter of 6.7nm under the influence of an applied magnetic field. The maximum enhancement in the thermal conductivity observed is 300% (k∕kf=4.0) at a particle loading of 6.3vol%. The increase in thermal conductivity is attributed to the effective conduction of heat through the chainlike structures formed in the nanofluid. This finding is consistent with the theoretical prediction of enhanced thermal conductivity in nanofluid containing fractal aggregates [R. Prasher et al., Appl. Phys. Lett.89, 143119 (2006)].

Effect of nanoparticle deposition on capillary wicking that influences the critical heat flux in nanofluids

Abstract: When nanofluids are boiled, nanoparticles are deposited on the heater surface, causing a significant critical heat flux (CHF) enhancement. The authors examined the effect of the surface wettability and the capillarity of the nanoparticle deposition layer on CHF. It is well known that the deposition of nanoparticles changes the surface wettability, but it also causes capillary wicking on a porous surface, whereby the supplied liquid effectively delays the irreversible growth of a dry patch. This study demonstrates that the outstanding CHF enhancement in nanofluids is the consequence of both the improved surface wettability and the capillarity of the nanoparticle deposition layer.