Showing papers on "Nanofluid published in 2003"

Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Abstract: Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

Temperature dependence of thermal conductivity enhancement for nanofluids

Abstract: Usual heat transfer fluids with suspended ultra fine particles of nanometer size are named as nanofluids, which have opened a new dimension in heat transfer processes. The recent investigations confirm the potential of nanofluids in enhancing heat transfer required for present age technology. The present investigation goes detailed into investigating the increase of thermal conductivity with temperature for nano fluids with water as base fluid and particles of Al 2 O 3 or CuO as suspension material. A temperature oscillation technique is utilized for the measurement of thermal diffusivity and thermal conductivity is calculated from it

The Role of Interfacial Layers in the Enhanced Thermal Conductivity of Nanofluids: A Renovated Maxwell Model

Abstract: Nanofluids, a new class of solid/liquid suspensions, offer scientific challenges because their measured thermal conductivity is one order of magnitude greater than predictions. It has long been known that liquid molecules close to a solid surface form layered solid-like structures, but little is known about the connection between this nanolayer and the thermal properties of the suspensions. Here, we have modified the Maxwell equation for the effective thermal conductivity of solid/liquid suspensions to include the effect of this ordered nanolayer. Because this ordered nanolayer has a major impact on nanofluid thermal conductivity when the particle diameter is less than 10 nm, a new direction is indicated for development of next-generation coolants.

Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer

Abstract: The present study is to enhance the critical heat flux (CHF) in pool boiling from a flat square heater immersed in nanofluid (water mixed with extremely small amount of nanosized particles). The test results show that the enhancement of CHF was drastic when nanofluid is used as a cooling liquid instead of pure water. The experiment was performed to measure and compare pool boiling curves of pure water and nanofluid at the pressure of 2.89 psia (Tsat=60 °C) using 1×1 cm2 polished copper surfaces as a boiling surface. The tested nanofluid contains alumina (Al2O3) nanoparticles dispersed in distilled and deionized water. Tested concentrations of nanoparticles range from 0 g/l to 0.05 g/l. The measured pool boiling curves of nanofluids saturated at 60 °C have demonstrated that the CHF increases dramatically (∼200% increase) compared to pure water case; however, the nucleate boiling heat transfer coefficients appear to be about the same.

Spreading of nanofluids on solids

Abstract: Suspensions of nanometre-sized particles (nanofluids) are used in a variety of technological contexts. For example, their spreading and adhesion behaviour on solid surfaces can yield materials with desirable structural and optical properties1. Similarly, the spreading behaviour of nanofluids containing surfactant micelles has implications for soil remediation, oily soil removal, lubrication and enhanced oil recovery. But the well-established concepts of spreading and adhesion of simple liquids do not apply to nanofluids2,3,4,5,6,7. Theoretical investigations have suggested that a solid-like ordering of suspended spheres will occur in the confined three-phase contact region at the edge of the spreading fluid, becoming more disordered and fluid-like towards the bulk phase8,9. Calculations have also suggested that the pressure arising from such colloidal ordering in the confined region will enhance the spreading behaviour of nanofluids10,11. Here we use video microscopy to demonstrate both the two-dimensional crystal-like ordering of charged nanometre-sized polystyrene spheres in water, and the enhanced spreading dynamics of a micellar fluid, at the three-phase contact region. Our findings suggest a new mechanism for oily soil removal—detergency.

Thermal conductivities of naked and monolayer protected metal nanoparticle based nanofluids: Manifestation of anomalous enhancement and chemical effects

Abstract: Thermal conductivities of two kinds of Au nanoparticles were measured in water and toluene media. The water soluble particles, 10–20 nm in mean diameter, made with citrate stabilization showed thermal conductivity enhancement of 5%–21% in the temperature range of 30–60 °C at a loading of 0.000 26 (by volume). The effect was 7%–14% for Au particles stabilized with a monolayer of octadecanethiol even for a loading of 0.011%. Comparatively lower thermal conductivity enhancement was observed for larger diameter Ag particles for significantly higher loading. Effective enhancement of 9%, even at vanishing concentrations, points to additional factors in the thermal conductivity mechanism in nanofluids. Results also point to important chemical factors such as the need for direct contact of the metal surface with the solvent medium to improve enhancement.

Nanofluids containing multiwalled carbon nanotubes and their enhanced thermal conductivities

Abstract: Multiwalled carbon nanotubes (CNTs) as produced are usually entangled and not ready to be dispersed into fluids. We treated CNTs by using a concentrated nitric acid to disentangle CNT aggregates for producing CNT nanofluids. Oxygen-containing functional groups have been introduced on the CNT surfaces and more hydrophilic surfaces have been formed during this treatment, which enabled to make stable and homogeneous CNT nanofluids. Treated CNTs were successfully dispersed into polar liquids like distilled water, ethylene glycol without the need of surfactant and into nonpolar fluid like decene with oleylamine as surfactant. We measured the thermal conductivities of these nanotube suspensions using a transient hot wire apparatus. Nanotube suspensions, containing a small amount of CNTs, have substantially higher thermal conductivities than the base fluids, with the enhancement increasing with the volume fraction of CNTs. For the suspensions with the same loading, the enhanced thermal conductivity ratios are re...

Aggregation structure and thermal conductivity of nanofluids

Abstract: Nanofluids are obtained by suspending metallic nanoparticles in conventional base liquids. Such a new class of heat-transfer fluid is superior to the base liquids in energy-transport performance, which depends on the distribution, volume fraction and thermal properties of the suspended nanoparticles. The theory of Brownian motion and the diffusion-limited aggregation model are applied to simulate random motion and the aggregation process of the nanoparticles. A theoretical model is developed to predict the thermal conductivity of nanofluids. Comparison between the theoretical and experimental results shows the validity and accuracy of the theoretical model.

Complex fluids: Spread the word about nanofluids

Abstract: Liquid spreads and wets a surface, but wetting behaviour changes if the liquid contains nanoparticles. New experiments point to an explanation and a way to create effective detergents for cleaning oil from a surface.

Improvement on thermal performance of a disk-shaped miniature heat pipe with nanofluid

Abstract: The present study used nanofluid as the working medium for a disk-shaped miniature heat pipe (DMHP) The nanofluid is a suspension with gold nanoparticles of an average diameter of 17 nm in an aqueous solution An experimental system was set up to measure the thermal resistance of the disk-shaped miniature heat pipe (DMHP) with both nanofluid and DI-water A mounting base is designed and integrated with the DMHP as a heat spreader for a laser diode TO can package The present mounting base is made of aluminum (6061 T6) The measured results show that the thermal resistance of the DMHP varies with the charge volume and the type of working medium At the same charge volume, a significant reduction in the thermal resistance of the DMHP can be found if nanofluid is used instead of DI-water

Method for synthesis of carbon nanotubes

Abstract: Carbon nanotubes are formed by chemical vapor deposition using metal nanoparticles as a growth substrate. Control over the size and properties of the carbon nanotubes is achieved by controlling the size of the metal nanoparticles in the growth substrate. The metal nanoparticles of a controlled size may be formed by a thermal decomposition reaction of a metal salt in a passivating solvent.

Study on the mechanism of heat conduction in nanofluid medium

Abstract: The heat conduction behaviour in a nanofluid fluid medium has many abnormal properties. Combined with the analysis based on microscale heat transfer theory and the physicochemical behaviours of nanofluid, the mechanism of heat conduction in nanofluid has been studied. The effects of nonlinear heat transfer in nanoparticle, micro convection are caused by the Brownian movement of nanoparticles, congregation of nanoparticles, and orderly array of liquid molecules at the interface between the nanoparticle surface and the base fluid.

Measurement of thermal conductivities of nanofluids

Abstract: The transient hot-wire method is used to measure the thermal conductivities of nanofluids.The experimental results show that nanofluids have remarkably higher thermal conductivities than those of conventional pure fluids.The thermal conductivities of nanofluids are not only influenced by the volume fraction of nanoparticles, but also by other factors such as properties and dimensions of nanoparticles.By analyzing the experimental results, a correlation of thermal conductivity of nanofluid is presented.

Stability of Nano-particle Suspensions

Abstract: Due to the increasing development of nanotechnology and its practical applications, the thermal conductivity of a nanoparticle suspension is becoming a frontier of nanofluid and its application in enhancing heat transfer It is highly expected that the addition of nanoparticles can significantly enhance the conductivity of the liquid without clogging, erosion, sedimentation and increase in pressure drop, which usually are encountered in common particle suspensions However, the stability of nanoparticle suspension is not still as good as expected in practical experiments Available investigations indicate that the stability can be improved choosing proper combination of nanoparticle and base fluid In this paper, a series of experimental investigations are conducted to observe the stability of nanoparticle suspensions with different combinations of nanoparticles and base fluids Experimental evidences demonstrate the influence of particle geometrical and physical properties, fluid properties, particularly viscosity on the stability The most important factors for the stability of nanoparticle suspensions are effective diameter of nanoparticle cluster and viscosity of base fluids Decreasing the effective diameter of nanoparticle cluster and increasing the viscosity of base fluids can significantly increase the stability of nanoparticle suspension

Determining the Effective Thermal Conductivity of a Nanofluid Using Brownian Dynamics Simulation

Abstract: A nanofluid is a fluid containing suspended solid particles, with sizes of the order of nanometers Normally the fluid has a low thermal conductivity compared to the suspended particles Therefore introduction of these particles into the fluid increases the effective thermal conductivity of the system It is of interest to predict the effective thermal conductivity of such a nanofluid under different conditions like varying particle volume fraction, varying particle size, changing fluid conductivity or changing fluid viscosity, especially since only limited experimental data are available Also, some controversy exists about the role of Brownian motion in enhancing the nanofluid’s thermal conductivity We have developed a novel technique to compute the effective thermal conductivity of a nanofluid using Brownian dynamics simulation, which has the advantage of being computationally less expensive than molecular dynamics We obtain the contribution of the nanoparticles towards the effective thermal conductivity using the equilibrium Green-Kubo method Then we combine that with the thermal conductivity of the base fluid to obtain the effective thermal conductivity of the nanofluid, and thus are able to show that the Brownian motion contributes greatly to the thermal conductivityCopyright © 2003 by ASME

Flow and heat transfer model of nanofluids based on lattice - boltzmann method

Abstract: The nanofluid is a two-phase suspension that consists of a base liquid and suspended nanoparticles A number of acting forces such as drag force, Brownian force, dispersion force and gravitational force affect flow behavior of the suspended nanoparticles, which complicates description of nanofluid flow The lattice Boltzmann (LB) method is used to develop a model for the nanofluid and to investigate flow structure of nanofluids and heat transfer inside the nanofluids with taking these forces into account Flow patterns and the temperature profiles of the sample nanofluid are simulated

Calculation of the thermal conductivity of nanofluids containing nanoparticles and carbon nanotubes

Abstract: A mathematical model for the calculation of thermal conductivity of nanofluids containing nanoparticles and nanotubes on the basis of statistical mechanics is presented. This appears to be the first such attempt published in the scientific literature. We focus on the influence of the liquid layer structure around nanoparticles as the reason for enhancement of the thermal conductivity of nanofluids. Analytical results of both the current analysis and statistical mechanics are compared with experimental data reported in the scientific literature, which show good agreement.