Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100 nm) Nanofluids have higher thermal conductivity' and single-phase heat transfer coefficients than their base fluids In particular the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter's In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid These are inertia, Brownian diffusion, thermophoresis, diffusioplwresis, Magnus effect, fluid drainage, and gravity We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies so an effect on turbulence intensity is also doubtful Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement A correlation structure that captures these effects is proposed

Convective Transport in Nanofluids

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.

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Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al2O3) and titanium dioxide (TiO2), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l04-I05 and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al2O3 and TiO2 particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt n...

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Effective thermal conductivity of mixtures of e uids and nanometer-size particles is measured by a steady-state parallel-plate method. The tested e uids contain two types of nanoparticles, Al 2O3 and CuO, dispersed in water, vacuum pump e uid, engine oil, and ethylene glycol. Experimental results show that the thermal conductivities of nanoparticle ‐e uid mixtures are higher than those of the base e uids. Using theoretical models of effective thermal conductivity of a mixture, we have demonstrated that the predicted thermal conductivities of nanoparticle ‐e uid mixtures are much lower than our measured data, indicating the dee ciency in the existing models when used for nanoparticle ‐e uid mixtures. Possible mechanisms contributing to enhancement of the thermal conductivity of the mixtures are discussed. A more comprehensive theory is needed to fully explain the behavior of nanoparticle ‐e uid mixtures. Nomenclature cp = specie c heat k = thermal conductivity L = thickness Pe = Peclet number P q = input power to heater 1 r = radius of particle S = cross-sectional area T = temperature U = velocity of particles relative to that of base e uids ® = ratio of thermal conductivity of particle to that of base liquid ¯ = .® i 1/=.® i 2/ ° = shear rate of e ow Ω = density A = volume fraction of particles in e uids Subscripts

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Thermal Conductivity of Nanoparticle -Fluid Mixture

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

We investigate the behavior of friction and heat transfer coefficients of water flowing turbulently in a relatively long (i.e., 950 diameter long) circular pipe. When a large heat flux was applied at the wall, the viscosity of water significantly decreased along the axial direction due to the increasing temperature of water. A concept of a redeveloping region was introduced, where the local heat transfer coefficient increased while the local friction coefficient decreased due to the above-mentioned viscosity change. We propose the use of local bulk-mean temperature to determine local Nusselt numbers by using local Reynolds (Re LB ) and Prandtl numbers (PT LB ), a method that automatically took into account the effect of axial viscosity change on the evaluation of local heat transfer coefficients. A new turbulent heat transfer correlation for the prediction of the local Nusselt number is given as Nu x = 0.00425 Re LB 0.079 Pr LB O.4 (μ w /μ h ) -0.11 .

Local Friction and Heat Transfer Behavior of Water in a Turbulent Pipe Flow With a Large Heat Flux at the Wall

There is an increasing interest in the study of direct plasma generated in liquid/ bioliquid as it fi nds more applications in both industry and academic research. For all the applications, it is important to get a better understanding of the key physical mechanisms of the breakdown process. In the present paper, streamer propagation during an electric breakdown process of dielectric liquid was analyzed quantitatively using two different mechanisms based on electrostatic expansion and local heating. It was proposed that at the early stage of the streamer propagation, the electrostatic force due to the charging of a liquid-gas interface under a high electric fi eld might be the major driving force for fi lament growth. Over a submicrosecond time scale, the local heating might dominate the streamer propagation process, and the growth of the fi lament could be caused by the continuous evaporation of liquid at the tip of the streamer. Analysis of linear instabilities that lead to the bushlike growth of the streamers was carried out. Both classic Rayleigh-Taylor instability and electric fi eld–induced instability were identifi ed. It was shown that with an increasing applied voltage, the electrostatic instability was enhanced, whereas the Rayleigh instability was suppressed.

Analysis of Streamer Propagation for Electric Breakdown in Liquid/Bioliquid

An implantable intravascular device for diverting blood flow from the aorta into at least one renal artery of a living being to effect a reduction in the being's blood pressure. The device comprises an anchoring means and a blood flow diverting means located on the anchoring means. The blood flow diverting element is arranged for movement between a stowed position and an extended position to effect the controlled diversion of blood into the at least one renal artery. A method of lowering blood pressure of a living being, the method comprising the steps of introducing a device into the aorta of a living being adjacent at least one renal artery whereupon the device increases the flow of blood to the at least one renal artery.

Device and method for reducing blood pressure

A drying structure and process has a wet or damp mass to be dried in a container. Hot dry gas is fed into the container to flow over and/or through the mass to be dried, and the vapor-containing gas which is produced by contact with the mass is discharged from the container to the input of a vortex tube. The vortex tube separates the humid vapor into a liquid which may be discharged, and a dried gas which is recirculated back to the container for further drying of the mass.

Young I. Cho

Papers

Local Friction and Heat Transfer Behavior of Water in a Turbulent Pipe Flow With a Large Heat Flux at the Wall

Analysis of Streamer Propagation for Electric Breakdown in Liquid/Bioliquid

Device and method for reducing blood pressure

Drying process with vortex tube

Use of RF electric fields for simultaneous mineral and bio-fouling control in a heat exchanger