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.

/pdf/investigation-on-convective-heat-transfer-and-flow-features-3a5qwo5efi.pdf

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

/pdf/thermal-conductivity-of-nanoparticle-fluid-mixture-54tj4jnmls.pdf

Thermal Conductivity of Nanoparticle -Fluid Mixture

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

An energy-saving refrigeration system circulates a mixture of R-134a, R-32 and R-125 whose composition is controlled using a vapor separator. A vortex generator is a preferred means to separate the mixture. For high thermal load operation, R-32 stays in the circulating line with increased cooling capacity, and R-134a and R-125 are stored in a storage tank. Conversely, for low thermal load operation, R-134a and R-125 stay in the circulating line with increased EER, and R-32 is stored in a storage tank. Multiple storage tanks can be used to control the composition of each refrigerant independently.

Energy saving refrigeration system using composition control with mixed refrigerants

A scanning capillary-tube viscometer (SCTV) determines the viscosity of an opaque liquid by measuring the changes in liquid level in two riser tubes. The current SCTV utilizes an optical sensor (i.e. charge-coupled device (CCD)) to measure the changes in the liquid level, where the liquid blocks the path of the light to the optical sensor. For a transparent liquid, one can use dye to make the liquid opaque. Hence, the present study investigated whether or not the addition of dye altered the viscosity of a transparent liquid such as distilled water. In this study, six different concentrations (0.5, 1, 2, 3, 4, and 7 vol.%) of dye were used. The present results demonstrated that the dye effect on the viscosity of dye–water solution was negligibly small as long as the dye concentration was less than 2 vol.%.

The effect of dye concentration on the viscosity of water in a scanning capillary-tube viscometer

Apparatus (20, 100) and methods for determining the deformability of the red blood cells of a living being. In one embodiment the apparatus (20) comprises a sampling unit (22) arranged to be coupled to a blood vessel (26) of the being for withdrawing a portion of the blood of the being at substantially the time that said portion of said blood is flowing through said vessel to calculate the deformability of red blood cells. In another embodiment the apparatus (100) comprises a viewing system (104) to provide direct visualization of the red blood cells and also includes a unit (24) for calculating blood viscosity to correlate that information with the perceived red blood cell deformability.

Apparatus and method for determining deformability of red blood cells of a living being

For producing controlled precipitation in fluid flowing through a pipe, a coil of electric wire is applied to the pipe segment in which this controlled precipitation is to be produced. This coil may be in the form of an induction coil wrapped around the pipe. Or the coil may be of a flat, rectangular spiral form. In any case, the coil is applied in such a way that the center of its curvature does not coincide with the pipe axis. Instead, this center of coil curvature is offset from the pipe axis. Preferably, this offset is approximately equal to the radius of the pipe.

Electronic scale reduction by eccentrically positioned coils

Four different heat transfer paths were separately investigated in a spirally-wound D-size cell. Thermal resistances of the relating components along each heat transfer path were calculated, and the total network of thermal resistances was constructed, resulting in the total thermal resistance of approximately 16.4 C/W at Tw = 40 C, and 12.2 C/W at Tw = 100 C. The amount of heat transferred along the radial direction, which is normal to the electrodes, was found to be about 91 percent of the total heat dissipation. Based on the present results obtained at the component level, the present D-size lithium cell may be operated at a C/2 discharge rate under normal discharge conditions below the cut-off voltage of 2.5 V.

Young I. Cho

Papers

Energy saving refrigeration system using composition control with mixed refrigerants

The effect of dye concentration on the viscosity of water in a scanning capillary-tube viscometer

Apparatus and method for determining deformability of red blood cells of a living being

Electronic scale reduction by eccentrically positioned coils

Thermal Modeling of High Rate Li ‐ SOCl2 Primary Cylindrical Cells