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

For producing controlled precipitation in fluid flowing through a pipe, a coil of electric wire, of flat, generally rectangular spiral shape, is sandwiched between flexible cover sheets. The resulting assembly is applied to the outer circumference of the pipe, with the lengthwise dimension of the coil extending perpendicularly to the flow inside the pipe. A time-varying electric current is applied to the coil to produce a corresponding magnetic field. This, in turn, causes induction which produces the controlled precipitation.

Electronic scale reduction technique

PurposeThe purpose of the present study was to examine the effect of hemodialysis procedures on the hemoconcentration status of end-stage renal disease (ESRD) patients.MethodsWe measured whole bloo...

Effect of hemodialysis on whole blood viscosity.

Aim: Homocysteine is associated with increased arterial resistance and eventually causes luminal reduction. The purpose of the present study was to evaluate an association between the plasma concentration of total homocysteine (tHcy) and stenosis or occlusion of the internal carotid artery (ICA) in patients with ischemic stroke.Methods: In total, 391 patients with ischemic stroke were evaluated from March 2007 to February 2008. The criterion for ICA stenosis or occlusion was set at greater than 50% luminal narrowing or complete obstruction in at least one ICA. Patients were assigned to one of three groups: normal ICA, ICA stenosis, and ICA occlusion.Results: ICA stenosis was found in 71 patients, whereas ICA occlusion in 22 patients (18.2% and 5.6%, respectively). Plasma tHcy was significantly higher in groups with ICA stenosis/occlusion with the highest value of ICA occlusion (14.6±1.0 μmol/L, p=0.025). A 1 μmol/L increase of tHcy showed an adjusted odds ratio of 1.12 (95% confidence intervals, 1.03-1.24, p=0.008) for ICA occlusion in a multivariate logistic model adjusted for all possible confounders, including age, sex, vascular risk factors, and stroke classifications.Conclusion: Elevated levels of tHcy were significantly associated with the ICA occlusion, independent of vascular risk factors and stroke subtypes.

Homocysteine and Internal Carotid Artery Occlusion in Ischemic Stroke

Benefit of filtration in physical water treatment for the mitigation of mineral fouling in heat exchangers

A two‐dimensional pulsatile hemodynamic analysis based on the finite‐element technique was performed on a minimally stenosed carotid artery to identify the possible explanation for the differences in the x‐ray and magnetic resonance carotid angiograms of a patient. The magnetic resonance angiogram was obtained by applying the maximum intensity projection algorithm to axial slices, acquired using the time‐of‐flight principle. The differences in the x‐ray and magnetic resonance depictions were interpreted based on velocity profile, wall shear stress, and streamline data provided by the hemodynamic analysis. The specific contribution of the stenosis was further isolated from that of the bifurcation by comparing the flow patterns within the stenotic artery with those of its normal counterpart. The Doppler spectral velocity wave form of the patient constituted the basis for the pulsatile flow velocity specification. The analysis took into consideration the non‐Newtonian viscosity of blood. The numerical procedure was validated through different convergence criteria and through shear stress comparisons. The importance of hemodynamic analyses in relation to magnetic resonance angiography was further discussed along with possible shortcomings of the technique.

Young I. Cho

Papers

Electronic scale reduction technique

Effect of hemodialysis on whole blood viscosity.

Homocysteine and Internal Carotid Artery Occlusion in Ischemic Stroke

Benefit of filtration in physical water treatment for the mitigation of mineral fouling in heat exchangers

Two-dimensional pulsatile hemodynamic analysis in the magnetic resonance angiography interpretation of a stenosed carotid arterial bifurcation