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

Effects of sarpogrelate on blood viscosity.

This paper presents the temporally resolved images of plasma discharge in de-ionized water. The discharge was produced by high voltage pulses with 0.3 ns rise time and 10 ns duration. The temporal resolution of the imaging system was one nanosecond. A unique three-stage process, including a fast ignition at the leading edge of the pulse, quenching at the plateau of the pulse, and self re-ignition at the trailing edge of the pulse, was observed in a single pulse cycle. The maximum measured propagation velocity of the plasma discharge was about 1000 km/s. The possibility of direct ionization in water under high reduced electric field conditions was discussed.

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Temporally resolved imaging on quenching and re-ignition of nanosecond underwater discharge

Study of the performance of physical water treatment with a solenoid coil to prevent mineral fouling. Part 2: Effect of air bubbles

Background: Common carotid artery (CCA) and internal carotid artery (ICA) are aligned linearly,
 but their hemodynamic role in ischemic stroke has not been studied in depth. Objectives:
 We aimed to investigate whether CCA and ICA endothelial shear stress (ESS) could
 be associated with the ischemic stroke of large artery atherosclerosis (LAA). Methods:
 We enrolled consecutive patients with unilateral ischemic stroke of LAA and healthy
 controls aged >60 years in the stroke center of Jeonbuk National University Hospital.
 All patients and controls were examined with carotid artery time-of-flight magnetic
 resonance angiography, and their endothelial signal intensity gradients (SIGs) were
 determined, as a measure of ESS. The effect of right or left unilateral stroke on
 the association between carotid artery endothelial SIG and ischemic stroke of LAA
 was assessed. Results:
 In total, the results from 132 patients with ischemic stroke of LAA and 121 controls
 were analyzed. ICA endothelial SIG showed significant and independent associations
 with the same-sided unilateral ischemic stroke of LAA, even after adjusting for the
 potential confounders including carotid stenosis, whereas CCA endothelial SIG showed
 a significant association with the presence of the ischemic stroke of LAA. Conclusion:
 Although CCA and ICA are located with continuity, the hemodynamics and their roles
 in large artery ischemic stroke should be considered separately. Further studies are
 needed to delineate the pathophysiologic roles of ESS in CCA and ICA for large artery
 ischemic stroke.

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Association of Carotid Artery Endothelial Signal Intensity Gradient with Unilateral Large Artery Ischemic Stroke

ABSIRAcr To identify the hemodynamic mechanisms associated with recurrence of intracranial aneurysms following the treatment with balloon or coil occlusion. Pulsatile flow in a terminal recurrent intracranial aneurysm was numerically simulated based on the physiological pulsatile flow observed in the middle cerebral artery. In order to quantify the reason for recurrence of terminal aneurysm. flow parameters including local maximum wall shear stress and pressure drop at the neck of the aneurysm was calculated. Though the maximum shear stress at the right neck of the aneurysm after the insertion of the balloon is significantly reduced. it is still two and a half times more than the normal maximum shear stress. It is observed that due to the residual neck left after the insertion of the balloon an inadequate reduction in shear stress is achieved at the right neck where the recurrence of aneurysm is initiated. It is clear that the structural alteration of the endothelial layers in the aneurysm neck, where the shear stress was high. was critical.

Young I. Cho

Papers

Effects of sarpogrelate on blood viscosity.

Temporally resolved imaging on quenching and re-ignition of nanosecond underwater discharge

Study of the performance of physical water treatment with a solenoid coil to prevent mineral fouling. Part 2: Effect of air bubbles

Association of Carotid Artery Endothelial Signal Intensity Gradient with Unilateral Large Artery Ischemic Stroke

Recurrence of balloon treated intracranial terminal aneurysm: a study with pulsatile blood flow