Young I. Cho

Bio: Young I. Cho is an academic researcher from Drexel University. The author has contributed to research in topics: Fouling & Blood viscosity. The author has an hindex of 42, co-authored 266 publications receiving 12349 citations. Previous affiliations of Young I. Cho include California Institute of Technology & Thomas Jefferson University Hospital.

Computer simulation of blood flow using short te magnetic resonance angiography

Abstract: Using computer simulation of pulsatile blood flow and magnetic resonance angiography (MRA), we investigated the hemodynamic factors leading to the formation and evolution of 1) atherosclerotic plaque in carotid arteries, and 2) aneurysms in the abdominal aorta. Phantom and patients were imaged by MRA, color Doppler and/or digital subtraction angiography (DSA). Computer modelling was carried out by finite element analysis to solve the Navier-Stokes equation. In the analyzed vessels, voxels representing pathology were digitally removed. Local wall shear stress and pressure were also calculated as a function of a cardiac cycle. There was general agreement between MRA, color Doppler, DSA and computer simulation in both phantom and in-vivo experiments. In MRA, the best results were achieved by short TE, thin slice 2D “time-of-flight” technique, which was least susceptible to the changes in velocity profiles, and best correlated with Doppler and computer simulation. The hemodynamic information obtained from analyzed carotid arteries predicted that during late systole, flow separation exists at the exact locations from where the plaque voxels were removed. We were also able to predict the location of abdominal aortic aneurysm and its evolution toward the distal vessel intima. In conclusion, MRA accurately depicted flow disturbances, and computer simulation of blood flow proved to be a good predictor of the development of vascular pathology.

Model for Development of Electric Breakdown in Liquids and Stability Analysis

Abstract: A semi-numerical model is presented for the development of electric breakdown in liquids based on nanosecond time scale. In this model, breakdown starts in the pre-existed bubble at the tip of the electrode. Formation of plasma immediately makes the surface of the bubble charged to the potential of the electrode, and the imbalance between the electrostatic force and surface tension makes the bubble elongate at high speed. Numerical estimations made using this model are in good agreement with published experimental data. Linear analysis also shows that the instability of the Rayleigh-Taylor type develops in the plasma-gaseous channel surface points with high curvature.

Dual capillary viscometer for newtonian and non-newtonian fluids

Abstract: An apparatus and method for measuring the viscosity of Newtonian/non-Newtonian fluids over a range of shear rates, especially low shear rates, by monitoring two rising columns of fluid that pass through respective capillaries having different lengths. Furthermore, a specialized column monitor is provided that uses multiple interrogation sources (e.g., lasers) and a single detector (e.g., a charge-coupled device array) to continuously monitor both columns of fluid substantially simultaneously. In particular, the system includes a Y-connector to form two flow paths and wherein each flow path includes a tube that includes a riser tube, a capillary tube of predetermined dimensions and a valve in each for controlling the fluid flow in each path. The specialized column monitor monitors fluid column movement in each of the riser tubes and an associated microprocessor analyzes these movements, along with the predetermined dimensions of the capillary tubes and riser tubes to determine the fluid viscosity.

Convective Transport in Nanofluids

Abstract: 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

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.

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

Abstract: 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...

Thermal Conductivity of Nanoparticle -Fluid Mixture

Abstract: 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