Kyo Sik Hwang

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Particle concentration and tube size dependence of viscosities of Al2O3-water nanofluids flowing through micro- and minitubes

An experimental and theoretical investigation has been performed on the effective viscosity of Al2O3-water nanofluids flowing through micrometer- and millimeter-sized circular tubes in the fully developed laminar flow regime. We have discovered that the effective viscosity of Al2O3-water nanofluids increases nonlinearly with the volume concentration of nanoparticles even in the very low range of 0.02–0.3vol% and strongly depends on the ratio of the nanoparticle diameter to the tube diameter. We have developed a modified Einstein model that accounts for the slip mechanism in nanofluids. The new model captures these new rheological features of nanofluids.

Effective thermal conductivities and viscosities of water-based nanofluids containing Al2O3 with low concentration

We experimentally investigated effective thermal conductivities and viscosities of water-based nanofluids containing Al₂O₃ (Al₂O₃-nanofluids) with low concentration from vol. 0.01% to 0.3%. Without surfactant, Al₂O₃-nanofluids are manufactured by two-step method which is widely used. To examine suspension and dispersion characteristics of Al₂O₃-nanofluids, zeta potential as well as transmission electron micrograph of Al₂O₃ nanoparticles is observed. The effective viscosities of Al₂O₃-nanofluids according to the temperature are measured by a viscometer of oscillating type. The transient hot wire method is used in this study to measure the effective thermal conductivities of Al₂O₃-nanofluids. Based on the results the maximum increase of effective viscosities of Al₂O₃-nanofluids is up to 2.9% while the maximum enhancement of effective thermal conductivities is up to 1.44%.

Round-robin test on thermal conductivity measurement of ZnO nanofluids and comparison of experimental results with theoretical bounds

Ethylene glycol (EG)-based zinc oxide (ZnO) nanofluids containing no surfactant have been manufactured by one-step pulsed wire evaporation (PWE) method. Round-robin tests on thermal conductivity measurements of three samples of EG-based ZnO nanofluids have been conducted by five participating labs, four using accurate measurement apparatuses developed in house and one using a commercial device. The results have been compared with several theoretical bounds on the effective thermal conductivity of heterogeneous systems. This study convincingly demonstrates that the large enhancements in the thermal conductivities of EG-based ZnO nanofluids tested are beyond the lower and upper bounds calculated using the models of the Maxwell and Nan et al. with and without the interfacial thermal resistance.

Flow and Thermal Characteristics of Condensing Steam in a Single Horizontal Mini-Channel of a Multiport Cylinder Dryer

In papermaking processes it is well known that the dryer uses the greatest amount of energy and has costly components compared with other devices. These dryers thus offer outstanding opportunities for energy savings and cost reduction. Recently, Argonne National Laboratory developed the concept of a multiport cylinder dryer in which the steam flows through “ports,” or longitudinally oriented mini-channels, which are close to the cylinder–dryer surface. It was reported that the innovative design minimized the condensate layer thickness and increased the dryer shells surface temperature. However, no systematically obtained experimental data are available. In this study we conducted experiments to investigate the flow and thermal characteristics of condensing steam in a single horizontal mini-channel, which represents a part of a multiport cylinder dryer. Based on the results, the flow pattern through a single horizontal mini-channel is presented. Also, we observed that the condensing heat transfer coefficient in the single horizontal mini-channel was about seven times higher than that of a conventional dryer with spoiler bars. Finally, the effects of the steam mass flux, system pressure, and aspect ratio of the mini-channel on the average condensing heat transfer coefficients as well as the pressure drops are presented.

Flow and Convective Heat Transfer Characteristics of Nanofluids With Various Shapes of Alumina Nanoparticles

This paper is to investigate flow and convective heat transfer characteristics of nanofluids with various shapes of Al2 O3 nanoparticles flowing through a uniformly heated circular tube under fully developed laminar flow regime. For the purpose, Al2 O3 nanofluids of 0.3 Vol.% with sphere, rod, platelet, blade and brick shapes are manufactured by a two-step method. Zeta potential as well as TEM image is experimentally obtained to examine suspension and dispersion characteristics of Al2 O3 nanofluids with various shapes. To investigate flow characteristics, the pressure drop of Al2 O3 nanofluids with various shapes are measured. In order to investigate convective heat transfer characteristics, the effective thermal conductivities of Al2 O3 nanofluids with various shapes, the temperature distribution at the tube surface and the mean temperature of nanofluids at the inlet are measured, respectively. Based on the experimental results, the convective heat transfer coefficient of Al2 O3 nanofluids with various shapes is compared with that of pure water and the thermal conductivity of Al2 O3 nanofluids with various shapes. Thus, the effect of nanoparticles shape on the flow and convective heat transfer characteristics flowing through a uniformly heated circular tube under fully developed laminar flow regime is experimentally investigated.Copyright

Flow Characteristics of Al2O3Nanofluids with Nanoparticles of Various Shapes

To study the flow characteristics of water-based nanofluids according to the shape of the nanoparticles, we measure the pressure drop in a fully developed laminar flow regime. Water-based nanofluids of 0.3 Vol.% with sphere-, rod-, platelet-, and brick-shaped nanoparticles are manufactured by the two-step method. Zeta potential is measured to examine the suspension and dispersion characteristics, and TEM image is considered to confirm the shape characteristics of the nanoparticles. The experimental results show that the pressure drop of nanofluids depends on the shape of the nanoparticles although the nanofluids has same volume fraction of nanoparticles. This is explained by the surface area per unit mass of the nanoparticles and the size of the nanoparticles suspended in the base fluids.

Flow Characteristics of Nanofluids According to Nanoparticles Shape

In this paper, flow characteristics of water-based Al2O3 nanofluids according to nanoparticles shape are experimentally investigated in fully developed laminar flow regime. Al2O3 nanofluids of 0.3 Vol. % with sphere-, rod-, blade-, platelet-and brick-shaped nanoparticles are manufactured by the two-step method. Nanoparticles shape dispersed in base fluid are also checked using TEM image. Zeta potential and sedimentation are measured to examine suspension and dispersion characteristics of Al2O3 nanofluids with nanoparticles of various shapes. Based on the experimental results, it is found that the pressure drop of Al2O3 nanofluids strongly depends on the shape of nanoparticles at the fixed volume fraction of 0.3%. We experimentally show that the pressure drop characteristics of Al2O3 nanofluids can be explained by both the surface area per unit mass and the size of nanoparticles which are related with the shape of nanoparticles.