Seok Pil Jang

Role of Brownian motion in the enhanced thermal conductivity of nanofluids

We have found that the Brownian motion of nanoparticles at the molecular and nanoscale level is a key mechanism governing the thermal behavior of nanoparticle–fluid suspensions (“nanofluids”). We have devised a theoretical model that accounts for the fundamental role of dynamic nanoparticles in nanofluids. The model not only captures the concentration and temperature-dependent conductivity, but also predicts strongly size-dependent conductivity. Furthermore, we have discovered a fundamental difference between solid/solid composites and solid/liquid suspensions in size-dependent conductivity. This understanding could lead to design of nanoengineered next-generation coolants with industrial and biomedical applications in high-heat-flux cooling.

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.

Effects of Various Parameters on Nanofluid Thermal Conductivity

The addition of a small amount of nanoparticles in heat transfer fluids results in the new thermal phenomena of nanofluids (nanoparticle-fluid suspensions) reported in many investigations. However, traditional conductivity theories such as the Maxwell or other macroscale approaches cannot explain the thermal behavior of nanofluids. Recently, Jang and Choi proposed and modeled for the first time the Brownian-motion-induced nanoconvection as a key nanoscale mechanism governing the thermal behavior of nanofluids, but did not clearly explain this and other new concepts used in the model. This paper explains in detail the new concepts and simplifying assumptions and reports the effects of various parameters such as the ratio of the thermal conductivity of nanoparticles to that of a base fluid, volume fraction, nanoparticle size, and temperature on the effective thermal conductivity of nanofluids. Comparison of model predictions with published experimental data shows good agreement for nanofluids containing oxide, metallic, and carbon nanotubes.

A Review of Thermal Conductivity Data, Mechanisms and Models for Nanofluids

Numerous studies have shown that nanofluids have superb physical properties, among which thermal conductivity has been studied most extensively but remains controversial. In this review article, we first present important milestones in experimental studies that show new features of the thermal conductivity of nanofluids, together with those that show no such special features. After a brief review of the physical mechanisms proposed to explain the thermal conductivity of nanofluids we present a critical review of the classical and new models used to predict the thermal conductivity behavior of nanofluids. We discuss some controversial issues such as data inconsistencies, the sufficiency and suitability of classical and new mechanisms, and the discrepancies between experimental data and model predictions. At the end of our review, we give some directions for future research in nanofluids and to aid researchers in resolving the controversial issues we are still facing in developing nanofluids with superior t...

Effects of the Darcy number, the Prandtl number, and the Reynolds number on local thermal non-equilibrium

Abstract In the present study a general criterion for local thermal equilibrium is presented in terms of parameters of engineering importance which include the Darcy number, the Prandtl number, and the Reynolds number. For this, an order of magnitude analysis is performed for the case when the effect of convection heat transfer is dominant in a porous structure. The criterion proposed in this study is more general than the previous criterion suggested by Carbonell and Whitaker, because the latter is applicable only when conduction is the dominant heat transfer mode in a porous medium while the former can be applied even when convection heat transfer prevails. In order to check the validity of the proposed criterion for local thermal equilibrium, the forced convection phenomena in a porous medium with a microchanneled structure subject to an impinging jet are studied using a similarity transformation. The effects of the Darcy number, the Prandtl number, and the Reynolds number on local thermal non-equilibrium are systematically studied by comparing the temperature of the solid phase with that of the fluid phase as each of these parameters is varied. The proposed criterion is also validated with the existing experimental and numerical results for convection heat transfer in various porous materials that include some of the parameters used in the criterion such as a microchannel heat sink with a parallel flow, a packed bed, a cellular ceramic, and a sintered metal. It is shown that the criterion presented in this work well-predicts the validity of the assumption of local thermal equilibrium in a porous medium.

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.

Experimental investigation of thermal characteristics for a microchannel heat sink subject to an impinging jet, using a micro-thermal sensor array

The present paper experimentally investigates the heat transfer enhancement of a microchannel heat sink subject to an impinging jet. In order to evaluate the cooling performance of the microchannel heat sink subject to an impinging jet, temperature distributions are measured by using a micro-thermal sensor array manufactured through simple and convenient microfabrication processes. Based on these experimental results, the thermal resistance of the microchannel heat sink subject to an impinging jet is obtained and compared with the thermal resistance as calculated numerically. In order to show the heat transfer enhancement of the microchannel heat sink subject to an impinging jet, its thermal resistance is compared with that of a microchannel heat sink with a parallel flow. Under the condition that the pumping power is 0.072 W, the thermal resistance of the microchannel heat sink subject to an impinging air jet is experimentally obtained to be 6.1 °C/W, which is enhanced by about 48.5% compared with that of the microchannel heat sink with a parallel flow. In addition, the microchannel heat sink subject to an impinging jet is shown to be superior to a manifold microchannel heat sink as a cooling device for advanced electronic equipment with high heat generation and compact size.

Experimental and numerical analysis of heat transfer phenomena in a sensor tube of a mass flow controller

In the present work, the heat transfer phenomena in the sensor tube of a mass flow controller (MFC) are studied using both experimental and numerical methods. A numerical model is introduced to predict the temperature profile in the sensor tube as well as in the gas stream. In the numerical model, the conjugate heat transfer problem comprising the tube wall as well as the gas stream is analyzed to fully understand the heat transfer interaction between the sensor tube and the fluid stream, using a single domain approach. This numerical model is further verified by experimental investigation. In order to describe the transport of heat energy in both the flow region and the sensor tube, the Nusselt number distributions at the interface between the tube wall and the gas stream as well as heatlines are presented from the numerical solution. Through this study several assumptions frequently used by previous investigators for their analytical models have been shown to be either irrelevant or physically unrealistic.

Fluid Flow and Thermal Characteristics of a Microchannel Heat Sink Subject to an Impinging Air Jet

In the present study, fluid-flow and heat-transfer characteristics of a microchannel heat sink subject to an impinging jet are experimentally investigated. In order to evaluate the cooling performance of a microchannel heat sink subject to an impinging jet under the condition of fixed pumping power, the pressure drop across the heat sink and temperature distributions at its base are measured. Specifically, a microthermal sensor array is fabricated and used to accurately measure temperature distributions at the base of the heat sink. Based on these experimental results, a correlation for the pressure drop across a microchannel heat sink subject to an impinging jet and a correlation for its thermal resistance are suggested. In addition, it is shown that the cooling performance of an optimized microchannel heat sink subject to an impinging jet is enhanced by about 21% compared to that of the optimized microchannel heat sink with a parallel flow under the fixed-pumping-power condition.

Free Convection in a Rectangular Cavity (Benard Convection) With Nanofluids

Investigators have been surprised with new thermal phenomena behind the recently discovered nanofluids, fluid with unprecedented stability of suspended nanoparticles although huge differences in the density of nanoparticles and fluid. For example, nanofluids have anomalously high thermal conductivities at very low volume fraction, strongly temperature-dependent and size-dependent conductivity, and three-fold higher critical heat flux than that of base fluids. In this paper, the thermal characteristics of free convection in a rectangular cavity with nanofluids such as water-based nanofluids containing 6nm copper and 2nm diamond are theoretically investigated with a new model of the thermal conductivity for nanofluids presented by Jang and Choi. In addition, based on theoretical results, the effects of various parameters such as the volume fraction, the temperature, and the size of nanoparticles on free convective instability and heat transfer characteristics in a rectangular cavity with nanofluids are suggested.Copyright