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.

A Benchmark Study on the Thermal Conductivity of Nanofluids

This paper presents a critical review of heat transfer applications of nanofluids. The effects of nanoparticle concentration, size, shape, and nanofluid flow rate on Nusselt number, heat transfer coefficient, thermal conductivity, thermal resistance, friction factor and pressure drop from numerous studies reported recently are presented. Effects of various geometric parameters on heat transfer enhancement of system using nanofluids have also been reviewed. Heat transfer devices covered in this paper include radiators, circular tube heat exchangers, plate heat exchangers, shell and tube heat exchangers and heat sinks. Various correlations used for experimental validation or developed in reviewed studies are also compiled, compared and analyzed. The pros and cons associated to the applications of nanofluids in heat transfer devices are presented in details to determine the future direction of research in this arena.

Recent advances in application of nanofluids in heat transfer devices: A critical review

https://bura.brunel.ac.uk/bitstream/2438/14115/5/Fulltext.pdf

Flow boiling in microchannels: Fundamentals and applications

Depleting energy resources have become the driving force for their conservation. Increasing the system efficiencies is one method by which sustainability of energy may be ensured, for which miniaturization has successfully provided solutions. Miniature heat exchangers, owing to their high thermal performance, have the potential to provide energy efficient systems. In addition, their characteristics of compactness, small size and lesser weight have attracted widespread applications. Various works on micro- and minichannel heat exchangers as heat sinks and heat exchangers have been reviewed in this paper. Currently employed fabrication techniques and different applications have been summarized. An overview of the single-phase thermo-hydraulic studies in micro- and minichannel heat sinks has been presented. Literatures related co-current, counter-current and cross-current micro- and minichannel heat exchangers have been discussed. Finally, the persisting lacunae of this technology drawn from the review have been pointed out.

Review of micro- and mini-channel heat sinks and heat exchangers for single phase fluids

Heat transfer and fluid flow in microchannels have been topics of intense research in the past decade. A critical review of the current state of research is presented with a focus on the future research needs. After providing a brief introduction, the paper addresses six topics related to transport phenomena in microchannels: single-phase gas flow, enhancement in single-phase liquid flow and flow boiling, flow boiling instability, condensation, electronics cooling, and microscale heat exchangers. After reviewing the current status, future research directions are suggested. Concerning gas phase convective heat transfer in microchannels, the antagonist role played by the slip velocity and the temperature jump that appear at the wall are now clearly understood and quantified. It has also been demonstrated that the shear work due to the slipping fluid increases the effect of viscous heating on heat transfer. On the other hand, very few experiments support the theoretical models and a significant effort should be made in this direction, especially for measurement of temperature fields within the gas in microchannels, implementing promising recent techniques such as molecular tagging thermometry (MTT). The single-phase liquid flow in microchannels has been established to behave similar to the macroscale flows. The current need is in the area of further enhancing the performance. Progress on implementation of flow boiling in microchannels is facing challenges due to its lower heat transfer coefficients and critical heat flux (CHF) limits. An immediate need for breakthrough research related to these two areas is identified. Discussion about passive and active methods to suppress flow boiling instabilities is presented. Future research focus on instability research is suggested on developing active closed loop feedback control methods, extending current models to better predict and enable superior control of flow instabilities. Innovative high-speed visualization and measurement techniques have led to microchannel condensation now being studied as a unique process with its own governing influences. Further work is required to develop widely applicable flow regime maps that can address many fluid types and geometries. With this, condensation heat transfer models can progress from primarily annular flow based models with some adjustments using dimensionless parameters to those that can directly account for transport in intermittent and other flows, and the varying influences of tube shape, surface tension and fluid property differences over much larger ranges than currently possible. Electronics cooling continues to be the main driver for improving thermal transport processes in microchannels, while efforts are warranted to develop high performance heat exchangers with microscale passages. Specific areas related to enhancement, novel configurations, nanostructures and practical implementation are expected to be the research focus in the coming years.

Heat Transfer in Microchannels—2012 Status and Research Needs

Optimization of short micro pin fins in minichannels

Advancements in electronic performance result in a decrease in device size and increase in power density. Because of these advancements, current cooling mechanisms for electronic devices are beginning to be ineffective. Within the localized hot spots, the materials of the components are reaching temperature values that can lead to improper functioning of the device. Many techniques have been successful in the past, such as heat sinks, cavities or grooves, micro pin-fins, etc., but still do not provide adequate cooling necessary to maintain temperature values low enough for the electronic components to operate. Microchannels, with their large heat transfer surface to volume ratio, cooled with either gas or liquid coolant, have shown some potential. By modifying the walls of the microchannel with fins, pins, or grooves, the cooling performance can be improved. A possible fin material used to increase the surface area of a microchannel is carbon nanotubes, which possess excellent thermal and mechanical prope...

J.F. Tullius

Papers

Optimization of short micro pin fins in minichannels

A Review of Cooling in Microchannels

Effect of Al2O3/H2O nanofluid on MWNT circular fin structures in a minichannel

Minichannels with carbon nanotube structured surfaces for cooling applications

Optimization of microstructured fins in minichannels