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

An introduction to heat transfer

In this review we focus on recent developments in applications of bio-inspired special wettable surfaces. We highlight surface materials that in recent years have shown to be the most promising in their respective fields for use in future applications. The selected topics are divided into three groups, applications of superhydrophobic surfaces, surfaces of patterned wettability and integrated multifunctional surfaces and devices. We will present how the bio-inspired wettability has been integrated into traditional materials or devices to improve their performances and to extend their practical applications by developing new functionalities.

Applications of Bio‐Inspired Special Wettable Surfaces

This paper explores the recent research developments in high-heat-flux thermal management. Cooling schemes such as pool boiling, detachable heat sinks, channel flow boiling, microchannel and mini-channel heat sinks, jet-impingement, and sprays, are discussed and compared relative to heat dissipation potential, reliability, and packaging concerns. It is demonstrated that, while different cooling options can be tailored to the specific needs of individual applications, system considerations always play a paramount role in determining the most suitable cooling scheme. It is also shown that extensive fundamental electronic cooling knowledge has been amassed over the past two decades. Yet there is now a growing need for hardware innovations rather than perturbations to those fundamental studies. An example of these innovations is the cooling of military avionics, where research findings from the electronic cooling literature have made possible the development of a new generation of cooling hardware which promise order of magnitude increases in heat dissipation compared to today's cutting edge avionics cooling schemes.

Assessment of high-heat-flux thermal management schemes

Superhydrophobic and superoleophobic surfaces have so far been made by roughening a hydrophobic material. However, no surfaces were able to repel extremely-low-energy liquids such as fluorinated solvents, which completely wet even the most hydrophobic material. We show how roughness alone, if made of a specific doubly reentrant structure that enables very low liquid-solid contact fraction, can render the surface of any material superrepellent. Starting from a completely wettable material (silica), we micro- and nanostructure its surface to make it superomniphobic and bounce off all available liquids, including perfluorohexane. The same superomniphobicity is further confirmed with identical surfaces of a metal and a polymer. Free of any hydrophobic coating, the superomniphobic silica surface also withstands temperatures over 1000°C and resists biofouling.

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Turning a surface superrepellent even to completely wetting liquids

This paper discusses the “inlet temperature difference” (ITD) based heat-exchanger (and its variants) design methodology frequently used by designers of electronic heat sinks. This is at variance with the accepted methodology recommended in standard heat-exchanger textbooks—the “log-mean temperature difference,” or the equivalent e-NTU design method. The purpose of this paper is to evaluate and discuss the ITD based design methodology. The paper shows that the ITD based method is an approximation at best. Variants of the method can lead to either under- or overprediction of the heat transfer rate. Its shortcomings are evaluated and designers are directed to the well established and accepted design methodology.

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Heat exchanger design methodology for electronic heat sinks

This paper studies enhancement of heat and mass transfer between a countercurrent, gravity-drained water film and air flowing in a vertical tube. The enhancement technique employed is spaced, transverse wires placed in the air boundary layer, near the air--water interface. Heat transfer correlations for turbulent, single-phase heat transfer in pipes having wall-attached spaced ribs are used to select the preferred wire diameter, and to predict the gas phase heat and mass transfer coefficients. Tests were run with two different radial placements of the rib roughness: (1) at the free surface of the liquid film, and (2) the base of the roughness displaced 0.51 mm into the air flow. The authors hypothesize that the best heat/mass transfer and friction performance will be obtained with the roughness at the surface of the water film. Experiments conducted with both roughness placements show that the authors' hypothesis is correct. The measured heat/mass transfer enhancement agreed very closely with the predicted values. A unique feature of the enhancement concept is that it does not require surface wetting of the enhancement device to provide enhancement.

Enhancement of Combined Heat and Mass Transfer in a Vertical-Tube Heat and Mass Exchanger

This work developed an analytical model for the friction factor with turbulent flow in internally finned channels. Such channels are an important class of enhanced heat transfer surfaces. Until this work, no analytical models for the turbulent friction factor have been proposed. The present model assumes the validity of the Law of the Wall and applies the logarithmic velocity distribution to the interfin and core regions of the flaw. Theoretically based friction factor equations are developed for internally finned circular tubes and rectangular channels. The model predicts Carnavos data for 21 internally finned tubes with +- 10 percent. Friction factor data were taken for five internally finned, rectangular channels. The analytical model predicts these data within +- 10 percent, except for the case of a very high fin.

Analytic Prediction of the Friction Factor for Turbulent Flow in Internally Finned Channels

This paper reports work on advanced cooling technology for servers. The air cooling load on the rack may be enhanced using highly compact Copper/Brass “flat tube” water-cooled heat exchangers that are integrated into the rack frame. The cooling water is supplied by a water chiller. Analysis shows that it is possible to provide cooling (UA/Afr ) in the range of 670–1000 W/m2 -K, where the m2 is the core frontal area. Also analyzed and compared are advanced technology CPU heat sinks — a thermo-syphon concept and a liquid micro-channel heat sink. The thermo-syphon may be used in a compact thermal-bus concept for heat removal from multiple CPUs. Used with boiling on an enhanced copper boiling surface in a thermo-syphon, heat loads in excess of 75 W/m2 -K are possible. The heat removed at each CPU in the chassis is rejected to water flow in a compact water cooled condenser. Performance results of the thermo-syphon concept are predicted, obstacles associated with increasing performance are discussed, and possible solutions are proposed. Performance predictions were also made for water cooled: (1) Copper micro-channels as an attached external sink and (2) Silicon micro-channels integral to the silicon CPU die. It is shown that micro-channels integrated into the silicon die do not offer significant advantage over copper micro-channels. This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.Copyright © 2005 by ASME

Ralph L. Webb

Papers

Heat exchanger design methodology for electronic heat sinks

Enhancement of Combined Heat and Mass Transfer in a Vertical-Tube Heat and Mass Exchanger

Analytic Prediction of the Friction Factor for Turbulent Flow in Internally Finned Channels

Advanced Technology for Server Cooling

Pool boiling of R-11 refrigerant and water on oxidized enhanced tubes